# Magnetic Structure and Propagation of Two Interacting CMEs from the Sun to
Saturn

###### Abstract

One of the grand challenges in heliophysics is the characterisation of coronal
mass ejection (CME) magnetic structure and evolution from eruption at the Sun
through heliospheric propagation. At present, the main difficulties are
related to the lack of direct measurements of the coronal magnetic fields and
the lack of 3D in-situ measurements of the CME body in interplanetary space.
Nevertheless, the evolution of a CME magnetic structure can be followed using
a combination of multi-point remote-sensing observations and multi-spacecraft
in-situ measurements as well as modelling. Accordingly, we present in this
work the analysis of two CMEs that erupted from the Sun on 28 April 2012. We
follow their eruption and early evolution using remote-sensing data, finding
indications of CME–CME interaction, and then analyse their interplanetary
counterpart(s) using in-situ measurements at Venus, Earth, and Saturn. We
observe a seemingly single flux rope at all locations, but find possible
signatures of interaction at Earth, where high-cadence plasma data are
available. Reconstructions of the in-situ flux ropes provide almost identical
results at Venus and Earth but show greater discrepancies at Saturn,
suggesting that the CME was highly distorted and/or that further interaction
with nearby solar wind structures took place before 10 AU. This work
highlights the difficulties in connecting structures from the Sun to the outer
heliosphere and demonstrates the importance of multi-spacecraft studies to
achieve a deeper understanding of the magnetic configuration of CMEs.

JGR-Space Physics

Space Sciences Laboratory, University of California–Berkeley, Berkeley, CA,
USA CPAESS, University Corporation for Atmospheric Research, Boulder, CO, USA
Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt,
MD, USA Department of Physics, University of Helsinki, Helsinki, Finland STFC
RAL Space, Rutherford Appleton Laboratory, Harwell Campus, Oxfordshire, UK
Solar–Terrestrial Centre of Excellence—SIDC, Royal Observatory of Belgium,
Brussels, Belgium Skobeltsyn Institute of Nuclear Physics, Moscow State
University, Moscow, Russia ESTEC, European Space Agency, Noordwijk,
Netherlands School of Physics and Astronomy, University of Leicester,
Leicester, UK National Institute of Information and Communications Technology
(NICT), Koganei, Japan LESIA, Observatoire de Paris, PSL, CNRS, UPMC,
Université Paris Diderot, Meudon, France LAM, Pythéas, Aix Marseille
Université, CNRS, CNES, Marseille, France Space Research Institute, Austrian
Academy of Sciences, Graz, Austria Institute of Geodesy, Graz University of
Technology, Graz, Austria Max Planck Institute for Solar System Research,
Göttingen, Germany Swedish Institute of Space Physics, Kiruna, Sweden The
Blackett Laboratory, Imperial College London, London, UK

Erika<EMAIL_ADDRESS>
We analyse the eruption of two coronal mass ejections on 28 April 2012 and
their evolution up to Saturn

We study and compare the flux rope magnetic structure at the Sun, at Venus, at
Earth, and at Saturn

We find a single flux rope structure at all planets, suggesting interaction of
the two eruptions in the inner heliosphere

##  1 Introduction

Coronal mass ejections (CMEs; e.g., Webb and Howard, 2012) are spectacular
eruptions of magnetic fields and plasma that are regularly launched from the
Sun throughout the heliosphere. Their magnetic structure, when they leave the
solar atmosphere, is that of a flux rope (e.g., Chen, 2011; Forbes, 2000;
Green et al., 2018; Klimchuk, 2001), i.e. consisting of a bundle of magnetic
fields wrapped about a central axis. After erupting, CMEs usually undergo
rapid acceleration and expansion in the low corona (e.g., Patsourakos et al.,
2010a, b; Temmer et al., 2008, 2010; Veronig et al., 2018) as a result of the
large energy release and the high internal pressure compared to that of the
ambient solar wind (Démoulin and Dasso, 2009). After the initial, impulsive
phase, i.e. from the outer corona outwards, CMEs generally expand in a self-
similar fashion (e.g., Good et al., 2019; Schwenn et al., 2005; Subramanian et
al., 2014; Vršnak et al., 2019). The speed at which CMEs propagate depends on
the speed of the surrounding solar wind flow, which has the effect of
accelerating slower CMEs and decelerating faster CMEs (e.g., Gopalswamy et
al., 2000; Vršnak and Žic, 2007). CME expansion in interplanetary space is
believed to usually take place up to ${\sim}10$–$15$ AU, where CMEs reach
pressure balance with the solar wind (e.g., Richardson et al., 2006; von
Steiger and Richardson, 2006). As CMEs propagate through the outer
heliosphere, they may interact with stream interaction regions (SIRs; e.g.,
Richardson, 2018) or with other CMEs to produce merged interaction regions
(MIRs; e.g., Burlaga et al., 1986, 1997), which are believed to dominate the
structure of the heliosphere at large heliocentric distances (e.g., Gazis et
al., 2006; von Steiger and Richardson, 2006).

In reality, the sparsity of observations throughout the heliosphere, together
with the fact that in-situ measurements typically sample a 1D trajectory
through a much larger structure, mean that many aspects of CME evolution are
yet to be fully understood (for recent reviews on CME evolution, see Luhmann
et al., 2020; Manchester et al., 2017). For example, it is unclear to which
extent 1D in-situ measurements are representative of the global CME structure
(e.g., Al-Haddad et al., 2011; Owens et al., 2017), mainly because of
distortions (e.g., Manchester et al., 2004; Owens, 2008; Savani et al., 2010)
and/or the particular sampling distance with respect to the CME nose and
central axis (e.g., Cane et al., 1997; Kilpua et al., 2011; Marubashi and
Lepping, 2007). As they propagate through the solar corona and interplanetary
space, CMEs are also known to experience deflections and rotations (e.g.,
Isavnin et al., 2014; Kay et al., 2015; Vourlidas et al., 2011; Wang et al.,
2004), which may significantly affect the magnetic configuration that is later
measured in situ (e.g., Palmerio et al., 2018; Yurchyshyn, 2008). Furthermore,
in addition to the difficulties in understanding CME evolution for single-CME
events, cases where CMEs interact with solar wind structures (e.g., Heinemann
et al., 2019; Rouillard et al., 2010; Winslow et al., 2016, 2021) or with
other CMEs (e.g., Dasso et al., 2009; Farrugia and Berdichevsky, 2004; Lugaz
et al., 2017a; Scolini et al., 2020) are not infrequent, thus complicating
things further.

The properties of interplanetary CMEs (or ICMEs; e.g., Kilpua et al., 2017)
have been studied mainly around 1 AU (e.g., Cane and Richardson, 2003; Jian et
al., 2006, 2018; Nieves-Chinchilla et al., 2018a; Nieves-Chinchilla et al.,
2019; Owens, 2018; Regnault et al., 2020; Richardson and Cane, 2010), i.e.
where continuous in-situ measurements of the solar wind have been available
for several decades and up to this day. In particular, the subset of ICME
ejecta known as magnetic clouds (Burlaga et al., 1981) have been analysed
extensively around Earth’s orbit, both in statistical (e.g., Huttunen et al.,
2005; Janvier et al., 2014a; Li et al., 2014, 2018; Lynch et al., 2003;
Nieves-Chinchilla et al., 2005; Nieves-Chinchilla and Viñas, 2008; Rodriguez
et al., 2016; Wood et al., 2017) and in case studies (e.g., Kilpua et al.,
2009; Lynch et al., 2010; Möstl et al., 2008, 2009a; Nieves-Chinchilla et al.,
2011). Magnetic clouds are characterised by enhanced magnetic field
magnitudes, a large and smooth rotation of the magnetic field over one
direction, low proton temperatures, and low plasma beta, and they are of
particular interest because their magnetic configuration corresponds to that
of a flux rope. Interestingly, the first study that identified and defined
magnetic clouds in the solar wind, i.e. that of Burlaga et al. (1981), was
performed using observations from several spacecraft between 1 and 2 AU, thus
highlighting the importance of well-separated multi-spacecraft measurements to
understand the intrinsic structure of CMEs in the heliosphere.

Indeed, studying the properties of CMEs at different radial distances as they
propagate throughout the heliosphere can provide paramount information and
insight into CME evolution. Heliocentric orbiters such as Helios 1/2 and
Ulysses have enabled long-term observation and analysis of ICMEs away from 1
AU and/or the ecliptic plane (e.g., Bothmer and Schwenn, 1994, 1998; Du et
al., 2010; Jian et al., 2008a; Lepri and Zurbuchen, 2004; Liu et al., 2005;
Richardson, 2014; Riley et al., 2000). Data from the more recently launched
Parker Solar Probe, BepiColombo, and Solar Orbiter have resulted in several
studies of CMEs measured between the Sun and 1 AU (e.g., Davies et al., 2021;
Korreck et al., 2020; Lario et al., 2020; Nieves-Chinchilla et al., 2020; Zhao
et al., 2020; Weiss et al., 2021). Considerable results were also achieved
using data from spacecraft, such as Pioneer 10/11 and Voyager 1/2, that were
launched on escape trajectories out of the solar system and that have provided
valuable information on the behaviour of ICMEs at progressively larger
heliocentric distances (e.g., Burlaga et al., 2001; Wang and Richardson, 2001,
2004). Finally, another important contribution in understanding CME properties
away from 1 AU has come from planetary missions, which usually spend some part
of their orbit outside of the planet’s bow shock, thus exposing their
instruments to the solar wind (e.g., Collinson et al., 2015; Good and Forsyth,
2016; Janvier et al., 2019; Jian et al., 2008b; Lee et al., 2017; Palmerio et
al., 2021; Winslow et al., 2015).

In-situ measurements of the same CMEs by multiple spacecraft at different
radial distances throughout the heliosphere enable studies of their
interplanetary evolution (e.g., Burlaga et al., 1981; Davies et al., 2020; de
Lucas et al., 2011; Good et al., 2018, 2019; Lugaz et al., 2020; Salman et
al., 2020; Vršnak et al., 2019). Additionally, solar and heliospheric remote-
sensing observations can be combined with multi-spacecraft in-situ data to
obtain a complete picture of the whole solar–heliospheric system when
characterising CME evolution (e.g., Asvestari et al., 2021; Kilpua et al.,
2019; Möstl et al., 2015; Nieves-Chinchilla et al., 2012; Palmerio et al.,
2021; Prise et al., 2015; Richardson et al., 2002; Rodriguez et al., 2008;
Rouillard et al., 2009). In particular, the use of heliospheric imagery has
been proven useful to connect CMEs at the Sun to their in-situ counterparts
(e.g., DeForest et al., 2013; Möstl et al., 2009b, 2017; Palmerio et al.,
2019; Rouillard, 2011; Srivastava et al., 2018). ICME structures can be
identified in situ using a number of signatures based on magnetic field,
plasma, compositional, and energetic particle data (e.g., Zurbuchen and
Richardson, 2006). Furthermore, since spacecraft may lack magnetic field
and/or plasma instruments (especially those not dedicated to studying the
solar wind), reduction in galactic cosmic rays (GCR) measurements known as
Forbush decreases (Forbush, 1937; Hess and Demmelmair, 1937) can be used as a
proxy for ICME passage (e.g., Cane, 2000; Dumbović et al., 2020; Freiherr von
Forstner et al., 2018, 2020; Papaioannou et al., 2020; Richardson and Cane,
2011; Winslow et al., 2018; Witasse et al., 2017).

Despite the amount of spacecraft scattered throughout the heliosphere and
decades of research efforts, the number of well-observed multi-spacecraft
events covering extended radial distances into the outer heliosphere is still
exiguous, even when considering both studies of the solar wind and
interplanetary magnetic field (e.g., Burlaga et al., 2001; Prise et al., 2015;
Witasse et al., 2017) as well as multi-planet observing campaigns designed to
track the auroral response of giant planetary magnetospheres to CME-driven
shocks between 5 and 30 AU (e.g., Branduardi-Raymont et al., 2013; Dunn et
al., 2021; Lamy et al., 2012, 2017; Lamy, 2020; Prangé et al., 2004). More so,
even fewer studies have focussed on the magnetic structure of CMEs in the
outer heliosphere. The Bastille event studied by Burlaga et al. (2001) at
Voyager 2 at 63 AU is considered the most distant magnetic cloud ever
observed, but its handedness was reported to be opposite to that of the
corresponding ejecta at 1 AU, thus highlighting the difficulties in
associating measurements that are separated by large radial distances.

This work represents the first in-depth study of the magnetic structure of a
(merged) flux rope from the Sun to ${\sim}10$ AU. We analyse here in detail
the eruption and evolution of two CMEs that left the Sun on 28 April 2012 and
were observed to interact in the inner heliosphere. The favourable position of
the spacecraft involved in this study allows us to follow the event in remote-
sensing imagery as it propagates away from the Sun and then to study its in-
situ signatures and magnetic structure at Venus, Earth, and Saturn. Figure 1
shows the position of various planets and spacecraft throughout the
heliosphere on the day of the eruptions under analysis. This article is
organised as follows. In Section 2, we enumerate the various instruments that
are involved in this study. In Section 3, we describe the CMEs under analysis
from a remote-sensing observational perspective. In Section 4, we estimate the
propagation of the CMEs and their impact at different locations using the
remote-sensing observations described in Section 3 as input. In Section 5, we
present the in-situ signatures and analyse the magnetic structure of the
interacting CMEs at different heliocentric distances. In Section 6, we discuss
different aspects of the event, especially in terms of CME propagation and
magnetic structure. Finally, in Section 7, we summarise our results and
present our conclusions.

Figure 1: Position of various planets and spacecraft until the orbits of (a)
Mars and (b) Saturn, on 28 April 2012. The orbits of all planets from Mercury
to Saturn are also indicated.

##  2 Spacecraft and Ground-based Data

We list here the fleet of instruments that are involved in this study,
starting from the Sun and moving outwards to Saturn. We combine remote-sensing
and in-situ data in order to follow and characterise the event at various
locations throughout the heliosphere.

Observations of the solar disc are made from three vantage points, namely
Earth and the twin Solar Terrestrial Relations Observatory (STEREO; Kaiser et
al., 2008) spacecraft. The STEREO mission comprises two identical spacecraft
that orbit the Sun close to 1 AU, one ahead of Earth in its orbit (STEREO-A)
and the other one trailing behind (STEREO-B, which has been out of contact
since late 2014). At the time of the events under analysis, STEREO-A and -B
were located ${\sim}113^{\circ}$ west and ${\sim}118^{\circ}$ east of the
Sun–Earth line, respectively. Extreme ultra-violet (EUV) images are provided
by the Atmospheric Imaging Assembly (AIA; Lemen et al., 2012) onboard the
Solar Dynamics Observatory (SDO; Pesnell et al., 2012) orbiting Earth and the
Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et
al., 2008) Extreme UltraViolet Imager (EUVI) onboard STEREO. Line-of-sight
magnetograms are available from Earth’s viewpoint only, and we use data from
the Helioseismic and Magnetic Imager (HMI; Scherrer et al., 2012) onboard SDO.

Coronagraph observations are made from the same three viewpoints. White-light
images from Earth are provided by the Large Angle and Spectrometric
Coronagraph (LASCO; Brueckner et al., 1995) C2 (2.2–6 $R_{\odot}$) and C3
(3.5–30 $R_{\odot}$) cameras onboard the Solar and Heliospheric Observatory
(SOHO: Domingo et al., 1995). Imagery from STEREO-A and -B is supplied by the
SECCHI/COR1 (1.5–4 $R_{\odot}$) and COR2 (2.5 – 15 $R_{\odot}$) coronagraphs.

White-light observations of the heliosphere are provided by the Heliospheric
Imagers (HI; Eyles et al., 2009) onboard the twin STEREO spacecraft. Each HI
instrument consists of two cameras, HI1 (4–24∘) and HI2 (18–88∘), that image
interplanetary space in the vicinity of the Sun–Earth line (the degrees
indicate the elongation in helioprojective radial coordinates).

In-situ measurements from Venus are provided by the Venus Express (VEX;
Svedhem et al., 2007) spacecraft. The instruments that we avail ourselves of
are the Magnetometer (MAG; Zhang et al., 2006) and the Analyser of Space
Plasmas and Energetic Atoms (ASPERA-4; Barabash et al., 2007). From the
ASPERA-4 package, we use data taken by the Ion Mass Analyser (IMA) and the
Electron Spectrometer (ELS) sensors. VEX terminated operations in early 2015
and was characterised by a 24-hour highly elliptical orbit, most of which was
in the solar wind and the remaining couple of hours were spent inside the bow
shock of Venus. MAG was operational at all times, whilst ASPERA-4 was turned
on for several hours close to periapsis and apoapsis.

In-situ measurements from Earth are provided by the Wind (Ogilvie and Desch,
1997) spacecraft, which orbits the Sun from Earth’s Lagrange L1 point.
Magnetic field and plasma (including solar wind electron distributions) data
are taken by the Magnetic Fields Investigation (MFI; Lepping et al., 1995) and
Solar Wind Experiment (SWE; Ogilvie et al., 1995) instruments. We also use
count rate data from the Neutron Monitor Database (NMDB), and specifically
from the Thule (THUL), Oulu (OULU), and South Pole (SOPO) observatories on
ground, as GCR proxies.

Finally, in-situ measurements from Saturn are provided by the Cassini (Matson
et al., 2002) spacecraft. We use data from the Cassini Magnetic Field
Investigation (MAG; Dougherty et al., 2004), the Radio and Plasma Wave Science
(RPWS; Gurnett et al., 2004) experiment, and the Magnetosphere Imaging
Instrument (MIMI; Krimigis et al., 2004). MIMI consists of three different
sensors, and we use data from the Low Energy Magnetospheric Measurement System
(LEMMS) sensor. The Cassini spacecraft was dismissed in 2017 through a
controlled entry into Saturn.

##  3 Remote-sensing Observations

In this section, we provide an overview of the solar events that originated on
28 April 2012 and follow them from the solar disc (Section 3.1) through white-
light imagery of the corona (Section 3.2) and heliosphere (Section 3.3).

###  3.1 Solar Disc Observations

The sequence of events analysed in this article commenced on 28 April 2012,
which was a day characterised by several eruptions. Here, we focus on the
three major solar events that originated on that day from the Earth-facing
disc. Figure 2a shows the approximate source regions of these CMEs as seen by
SDO (i.e., from Earth’s perspective). For a complete set of observations of
the front-sided eruptions of 28 April 2012 from three viewpoints (STEREO-A,
Earth, and STEREO-B), see Movie S1.

Figure 2: Eruptive events on 28 April 2012 as seen in solar disc imagery from
SDO. (a) Overview of the solar disc in the 304 Å channel prior to the
eruptions, with the approximate source regions of the three CMEs marked in
chronological order as ‘CME0’, ‘CME1’, and ‘CME2’, respectively. (b) Base-
difference image in the 211 Å channel overlaid with magnetogram contours (red:
positive polarity, blue: negative polarity). The footpoints of CME1 are
indicated with green circles. (c) Image in the 171 Å channel showing the post-
eruption arcade signatures of CME1. The global neutral lines (calculated from
smoothed magnetogram data taken at the same time) are overlaid in red, and the
main polarity inversion line involved in the eruption is indicated with a
white arrow. (d–e) Same as panels (b–c), but for CME2.

The first major eruption of the day, marked as ‘CME0’ in Figure 2a, had its
onset at ${\sim}$10:00 UT and originated from a quiet-Sun, J-shaped filament
located in the southwestern quadrant of the solar disc (approximately at
S25W40). Given the large scale of the resulting eruption and the proximity of
the filament to the source of the next CME (CME1), it is possible that CME0
triggered the chain of subsequent events (e.g., Schrijver and Title, 2011;
Török et al., 2011). However, this CME was seen to deflect significantly
towards the southwest already in the lower corona (see Movie S1 and Section
3.2), making it highly unlikely to interact with the following CMEs or to
encounter any observer close to the ecliptic plane. Hence, CME0 will be
disregarded for the rest of this study.

The following eruption, marked as ‘CME1’ in Figure 2a, had its onset at
${\sim}$13:00 UT and originated from a quiet-Sun, U-shaped filament located in
the southeastern quadrant of the solar disc (approximately at S45E15).
Although this CME erupted from higher latitudes than CME0, it deflected
towards the equator (see Movie S1 and Section 3.2), making it more likely to
exhibit a significant component along the ecliptic plane and to interact with
the following CME2. Hence, we proceed to evaluate the magnetic structure of
the flux rope associated with CME1, also known as the “intrinsic flux rope
type”. This can be achieved via a combination of multi-wavelength, remote-
sensing observations that yield information on the chirality (or handedness),
tilt, and axial field direction of a flux rope during eruption (see Palmerio
et al., 2017, and references therein for a summary of the available proxies).
The eruption of CME1 was not characterised by a clear pair of coronal dimmings
(usually tracers of a flux rope’s footpoints; e.g., Thompson et al., 2000;
Zhukov and Auchère, 2004), but rather by several patches of diffuse dimming
regions. Hence, we estimate the CME footpoints to be simply located at either
end of the erupting filament’s spine, noting however that these are
approximate locations because of possible projection effects. The resulting
footpoints are shown in Figure 2b, according to which CME1 was rooted in a
positive (negative) polarity to the west (east). Furthermore, we observe the
“roll effect” (i.e., the sense of bending and twisting) of the filament
material off limb in STEREO imagery (see Movie S1), which can be used as a
chirality proxy (e.g., Martin, 2003; Panasenco and Martin, 2008). We determine
that the filament rolled in a right-handed sense, which is also confirmed by
the presence of a left-bearing barb (signature of a right-handed CME; e.g.,
Martin, 1998) that we could identify in H$\alpha$ imagery of the solar disc
from the Big Bear Solar Observatory (not shown). For the the flux rope tilt,
we consider the inclination of the corresponding polarity inversion line and
post-eruption arcade (e.g., Marubashi et al., 2015), shown in Figure 2c. Both
are inclined ${\sim}15^{\circ}$ counterclockwise with respect to the solar
equator, implying thus a low-inclination flux rope. A right-handed, low-
inclination flux rope with an eastward axial field yields a north–east–south
(NES) type, following the scheme of Bothmer and Schwenn (1998) and Mulligan et
al. (1998).

Finally, the last eruption, marked as ‘CME2’ in Figure 2a, had its onset at
${\sim}$18:00 UT and originated from active region (AR) 11469 (approximately
at S15E20), in the vicinity of the eastern footpoint of the CME1 filament.
Given the close location of the source region with respect to the centre of
the solar disc and noting no major deflections during early evolution (see
Movie S1 and Section 3.2), we can expect CME2 to be likely Earth-directed.
Hence, we again estimate the intrinsic flux rope type associated with this
eruption. Low-coronal signatures of the CME included J-shaped ribbons (shown
in Figure 2d), proxies of right-handed chirality (e.g., Démoulin et al.,
1996). Together with coronal dimming pairs, flare ribbons are also tracers of
where a flux rope is rooted (e.g., Aulanier et al., 2012; Janvier et al.,
2014b), resulting in the footpoints indicated in Figure 2d and showing the
northern (southern) leg anchored to the positive (negative) magnetic polarity.
The polarity inversion line and post-eruption arcade are both inclined
${\sim}55^{\circ}$ counterclockwise with respect to the solar equator,
indicating a high-to-intermediate inclination flux rope. A right-handed, high-
inclination flux rope with a southward axial field yields an east–south–west
(ESW) type, whilst its low-inclination counterpart would be a NES type.

###  3.2 Coronagraph Observations

After erupting, the 28 April 2012 CMEs appeared in coronagraph imagery from
three locations (STEREO-A, SOHO, and STEREO-B). We provide here an overview of
white-light observations of the events throughout the solar corona, with a
particular focus on the derivation of geometric and kinematic parameters for
the eruptions that we deemed in Section 3.1 to be possibly Earth-directed
(i.e., CME1 and CME2, shown in Figure 3). For a complete set of the
coronagraph observations from the three available viewpoints, see Movie S2.

Figure 3: Coronagraph observations and GCS reconstructions for CME1 and CME2.
(a–c) CME1 shown in difference images from the COR2-A, C3, and COR2-B
telescopes, respectively. Background images are taken 1 hour prior to each of
the main images. (d–f) Same images as (a–c), with the GCS wireframe overlaid
in red. (g–i) Same as (a–c), but for CME2. (j–l) Same as (d–f), but for CME2.
All images are taken within 6 minutes of the times reported under each column.

As mentioned in Section 3.1, CME0 (the first to erupt and visible in COR2-A
imagery starting at 12:24 UT on 28 April 2012) was seen to propagate mainly
below the ecliptic plane (STEREO views) and towards the west (SOHO view),
strongly suggesting that the eruption was directed away from the Sun–Earth
line and from the following CMEs. This event was particularly faint in
STEREO-B imagery, which is not surprising since the CME was propagating fully
away with respect to the plane of the sky.

CME1 emerged in coronagraph imagery a few hours after CME0 (around 18:24 UT in
COR2-A data) and was seen to propagate at significantly lower latitudes than
the previous eruption (see also Section 3.1). In addition, the main “bulk” of
CME1 was preceded by a smaller feature at its southern edge, resulting in a
double-lobe structure visible from all viewpoints shown in Movie S2. We note
that asymmetric and complex white-light morphologies of CMEs that originated
from filament eruptions have been reported in previous studies (e.g., Palmerio
et al., 2021; Yang et al., 2012; Zhu et al., 2014). In the case under study,
it is likely that the double-lobe structure seen in STEREO imagery resulted
from an uneven eruption and disconnection of the corresponding filament legs
(e.g., Liu et al., 2009; Tripathi et al., 2006; Vourlidas et al., 2011).
Furthermore, we note that images of CME1 from the SOHO viewpoint (see e.g.
Figure 3b) are particularly complex to interpret because of an almost-
simultaneous back-sided CME (clearly visible in Figure 3a,c) overlapping due
to projection effects. Overall, CME1 did not appear to evolve drastically
throughout the solar corona, suggesting that its orientation was maintained
similar to that prior to eruption (cf. the low-inclination configuration that
was inferred from the analysis of solar disc imagery in Section 3.1).

Finally, CME2 appeared in coronagraph imagery a few hours after CME1 (around
22:24 UT in COR2-A data) and was seen to propagate mainly along the ecliptic
plane. It was the faintest of the three eruptions in white-light data from all
available viewpoints (see Movie S2), possibly because of its passage through
an already disturbed corona due to the preceding CME1. The morphology of the
eruption in coronagraph images from the two STEREO viewpoints (see also Figure
3g–i) is reminiscent of a low-inclination flux rope seen edge on (see, e.g.,
Howard et al., 2017; Krall and St. Cyr, 2006; Thernisien et al., 2006),
suggesting that the CME rotated slightly upon eruption (cf. the high-to-
intermediate-inclination configuration that was inferred from the analysis of
solar disc imagery in Section 3.1).

In order to obtain quantitative estimates of the geometric and kinematic
parameters of CME1 and CME2 through the solar corona, we reconstruct both
eruptions using the Graduated Cylindrical Shell (GCS; Thernisien et al., 2009;
Thernisien, 2011) model. In the GCS model, a parameterised shell reminiscent
of a croissant with its legs attached to the Sun is manually fitted to nearly-
simultaneous white-light images until its morphology best matches the observed
features. GCS model results are shown in Figure 3d–f for CME1 and Figure 3j–l
for CME2. According to our reconstruction results, CME1 had a propagation
direction of ($\theta$,$\phi$) = ($-12^{\circ}$,$-18^{\circ}$) and a tilt
($\gamma$) of $5^{\circ}$ with respect to the solar equatorial plane, whilst
the corresponding values for CME2 are ($\theta$,$\phi$) =
($-8^{\circ}$,$-24^{\circ}$) and $\gamma=15^{\circ}$ (here, $\theta$ and
$\phi$ are expressed in Stonyhurst coordinates; e.g., Thompson, 2006, and a
positive $\gamma$ is assumed for counterclockwise rotations). Furthermore, we
determine the speeds ($v$) of both CMEs through the outer corona by performing
GCS reconstructions at two separate times separated by 1 hour, resulting in
$v=405.8$ km$\cdot$s-1 for CME1 and $v=309.2$ km$\cdot$s-1 for CME2. Overall,
both CMEs propagated slightly towards the southeast as seen from Earth, had
low speeds, and were characterised by a low tilt of their axes with respect to
the solar equatorial plane. In terms of their resulting magnetic configuration
through the corona, it can be assumed that CME1 maintained its NES flux rope
type determined in Section 3.1, whilst CME2 rotated from its higher-
inclination, ESW type inferred in Section 3.1. The shortest “path” from
$\gamma=55^{\circ}$ to $\gamma=5^{\circ}$ corresponds to a clockwise motion of
the CME axis, which is the expected sense of rotation for a right-handed flux
rope (e.g., Green et al., 2007; Lynch et al., 2009). Hence, these results
indicate that both CME1 and CME2 featured a NES flux rope configuration when
they left the fields of view of the coronagraphs employed in this study.

###  3.3 HI Observations

After leaving the field of view of the COR2 coronagraphs, CME1 and CME2
appeared in images from the HI cameras onboard both STEREO spacecraft. For a
complete set of observations from both HI1 cameras, see Movie S3. An overview
of HI observations is shown in Figure 4. CME1 first emerged in HI1-A data at
20:49 UT on 28 April 2012 and in HI1-B data at 22:49 UT on the same day. The
complex morphology of CME1, mentioned already in Section 3.2, is evident in
heliospheric imagery as well. Furthermore, it is difficult to clearly
distinguish the front of CME2 amongst the material of the preceding CME1,
making the two eruptions appear as a merged, complex structure as they
propagate away from the Sun. Figure 4a–b shows a snapshot of the complex
structure observed in HI1 imagery, whilst Figure 4c–d shows time–elongation
maps constructed using data from both HI1 and HI2 cameras, together with the
corresponding tracks of the merged CME.

Figure 4: CME1 and CME2 seen as a single structure in images from the
STEREO/SECCHI/HI cameras. (a–b) The CME seen in running-difference images
taken with the (a) HI1-A and (b) HI1-B cameras. The locations of Mercury,
Venus, and Mars are marked in panel (a). (c–d) Time–elongation maps from (c)
STEREO-A and (d) STEREO-B. The CME is tracked in red. The maps are constructed
along position angles of $90^{\circ}$ for STEREO-A and $260^{\circ}$ for
STEREO-B.

We note that CME1 and CME2 are also indicated as a single CME in the
HELiospheric Cataloguing, Analysis and Techniques Service (HELCATS)
catalogues, based on observations made with the STEREO/SECCHI/HI cameras. This
event is included in the HICAT catalogue (Harrison et al., 2018), which was
compiled through visual inspection of HI1 images, and in the HIGeoCAT
catalogue (Barnes et al., 2019), which was compiled using time–elongation maps
and by applying single-spacecraft fitting techniques to derive CME kinematic
properties. In both catalogues, CMEs are identified using single-spacecraft
data, hence the STEREO-A and STEREO-B observations are presented separately.
Amongst the fitting techniques reported in HIGeoCAT, we consider here the
results obtained with the Self-Similar Expansion fitting technique (SSEF;
Davies et al., 2012; Möstl and Davies, 2013) with a fixed half-width of
$30^{\circ}$ applied to time–elongation single-spacecraft data. In the SSEF
model, CMEs are assumed to have a circular front and to propagate radially
with constant speed and half-width. SSEF results based on STEREO-A
observations report a propagation direction of ($\theta$,$\phi$) =
($-16^{\circ}$,$-9^{\circ}$) and a speed of $405$ km$\cdot$s-1; whilst SSEF
results based on STEREO-B observations report a propagation direction of
($\theta$,$\phi$) = ($-16^{\circ}$,$1^{\circ}$) and a speed of $791$
km$\cdot$s-1. Whilst the values for propagation latitude and longitude are
consistent with each other and also with the GCS results shown in Section 3.2,
the SSEF-B speed is approximately twice as large as the SSEF-A one, which is
on the other hand basically identical to the speed for CME1 derived using GCS
reconstructions.

##  4 CME Propagation Modelling

In this section, we estimate the impact locations and arrival times of CME1
and CME2 across the heliosphere. In order to achieve this, we employ two
different propagation models that are based on different physical assumptions
and observational inputs. The first model (presented in Section 4.1) is based
uniquely on HI observations (see also Section 3.3) and treats CME1 and CME2 as
a single, merged structure. In the second model (presented in Section 4.2)
CME1 and CME2 are inserted separately and their input parameters are derived
from coronagraph observations (see also Section 3.2).

###  4.1 SSSE Model

The first CME propagation model that we employ in this study is based on HI
observations and fittings such as the SSEF results presented in Section 3.3.
The SSEF results reported in the HELCATS catalogues, however, consider
observations made with each spacecraft separately. Since the merged CME1 and
CME2 were observed from both STEREO probes, we use here the two-spacecraft
version of the SSE model, i.e. the Stereoscopic Self-Similar Expansion (SSSE;
Davies et al., 2013) model. With the aid of time–elongation data (see Figure
4) from both spacecraft, we initially triangulate the CME front as it
propagates away from the Sun assuming a circular cross-section and fixed half-
width (set in this case as $\omega/2=60^{\circ}$, since the model was found by
Barnes et al., 2020, to perform better with angular extents $\gg 30^{\circ}$).
Then, in order to propagate the CME beyond its last observation time in both
HI cameras, we fit a second-order polynomial to the CME apex as a function of
time, assuming a constant propagation direction. As a result, the merged CME
is predicted to impact Venus (2012-05-01T22:19), Earth (2012-05-04T15:08),
Mars (2012-05-05T16:21), and Saturn (2012-06-08T14:52). Figure 5 shows the
position of the tracked CME front (top row) and the impact location at the
four planets with respect to the apex (bottom row).

Figure 5: Schematic representation of the SSSE method applied to the merged
CME1 and CME2 observed in HI data. (a–d) Position of the CME front (red
circle) within the ecliptic plane triangulated from the observed leading edge
in the HI1 cameras (dashed lines), shown at $\sim$12-hour intervals. (e–h) CME
position extrapolated from the last observations to predict arrival times at
four planets: (e) Venus, (f) Earth, (g) Mars, and (h) Saturn. Plots are shown
in the Heliocentric Earth Ecliptic (HEE) coordinate system.

###  4.2 Enlil Simulation

The second CME propagation model that we use in this work is the 3D
heliospheric magnetohydrodynamic (MHD) Enlil (Odstrcil, 2003; Odstrcil et al.,
2004) model. Enlil uses the Wang–Sheeley–Arge (WSA; Arge et al., 2004) coronal
model to generate a background solar wind from its inner boundary (placed at
$21.5$ R⊙ or 0.1 AU) onwards. Here, we set the outer boundary of the
simulation domain at 10 AU. CMEs are launched through the heliospheric domain
at the inner boundary as spherical hydrodynamic structures, i.e. lacking an
internal magnetic field. The input parameters for CME1 and CME2 are entirely
derived from the GCS reconstructions reported in Section 3.2. The two CMEs
that we inject have an elliptical cross-section and their angular extent is
obtained by “cutting” a slice out of the GCS shell (see Figure 3 and
Thernisien, 2011). The speeds and injection times are derived from the last
observations in coronagraph data and by propagating the CMEs up to $21.5$ R⊙
under the assumption of constant speed. As a result, CME1 is launched on 29
April 2012 at 04:32 UT, at ($\theta$,$\phi$) = ($-12^{\circ}$,$-18^{\circ}$)
and with axis tilt $\gamma=5^{\circ}$, speed $v=405.8$ km$\cdot$s-1, and half-
angular width ($R_{\mathrm{max}}$,$R_{\mathrm{min}}$) =
($41.6^{\circ}$,$16.9^{\circ}$); CME2 is launched on 29 April 2012 at 12:42
UT, at ($\theta$,$\phi$) = ($-8^{\circ}$,$-24^{\circ}$) and with
$\gamma=15^{\circ}$, $v=309.2$ km$\cdot$s-1, and
($R_{\mathrm{max}}$,$R_{\mathrm{min}}$) = ($44.8^{\circ}$,$20.5^{\circ}$).
Three screenshots from the simulation, corresponding to when the merged CME
was at approximately 1, 5, and 10 AU, are shown in Figure 6. We note that,
although CME2 is slightly slower than CME1, the two eruptions merge early on
and appear to propagate away from the Sun as a single structure, possibly due
to solar wind preconditioning caused by the prior CME (i.e., CME1), which is a
phenomenon seen both in observations (e.g., Liu et al., 2014, 2019; Temmer and
Nitta, 2015) and in simulations (e.g., Desai et al., 2020; Scolini et al.,
2020). The resulting CME-driven shock and/or sheath is predicted to impact
Venus (2012-05-01T19:35), Earth (2012-05-02T06:15), Mars (2012-05-05T23:33),
and Saturn (2012-06-10T14:10). The merged ejecta is predicted to impact Venus
(2012-05-02T00:42), Earth (2012-05-02T13:53), Mars (2012-05-06T06:31), and
Saturn (2012-06-11T14:26).

Figure 6: Screenshots from the WSA–Enlil+Cone simulation. The parameter shown
in the plots is the solar wind radial speed in the ecliptic plane on (a) 2 May
2012, (b) 20 May 2012, and (c) 10 June 2012. The merged CME ejecta is
represented with a black countour.

##  5 In-situ Measurements

In this section, we present and analyse in-situ data from Venus at 0.7 AU
(Section 5.1), Earth at 1.0 AU (Section 5.2), and finally Saturn at 9.7 AU
(Section 5.3). We remark that, although both propagation models shown in
Section 4 estimated an impact at Mars, the main focus of this work is the
study of the magnetic structure of the CMEs from eruption through heliospheric
propagation. Since there was no spacecraft equipped with a solar wind-sampling
magnetometer in orbit around Mars at the time of the events presented here,
this location is not included in our investigation.

###  5.1 Measurements at Venus

The first impact location predicted by both models presented in Section 4 is
Venus. On 28 April 2012, Venus was located ${\sim}25^{\circ}$ east of the
Sun–Earth line, at 0.72 AU (see Figure 1). In-situ measurements at Venus taken
around the expected arrival time of CME1 and CME2 are shown in Figure 7,
revealing the passage of a clear, albeit weak, interplanetary disturbance. In
particular, two sudden increases in the magnetic field magnitude (at
2012-05-01T09:50 and 2012-05-02T00:39, marked by solid lines in Figure 7) may
correspond to two interplanetary shocks, but it is not possible to establish
this with certainty because of the lack of high-cadence plasma data.
Nevertheless, the solar wind speed displays an increase after each magnetic
field jump, suggesting that the two structures may indeed coincide with
shocks. These are followed by a period of enhanced magnetic field featuring a
rotation in the $\theta_{B}$ component and a steady $\phi_{B}$ (shaded area in
Figure 7), characteristic of a flux rope configuration. The exact boundaries
(i.e., leading and trailing edge) of this magnetic ejecta are not
straightforward to identify, in particular because the rotation seem to extend
beyond the enhancement in the magnetic field magnitude (see also the dash-
dotted line in Figure 7, which appears to mark the beginning of a smoothly
rotating region but is followed by a data gap). Hence, the identified ICME
ejecta region (from 2012-05-02T15:14 to 2012-05-03T20:56) is based largely on
the magnetic field magnitude (assumed to be greater than its surrounding
material). Furthermore, it is unclear whether this structure corresponds to a
single ejecta or to the merged CME1 and CME2, especially because of the lack
of high-cadence plasma measurements and a data gap in the middle of the flux
rope. It has been shown, in fact, that the interaction of two CMEs may result
in an ejecta that resembles a coherent, isolated magnetic flux rope (e.g.,
Kilpua et al., 2019; Lugaz and Farrugia, 2014).

Figure 7: Measurements at Venus around the expected arrival time of the merged
CME1 and CME2. The parameters shown are: (a) magnetic field magnitude, (b)
magnetic field components in Venus Solar Orbital (VSO) Cartesian coordinates,
(c) $\theta$ and (d) $\phi$ angles of the magnetic field in VSO angular
coordinates, (e) solar wind speed, and (f) proton and (g) electron energy
distribution. The solid grey lines indicate two possible interplanetary
shocks. The dash-dotted blue line represents a possible start of the magnetic
field rotation. The shaded green area marks the period characterised by flux
rope signatures and enhanced magnetic field, corresponding to a magnetic
ejecta. The magenta curves over magnetic field data within the ejecta show
fitting results from applying the Elliptic–Cylindrical flux rope model of
Nieves-Chinchilla et al. (2018b).

Visual inspection of the magnetic field components within the ejecta reveals a
(weak) rotation from north to south and a constant eastward direction,
corresponding to a right-handed, NES flux rope type. This was also the
magnetic configuration of both CME1 and CME2 in the outer corona as inferred
from the analysis of solar disc and coronagraph imagery (see Sections
3.1–3.2). We also fit the structure using the Elliptic–Cylindrical (EC)
analytical model of Nieves-Chinchilla et al. (2018b), which is able to
describe a magnetic flux rope topology with a distorted cross-section, which
may result from interactions with e.g. the solar wind. The results are shown
over the magnetic field measurements in in Figure 7 and the full set of
parameters obtained are reported in Table S1. Apart from vector magnetic field
data, the model requires as input the CME average speed, for which we choose a
value $v=333$ km$\cdot$s-1 based on the three data points available. According
to the EC model, the flux rope is right-handed and has axis orientation
($\theta$,$\phi$) = ($-10^{\circ}$,$128^{\circ}$), fully consistent with a
NES-type structure. Furthermore, its cross-section is rather distorted, with a
distortion parameter $\delta=0.47$ ($\delta$ is defined to be 1 for a circular
cross-section and 0 for maximum distortion).

###  5.2 Measurements at Earth

The next impact location predicted by the models presented in Section 4 is
Earth, situated at 1.01 AU on 28 April 2012 (see Figure 1). In-situ
measurements at Earth taken around the expected arrival time of CME1 and CME2
are shown in Figure 8. The sequence of events starts with an interplanetary
shock (at 2012-05-03T01:01, marked by a solid vertical line in Figure 8),
followed by a decreasing magnetic field profile similarly to the first
structure encountered at Venus (cf. Figure 7). We note that the shock is
remarkably slow ($v\sim 300$ km$\cdot$s-1), which has however been shown to be
possible for slow CMEs characterised by significant expansion that travel
through a slow upstream solar wind with a low magnetosonic speed (Lugaz et
al., 2017b). We do not find signatures of a second shock at Earth. A second
sharp increase in the magnetic field magnitude (similar to the one detected at
Venus) is not associated with a solar wind speed jump and, furthermore,
density and temperature feature a decrease. This suggests that the second
discontinuity at Venus was also not a shock or that the second shock
dissipated between 0.7 and 1.0 AU. The structure (between 2012-05-04T03:25 and
2012-05-05T11:23) following the second sharp magnetic field increase displays
clear signatures of a magnetic cloud (shaded region in Figure 8), albeit with
a low magnetic field strength and several complex characteristics. In
particular, the centre of the identified flux rope (between the dashed orange
lines in Figure 8) features a region characterised by less smooth magnetic
field, an irregular speed profile, and enhanced density and plasma beta.
Furthermore, the proton temperature is relatively high throughout the first
portion of the ejecta and finally drops below expected levels only after the
high-beta region (the same trend is observed for alpha particles, not shown
here). Unfortunately, heavy ion composition or charge state data from the
Advanced Composition Explorer (ACE) spacecraft are not available because of a
data gap. Nevertheless, these characteristics are consistent with the merging
CMEs or multiple magnetic clouds scenario described by, e.g., Lugaz and
Farrugia (2014) and Wang et al. (2003), in which the central portion
represents the interaction region between the two original ejecta (in this
case, CME1 and CME2). Finally, we note that the interplanetary disturbance is
associated with a weak (${\sim}2$% variation) Forbush decrease, registered at
all neutron monitors considered here but with different onset times, ranging
from the arrival of the interplanetary shock to the beginning of the magnetic
field rotation (dash-dotted line in Figure 8).

Figure 8: Measurements at Earth around the expected arrival time of the merged
CME1 and CME2. The parameters shown are: (a) magnetic field magnitude, (b)
magnetic field components in Geocentric Solar Ecliptic (GSE) Cartesian
coordinates, (c) $\theta$ and (d) $\phi$ angles of the magnetic field in GSE
angular coordinates, (e) solar wind speed, (f) proton density, (g) proton
temperature (with the expected temperature defined by Richardson and Cane,
1995, shown in red), (h) plasma beta, (i) pitch angle distribution, and (j)
galactic cosmic rays percentage variation (measured by three different neutron
monitors on ground). The solid grey line marks the arrival of the
interplanetary shock. The dash-dotted blue line represents the beginning of
the magnetic field rotation. The shaded grey area marks the magnetic cloud,
with an interaction region delimited by dashed orange lines. The magenta
curves over magnetic field data within the ejecta show fitting results from
applying the Elliptic–Cylindrical flux rope model of Nieves-Chinchilla et al.
(2018b).

The magnetic cloud boundaries identified here coincide with the ones defined
in the NASA–Wind ICME list (Nieves-Chinchilla et al., 2018a). Visual
inspection of the magnetic field components within the ejecta reveal a very
similar configuration to the one encountered at Venus (see Section 5.1), i.e.
characterised by a north–south rotation of the helical field and an eastward
axial field, forming a NES-type flux rope. This picture is consistent with
CME1 and CME2 (both of NES type in the outer corona, see Section 3.2)
reconnecting in interplanetary space before reaching Venus and coalescing into
a single NES flux rope. Again, we fit the structure using the EC model and
assuming a CME average speed of $v=310$ km$\cdot$s-1 directly from in-situ
data. The results are shown over the magnetic field measurements in Figure 8
and the full set of parameters obtained are reported in Table S1. The
resulting flux rope is right-handed, has axis orientation ($\theta$,$\phi$) =
($2^{\circ}$,$145^{\circ}$), and its distortion parameter is $\delta=0.46$. We
note that these results are compatible with those at Venus, i.e. consistent
with a NES flux rope that is rather distorted.

###  5.3 Measurements at Saturn

Finally, the last location where an arrival of CME1 and CME2 is predicted is
Saturn, which was positioned at 9.73 AU from the Sun and ${\sim}12^{\circ}$
east of Earth on 28 April 2012 (see Figure 1). Even though CMEs usually take a
relatively long time to reach ${\sim}10$ AU (about a month; e.g., Prangé et
al., 2004), Saturn’s 29-year orbit results in the planet moving about
$1^{\circ}$ in longitude per month and, thus, can be considered essentially
fixed in space throughout the analysed period. In-situ measurements at Saturn
taken around the expected arrival time of CME1 and CME2 are shown in Figure 9.
Providentially, Cassini exited the Kronian magnetosheath between 9–16 June
2012 and was fully immersed in the solar wind during most of this time
interval. On the other hand, Cassini’s plasma spectrometer was permanently
turned off on 2 June 2012 (i.e., just a week before the start of the data
shown in Figure 9) due to short circuits in the instrument, hence there are no
measurements such as solar wind speed and proton density available.
Nevertheless, we complement magnetic field data with particle measurements and
radio observations of Saturn’s Kilometric Radiation (SKR; e.g., Kaiser et al.,
1984; Lamy et al., 2008; Warwick et al., 1981). The SKR is Saturn’s primary
radio emission and is generated by auroral electrons that are accelerated
along field lines rooted around Saturn’s auroral oval (e.g., Lamy et al.,
2009; Lamy, 2017). The emission is highly dependent on solar wind conditions
(e.g., Bradley et al., 2020; Clarke et al., 2009; Desch, 1982; Kurth et al.,
2005; Lamy et al., 2018), and previous studies have reported SKR enhancements
concurrently with the passage of ICMEs (e.g., Crary et al., 2005; Palmaerts et
al., 2018) and SIRs (e.g., Badman et al., 2008; Kurth et al., 2016).

Figure 9: Measurements at Saturn around the expected arrival time of the
merged CME1 and CME2. The parameters shown are: (a) magnetic field magnitude,
(b) magnetic field components in Kronocentric Solar Orbital (KSO) Cartesian
coordinates, (c) $\theta$ and (d) $\phi$ angles of the magnetic field in KSO
angular coordinates, (e) electron (including penetrating GCR protons) and (f)
proton count rates (with the corresponding LEMMS channels and proton energy
ranges indicated), and (g) dynamic spectrum of SKR spectral flux density
normalised to 1 AU. Green-shaded regions correspond to periods in which
Cassini was inside the Kronian magnetosheath. The beginning of the period
featuring magnetic field rotation is marked with the dash-dotted blue line,
and the period of enhanced magnetic field magnitude is shaded in grey, with a
possible interaction region indicated by the dashed orange lines. The magenta
curves over magnetic field data within the ejecta show fitting results from
the Elliptic–Cylindrical flux rope model of Nieves-Chinchilla et al. (2018b).

First of all, the magnetic field measurements shown in Figure 9 reveal the
passage of a (weak) disturbance, since the (ambient) interplanetary magnetic
field around Saturn’s orbit has usual magnitudes below 0.5 nT (e.g., Jackman
et al., 2008; Echer, 2019). In this case, the magnetic field magnitude reaches
values just under 1 nT, which is rather low (cf. the ${\sim}$2 nT measurements
reported by Witasse et al., 2017, for an ICME in 2014 November), but is on the
other hand consistent with the “weakness” of the transient measured at Earth
(${\sim}$7.5 nT, see Figure 8). We do not find signatures of an interplanetary
shock, but we could identify a period of smoothly rotating magnetic field
(starting at 2012-06-10T21:24, marked by the dash-dotted blue line in Figure
9) that includes the interval of enhanced magnetic field magnitude (between
2012-06-11T21:24 and 2012-06-15T03:50, shaded grey region in Figure 9). We
note that the central portion of this interval (bounded between the dashed
orange lines in Figure 9) is characterised by a more irregular $\theta_{B}$
component that is reminiscent of the one encountered at Earth (see Figure 8),
i.e. indicating a possible interaction region. In order to confirm that an
interplanetary disturbance has indeed impacted Saturn, we also examine
energetic particle observations. In this regard, Roussos et al. (2018, 2020)
showed that Cassini observations of solar energetic particle (SEP) and GCR
transients can be used to identify disturbed solar wind conditions around
Saturn, especially due to CMEs and SIRs. This is because SEPs and GCRs are
able to penetrate Saturn’s magnetosphere, hence they can be monitored at all
times (e.g., Roussos et al., 2008, 2011). During our period of interest, we
find signatures of a Forbush decrease in the GCR data that are indirectly
monitored with the LEMMS electron channel E6 (due to penetrating GCR protons;
Roussos et al., 2019). This event was included by Roussos et al. (2018) in
their list of SEP and GCR transients at Saturn between 2004 and 2016. The
decrease commenced around 10 June 2012 and reached its minimum around 14 June,
concurrently with an increase in the LEMMS proton channels A5/A6 (the recovery
phase of the Forbush decrease follows the plotted interval and is shown in its
full extent by Roussos et al., 2018). Furthermore, SKR data feature increased
emission between 2012-06-10T13:08 and 2012-06-14T02:16, i.e. over an interval
that is in agreement with the Forbush decrease. The temporal extent of the SKR
intensification is consistent with enhancements triggered by solar wind
pressure fronts, as opposed to transient ones triggered by planetary rotation
(lasting a few hours; e.g., Reed et al., 2018). Finally, we note a peak in
proton counts occurring on 9 June 2012 shortly before the interval of enhanced
SKR emission. It is unclear whether this structure has a magnetospheric or
solar origin, especially since it was observed when Cassini was inside the
Kronian magnetosheath. Nevertheless, the features detected during 10–14 June
2012 (in magnetic field, particles, and SKR) occur approximately at the same
time, and strongly suggest that a solar transient, and in particular an ICME,
impacted Saturn during the observed period.

The magnetic configuration of the entire structure characterised by smoother
magnetic field components (i.e., from the dash-dotted blue line to the end of
the shaded grey area in Figure 9) exhibits a larger rotation than that
expected for an axial-symmetric flux rope (the $\phi_{B}$ component rotates as
west–east–west, resulting in a $360^{\circ}$ rotation in the $B_{Y}$–$B_{Z}$
hodogram). Similar events displaying a rotation greater than $180^{\circ}$ in
the magnetic field were analysed at 1 AU by Nieves-Chinchilla et al. (2019),
who suggested that such events can be interpreted as flux ropes with
significant curvature and/or distortion or as more complex topologies, such as
a spheromak or a double flux rope. Since it is not unusual for interplanetary
transients to interact and possibly merge by 1 AU, one may expect even more
interaction and complexity in structures detected in the outer heliosphere
(e.g., Hanlon et al., 2004; Prise et al., 2015). Hence, it is not possible to
establish with certainty whether the observed configuration stems from a
single, intrinsically complex structure, from the interaction of different
structures, or if the region preceding the period of enhanced magnetic field
magnitude corresponds to a “smoothed” sheath. Nevertheless, considering only
the shaded grey region in Figure 9, visual inspection of the magnetic field
yields an ESW flux rope type, still right-handed but corresponding to a
${\sim}90^{\circ}$ counterclockwise rotation of the configuration found at
Venus and Earth (NES). Again, we fit the structure using the EC model. In this
case, due to the lack of plasma data at Saturn, we assume an average CME speed
of $v=380$ km$\cdot$s-1 by considering the transit times of the flux rope
leading and trailing edges from Earth to Saturn. Fitting results are shown
over the magnetic field measurements in in Figure 9 and the full set of
parameters obtained are reported in Table S1. The resulting structure is
right-handed (as expected), but its axis has orientation ($\theta$,$\phi$) =
($-9^{\circ}$,$3^{\circ}$), i.e. consistent with a low-inclination flux rope.
Although the reconstructed magnetic field components fit well to the data, the
observed east–west rotation is attributed in the model to a crossing more
parallel to the central axis of a low-inclination flux rope, rather than a
crossing perpendicular to the axis of a high-inclination one. Furthermore, the
resulting distortion parameter is $\delta=0.21$, indicating that the structure
is highly distorted.

##  6 Discussion

In this section, we synthesise the multi-spacecraft observations, modelling
results, and interpretations presented in Sections 3, 4, and 5 and discuss
them in the context of two main aspects: the propagation of the interacting
CME1 and CME2 to 10 AU (Section 6.1) and the evolution of their magnetic
structure from the Sun to Saturn (Section 6.2).

###  6.1 CME Propagation

CME1 and CME2 erupted ${\sim}$5 hours apart from the southeastern quadrant of
the Earth-facing disc, and their source regions were separated by
${\sim}30^{\circ}$ in latitude (see Section 3.1). Through the solar corona
(see Section 3.2), CME1 could be seen to deflect significantly towards the
solar equator (its apex position changed from approximately S45E15 to S12E18),
whilst CME2 propagated radially (its apex direction could be considered
basically unchanged, from S15E20 at the Sun to S12E24 in the corona). This
resulted in the two CMEs travelling away from the Sun in close succession and
on a very similar trajectory. Despite CME2 appearing slightly slower (by
${\sim}100$ km$\cdot$s-1) than CME1 at an altitude of ${\sim}15$ $R_{\odot}$,
the two eruptions could not be clearly distinguished in HI imagery (see
Section 3.3), possibly indicating that they interacted somewhere in the HI1
field of view. This outcome occurs also in the Enlil simulation (Section 4.2),
where the CMEs appear as a merged structure throughout the heliospheric domain
even if they were inserted separately at the inner boundary of $21.5$
$R_{\odot}$. We suggested that this scenario can be attributed to solar wind
preconditioning (e.g., Temmer et al., 2017), which allowed CME2 to travel
through a rarefied background experiencing little to no drag and thus to run
into CME1. As a consequence, we considered CME1 and CME2 as a single,
interacting structure when estimating their propagation throughout the
heliosphere.

In Section 4, we used two models to evaluate the arrival times of the
interacting CME1 and CME2 at different locations and to aid interpretation of
the in-situ measurements shown in Section 5. The two propagation models that
we employed are substantially different in their physics, assumptions, and
observational input: the SSSE model (Section 4.1) consists of a 2D circular
cross-section reconstructed using HI data and is then propagated outwards
assuming constant acceleration, whilst Enlil (Section 4.2) is a full 3D MHD
model of the heliosphere in which we inserted CME1 and CME2 as hydrodynamic
pulses and with input parameters based on coronagraph imagery. Nevertheless,
both techniques estimated impacts at the same locations (Venus, Earth, Mars,
and Saturn) and within reasonable temporal windows compared to the actual in-
situ observations. In particular, the predicted arrival time at Saturn was
remarkably accurate (with same-day precision) in the case of Enlil, and about
3 days off for SSSE (corresponding to a 7% error for a ${\sim}$43-day
propagation). We note that, regardless of these encouraging results, both
models have made a number of simplifying assumptions. For example, the
treatment of CMEs as hydrodynamic pulses within Enlil is likely to affect the
predicted arrival times due to the absence of internal magnetic forces that
can contribute to the acceleration profile. However, as a CME travels away
from the Sun these forces are expected to be less influential, especially at
radial distances of a few AU and through the outer heliosphere. The SSSE
model, on the other hand, treats CMEs as self-similarly propagating spherical
fronts under the assumption of constant half-width and acceleration, from
which it would be unrealistic to expect precise estimates at 10 AU.
Nevertheless, these results suggest that even simplifying models can be used
to successfully approximate a CME arrival time window up to the outer
heliosphere.

As a further indication of the solar wind propagation that occurred between 1
and 10 AU, we employ the 1D numerical MHD model of Tao et al. (2005), which
uses in-situ measurements from Earth or the STEREO spacecraft to estimate the
interplanetary conditions further out in the heliosphere, such as the orbits
of Jupiter (e.g., Dunn et al., 2020), Saturn (e.g., Provan et al., 2015), and
Uranus (e.g., Lamy et al., 2017). In this case, we use data at Earth (closest
to Saturn in longitude, see Figure 1) and propagate the measurements up to
${\sim}$10 AU along the Sun–Earth line. The results for speed and dynamic
pressure are shown in Figure 10. The ICME at Earth was followed by a high-
speed stream (HSS), which is estimated to have arrived at Saturn around 2012
June 10. The solar wind preceding this HSS (which includes the ICME
measurements shown in Figure 8) is expected to be caught up by the fast stream
around 25 May 2012 at a heliocentric distance of ${\sim}5$ AU, possibly
resulting in compression and acceleration of the ICME from behind (this is
consistent with the Enlil simulation, see Figure 6b). The arrival time of
these features at 10 AU matches quite well with the Enlil results (see Section
4.2) and the in-situ measurements at Saturn (see Section 5.3), further
confirming the likelihood of our connection.

Figure 10: Solar wind propagated from 1 to 9.75 AU using the Tao et al. (2005)
model. The top panels show solar wind speed, whilst the bottom panels show
dynamic pressure. Results are shown in both line (left) and contour (right)
formats. In the line plots, the values for speed and dynamic pressure beyond 1
AU are progressively shifted towards lower values for visibility (values at
different heliocentric distances can be read in the contour plots). In the
line plots, the blue lines show measurements at Earth, the red lines show
solar wind propagated to 9.75 AU along the Sun–Earth line, and the black lines
at the bottom show model results shifted to Saturn’s position. In all panels,
the downward-pointing arrows mark the ICME ejecta arrival time at Earth (see
Figure 8), whilst the upward-pointing arrows mark the ICME ejecta arrival time
at Saturn (see Figure 9).

We also explored the level of interaction and possible merging between CME1
and CME2 through interplanetary space. We emphasise that, whilst the Enlil
simulation estimated the two eruptions to merge well before reaching Mercury’s
orbit, it is not possible to fully model the nature and outcome of their
interaction due to the lack of an internal magnetic field in the purely
hydrodynamic representation of CMEs. Hence, our interpretation is entirely
based on the in-situ observations presented in Section 5. At Venus, we did not
find clear indications of interaction, possibly because of the lack of high-
cadence plasma data and a data gap exactly at the centre of the flux rope
interval that we identified (see Figure 7). At Earth, a seemingly single
magnetic cloud structure displayed less smooth magnetic field, variable speed,
and enhanced plasma parameters at its centre, which could be considered
signatures of the interaction between CME1 and CME2 (see Figure 8). At Saturn,
the region that we selected as the magnetic ejecta featured similar magnetic
field characteristics at its centre as the ones encountered at Earth (see
Figure 9). However, we could not determine with confidence whether the
observed structure corresponded entirely to the ejecta detected at Venus and
Earth, or whether additional material was added to it via further interaction
with nearby structures in the solar wind. At all locations, fitting the entire
ejecta interval as a single flux rope yielded the best results, suggesting
that, if interaction was present, it resulted in the two eruptions slowly
merging and travelling as a single magnetic structure, rather than in more
extreme outcomes such as the second CME overcoming and compressing the first.
This is plausible, considering that CME1 and CME2 were ejected close in time
and with comparable speeds. An alternative interpretation is that CME1 skimmed
the inner planets and its signatures were observed following the first
interplanetary shock, and the entire ejecta intervals at Venus and Earth
belonged on the other hand to CME2.

###  6.2 CME Magnetic Structure

Both CME1 and CME2 erupted from the Sun as right-handed flux ropes, but with
slightly different orientations according to our analysis of solar disc
imagery (Section 3.1): CME1 featured a low-inclination NES type, whilst CME2
displayed a high-to-intermediate inclination between an ESW and a NES type. In
the solar corona, CME1 was observed to largely maintain its orientation,
whilst CME2 appeared to have slightly rotated in a clockwise direction, from
which we inferred that both eruptions consisted of NES-type flux ropes based
on coronagraph imagery and reconstructions (Section 3.2). The two CMEs were
hard to distinguish as separate structures in HI data (Section 3.3),
suggesting that they interacted in some capacity in the inner heliosphere. At
Venus (Section 5.1), we found a single flux rope signature of NES type (based
on both visual inspection and flux rope fitting), which did not display
evident indications of interaction (possibly because of lacking high-cadence
plasma data and magnetic field measurements at the centre of the structure).
Nevertheless, the whole interplanetary distubance (including shock and sheath
material) was rather long in duration (${\sim}$2.5 days), suggesting that it
may be linked to more than one parent eruption. At Earth (Section 5.2), we
again found a long-duration (${\sim}$2.5 days) disturbance culminating in a
seemingly single magnetic cloud of NES type (based on both visual inspection
and flux rope fitting). However, we observed characteristic interaction
signatures at the centre of the flux rope, including enhanced plasma density,
temperature, and beta, as well as an irregular speed profile and a less smooth
magnetic field in the $\theta_{B}$ direction. Finally, at Saturn (Section
5.3), we found an extended (${\sim}$4 days in duration) disturbance mostly
characterised by smoothly rotating field. We associated a period of enhanced
magnetic field magnitude with a magnetic ejecta, noting that it featured at
its centre an irregular magnetic field profile (especially in $\theta_{B}$)
reminiscent of the one encountered at Earth and attributed to interaction
signatures. Its corresponding flux rope type was determined to be ESW based on
visual inspection, whilst flux rope fitting yielded a low-inclination
structure with its axis pointing back towards the Sun. The chirality was found
to be consistently right-handed at all in-situ locations.

The overall picture extrapolated from the set of observations described above
is that CME1 and CME2 started to interact in the inner heliosphere (roughly in
the HI1 field of view and before the orbit of Venus). The distinct NES ejecta
measured at Venus and Earth suggests that reconnection between the trailing
edge of CME1 and the leading edge of CME2 resulted in a single flux rope that
maintained the original orientation of its constituent parts. The resulting
magnetic structure of interacting CMEs has been analysed via simulations by,
e.g., Lugaz et al. (2013) and Schmidt and Cargill (2004). Two flux ropes with
the same twist and orientation (in this case, NES) are favourably configured
for magnetic reconnection during interaction, at least at the interface
between the two, which is where antiparallel fields meet. These conclusions
were also found by Kilpua et al. (2019), who analysed two interacting CMEs in
June 2012 that left the solar corona as NES types and were observed at Venus
at the beginning of their interaction and at Earth as a single NES flux rope.
The structure observed at Saturn, if corresponding in its entirely to the
merged flux ropes detected in the inner heliosphere, would be consistent
either with a modest (${<}90^{\circ}$) counterclockwise rotation in the
north–south direction (based on the ESW configuration retrieved from visual
inspection) or with a ${\sim}130^{\circ}$ rotation along the equatorial plane
(based on flux rope fitting) between 1 and 10 AU, possibly as a result of the
pressure exerted by a following HSS (see also the discussion in Section 6.1).

Finally, in order to investigate how the observed structure at Saturn relates
to the ones at Venus and Earth, we employ the magnetic field mapping technique
of Good et al. (2018), used to determine the arrival time and magnetic field
of different plasma parcels in an ejecta between radially separated
spacecraft. The technique takes into account expansion as a CME travels
through interplanetary space, and assumes that each ejecta features a
monotonically increasing/decreasing speed profile determined by its leading
and trailing edge speeds (calculated on the basis of their arrival times from
one location to the next). Magnetic field mapping results for two different
structures are shown in Figure 11. The full identified magnetic ejecta at
Venus, Earth, and Saturn (i.e., the shaded grey areas in Figures 7, 8, and 9)
is mapped in panel (b), whilst in panel (a) the smaller magnetic structure
preceding the ejecta (i.e., from the dash-dotted blue lines to the ejecta
leading edges in Figures 7 and 9) is mapped between Venus and Saturn. In both
panels, the measurements bounded by solid vertical lines are mapped according
to the Good et al. (2018) technique, whilst the preceding and following data
are propagated at the resulting leading and trailing edge speeds,
respectively. In Figure 11a, it is evident that, despite a data gap at Venus,
the magnetic field mapping shows good agreement between the two spacecraft in
all the magnetic field components. We also note that, by propagating the
following measurements at Saturn at the resulting trailing edge speed (387
km$\cdot$s-1), good agreement between the two data sets continues to be
displayed roughly until the ejecta trailing edge at Venus and the start of the
identified interaction region at Saturn (i.e., close to the first dashed
orange line in Figure 9). This would suggest that the original ejecta
resulting from the interaction of CME1 and CME2 may be linked to approximately
the first third of the flux rope interval identified at Saturn, and that the
full ejecta may on the other hand correspond to a MIR, which as mentioned in
the Introduction is a dominating structure in the outer heliosphere.
Contrarily, the magnetic field mapping in Figure 11b shows good agreement in
all the magnetic field components between Venus and Earth, but a substantial
difference in the $\phi_{B}$ component at Saturn in the second half of the
ejecta (which rotates from east to west). This is however not surprising,
since the structure at Saturn was found to display a different orientation
from that at Venus and Earth. Furthermore, the central, more turbulent portion
of magnetic field measurements found at Earth and Saturn appears to match
relatively well. According to these results, it is not possible to establish
with certainty the relationship between measurements at Saturn and those at
Venus and Earth (i.e., whether the original flux rope corresponds to the full
ejecta interval at Saturn or to a portion of it). We emphasise that the
magnetic field mapping technique of Good et al. (2018) assumes monotonically
increasing/decreasing speed, which is not a realistic assumption as far from
the Sun as 10 AU, where it is likely for a CME to have accelerated/decelerated
multiple times via continued interactions with the ambient solar wind (evident
also in the high distortion parameter found at Saturn). Nevertheless, this
method appears to work well at least for a qualitative comparison of
structures observed in the inner and outer heliosphere.

Figure 11: Magnetic field mapping between (a) Venus and Saturn for the small
structure preceding the ejecta, and (b) Venus, Earth, and Saturn for the full
ICME ejecta, using the method in Good et al. (2018). In (a) measurements at
Saturn have been temporally shifted and scaled to match those at Venus. The
dashed line marks the identified ejecta trailing edge at Venus. In (b),
measurements at Venus and Saturn have been temporally shifted and their
magnitudes have been scaled to match data taken near Earth.

##  7 Conclusions

In this work, we have analysed the eruption and evolution of two CMEs (CME1
and CME2) that left the Sun on 28 April 2012 just a few hours apart. After
observing their early evolution through the solar corona, we found indications
of interaction in HI imagery, and we eventually identified signatures of a
single ejecta at the in-situ locations that we considered (Venus, Earth, and
Saturn), suggesting that the two eruptions had merged. This study represents
the first detailed analysis of the magnetic structure of CME flux ropes from
the Sun to 10 AU, taking advantage of remote-sensing observations of the Sun,
its corona, and interplanetary space, as well as in-situ measurements at three
planets. Whilst the chirality of CME1 and CME2 at the Sun and of the flux rope
ejecta detected in situ was found to be consistently right-handed, the flux
rope type presented some changes. At the Sun, CME1 erupted as a NES flux rope
and CME2 displayed a high-to-intermediate inclination (between ESW and NES).
At Venus and Earth, the single observed ejecta was of type NES, whilst at
Saturn we found a structure featuring an ESW rotation that is consistent with
a rotation on either the meridional or the equatorial plane. We could not
determine with certainty whether the ejecta that we identified at Saturn
consisted entirely of material from CME1 and CME2, or whether additional
material was gathered on the way to 10 AU, thus forming a MIR.

The CME propagation models that we used to estimate the impact locations of
CME1 and CME2 (which were treated as a single, merged structure) across the
heliosphere provided useful insights necessary to interpret the in-situ
observations, and were reasonably well-timed compared to the actual
measurements. However, in-depth understanding of how CME magnetic fields
evolve throughout interplanetary space and interact with other CMEs or the
ambient solar wind still remains an arduous task. In the case of the events
under study, an “intermediate” observer between Earth and Saturn, perhaps
around 5 AU, may have helped shed more light on the evolution of solar
transients beyond 1 AU, especially as it may have been possible to catch the
ICME ejecta at the beginning of its interaction with the following HSS.
Nevertheless, this study remarks the importance of multi-point studies of CMEs
and the advantages to be gained by analysing data from both heliospheric and
planetary missions, which are necessary steps to undertake in order to deepen
our current understanding of the structure and evolution of solar transients
in the outer heliosphere.

## Data Availability Statement

The HELCATS catalogues are available at https://www.helcats-fp7.eu. Images and
additional information on the 28 April 2012 CME(s) are available at
https://www.helcats-fp7.eu/catalogues/event_page.html?id=HCME_A__20120428_01
(STEREO-A viewpoint) and https://www.helcats-
fp7.eu/catalogues/event_page.html?id=HCME_B__20120428_02 (STEREO-B viewpoint).
Enlil simulation results have been provided by the Community Coordinated
Modeling Center (CCMC) at NASA Goddard Space Flight Center through their
public Runs on Request system (http://ccmc.gsfc.nasa.gov). The full Enlil
simulation results are available at
https://ccmc.gsfc.nasa.gov/database_SH/Erika_Palmerio_031021_SH_3.php (run id:
Erika_Palmerio_031021_SH_3). The NASA–Wind ICME list can be found at
https://wind.nasa.gov/ICMEindex.php. Solar disc and coronagraph data from SDO,
SOHO, and STEREO are openly available at the Virtual Solar Observatory (VSO;
https://sdac.virtualsolar.org/). These data were processed and analysed trough
SunPy (SunPy Community et al., 2015, 2020), IDL SolarSoft (Freeland and Handy,
1998), and the ESA JHelioviewer software (Müller et al., 2017). Level-2
processed STEREO/HI data were obtained from the UK Solar System Data Centre
(UKSSDC; https://www.ukssdc.ac.uk/solar/stereo/data.html). VEX data are openly
available at ESA’s Planetary Science Archive (PSA; Besse et al., 2018),
accessible at https://archives.esac.esa.int/psa. These data were processed and
analysed with the aid of the irfpy library
(https://irfpy.irf.se/irfpy/index.html). Wind data are publicly available at
NASA’s Coordinated Data Analysis Web (CDAWeb) database
(https://cdaweb.sci.gsfc.nasa.gov/index.html/). NMDB data are publicly
available at http://www.nmdb.eu. Cassini data are openly available at the
Planetary Plasma Interactions (PPI) Node of NASA’s Planetary Data System
(PDS), accessible at https://pds-ppi.igpp.ucla.edu. All in-situ spacecraft
data can also be found on the Automated Multi-Dataset Analysis (AMDA; Génot et
al., 2021) tool at the address http://amda.irap.omp.eu. Solar wind data
propagated with the Tao et al. (2005) model can be browsed at the AMDA website
and at the HelioPropa tool, accessible at http://heliopropa.irap.omp.eu.

###### Acknowledgements.

E. Palmerio is supported by the NASA Living With a Star (LWS) Jack Eddy
Postdoctoral Fellowship Program, administered by UCAR’s Cooperative Programs
for the Advancement of Earth System Science (CPAESS) under award no.
NNX16AK22G. E. Kilpua acknowledges the SolMAG project (ERC-COG 724391) funded
by the European Research Council (ERC) in the framework of the Horizon 2020
Research and Innovation Programme, Academy of Finland project SMASH 310445,
and the Finnish Centre of Excellence in Research of Sustainable Space (Academy
of Finland grant no. 312390). A. Zhukov thanks the Belgian Federal Science
Policy Office (BELSPO) for the provision of financial support in the framework
of the PRODEX Programme of the European Space Agency (ESA) under contract no.
4000117262. L. Jian acknowledges support from the NASA LWS and Heliophysics
Supporting Research (HSR) programs. G. Provan and B. Sánchez-Cano acknowledge
support through UK‐STFC Grant ST/S000429/1. L. Lamy acknowledges support from
CNES and CNRS/INSU programs of planetology and heliophysics. C. Möstl thanks
the Austrian Science Fund (FWF): P31521-N27. A. Masters was supported by a
Royal Society University Research Fellowship. We acknowledge support from the
European Union FP7-SPACE-2013-1 programme for the HELCATS project (grant no.
606692). The HI instruments on STEREO were developed by a consortium that
comprised the Rutherford Appleton Laboratory (UK), the University of
Birmingham (UK), Centre Spatial de Liège (CSL, Belgium) and the Naval Research
Laboratory (NRL, USA). The STEREO/SECCHI project, of which HI is a part, is an
international consortium led by NRL. We recognise the support of the UK Space
Agency for funding STEREO/HI operations in the UK. The WSA model was developed
by C. N. Arge (currently at NASA/GSFC), and the Enlil model was developed by
D. Odstrcil (currently at GMU). We thank the model developers, R. Colaninno,
and the CCMC staff. We acknowledge the NMDB, founded under the European
Union’s FP7 programme (contract no. 213007), for providing neutron monitor
data. Finally, we thank the instrument teams of all the spacecraft involved in
this study.

## References

  * Al-Haddad et al. (2011) Al-Haddad, N., Roussev, I. I., Möstl, C., Jacobs, C., Lugaz, N., Poedts, S., and Farrugia, C. J. (2011), On the Internal Structure of the Magnetic Field in Magnetic Clouds and Interplanetary Coronal Mass Ejections: Writhe versus Twist, ApJ, 738(2), L18, 10.1088/2041-8205/738/2/L18.
  * Arge et al. (2004) Arge, C. N., Luhmann, J. G., Odstrcil, D., Schrijver, C. J., and Li, Y. (2004), Stream structure and coronal sources of the solar wind during the May 12th, 1997 CME,  Journal of Atmospheric and Solar-Terrestrial Physics, 66, 1295–1309, 10.1016/j.jastp.2004.03.018.
  * Asvestari et al. (2021) Asvestari, E., Pomoell, J., Kilpua, E., Good, S., Chatzistergos, T., Temmer, M., Palmerio, E., Poedts, S., and Magdalenic, J. (2021), Modelling a multi-spacecraft coronal mass ejection encounter with EUHFORIA, A&A, 652, A27, 10.1051/0004-6361/202140315.
  * Aulanier et al. (2012) Aulanier, G., Janvier, M., and Schmieder, B. (2012), The standard flare model in three dimensions. I. Strong-to-weak shear transition in post-flare loops, A&A, 543, A110, 10.1051/0004-6361/201219311.
  * Badman et al. (2008) Badman, S. V., Cowley, S. W. H., Lamy, L., Cecconi, B., and Zarka, P. (2008), Relationship between solar wind corotating interaction regions and the phasing and intensity of Saturn kilometric radiation bursts,  Annales Geophysicae, 26(12), 3641–3651, 10.5194/angeo-26-3641-2008.
  * Barabash et al. (2007) Barabash, S., Sauvaud, J.-A., Gunell, H., Andersson, H., Grigoriev, A., Brinkfeldt, K., Holmström, M., Lundin, R., Yamauchi, M., Asamura, K., Baumjohann, W., Zhang, T. L., Coates, A. J., Linder, D. R., Kataria, D. O., Curtis, C. C., Hsieh, K. C., Sandel, B. R., Fedorov, A., Mazelle, C., Thocaven, J.-J., Grande, M., Koskinen, H. E. J., Kallio, E., Säles, T., Riihela, P., Kozyra, J., Krupp, N., Woch, J., Luhmann, J., McKenna-Lawlor, S., Orsini, S., Cerulli-Irelli, R., Mura, M., Milillo, M., Maggi, M., Roelof, E., Brandt, P., Russell, C. T., Szego, K., Winningham, J. D., Frahm, R. A., Scherrer, J., Sharber, J. R., Wurz, P., and Bochsler, P. (2007), The Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) for the Venus Express mission, Planet. Space Sci., 55, 1772–1792, 10.1016/j.pss.2007.01.014.
  * Barnes et al. (2019) Barnes, D., Davies, J. A., Harrison, R. A., Byrne, J. P., Perry, C. H., Bothmer, V., Eastwood, J. P., Gallagher, P. T., Kilpua, E. K. J., and Möstl, C. (2019), CMEs in the Heliosphere: II. A Statistical Analysis of the Kinematic Properties Derived from Single-Spacecraft Geometrical Modelling Techniques Applied to CMEs Detected in the Heliosphere from 2007 to 2017 by STEREO/HI-1, Sol. Phys., 294(5), 57, 10.1007/s11207-019-1444-4.
  * Barnes et al. (2020) Barnes, D., Davies, J. A., Harrison, R. A., Byrne, J. P., Perry, C. H., Bothmer, V., Eastwood, J. P., Gallagher, P. T., Kilpua, E. K. J., Möstl, C., Rodriguez, L., Rouillard, A. P., and Odstrčil, D. (2020), CMEs in the Heliosphere: III. A Statistical Analysis of the Kinematic Properties Derived from Stereoscopic Geometrical Modelling Techniques Applied to CMEs Detected in the Heliosphere from 2008 to 2014 by STEREO/HI-1, Sol. Phys., 295(11), 150, 10.1007/s11207-020-01717-w.
  * Besse et al. (2018) Besse, S., Vallat, C., Barthelemy, M., Coia, D., Costa, M., De Marchi, G., Fraga, D., Grotheer, E., Heather, D., Lim, T., Martinez, S., Arviset, C., Barbarisi, I., Docasal, R., Macfarlane, A., Rios, C., Saiz, J., and Vallejo, F. (2018), ESA’s Planetary Science Archive: Preserve and present reliable scientific data sets, Planet. Space Sci., 150, 131–140, 10.1016/j.pss.2017.07.013.
  * Bothmer and Schwenn (1994) Bothmer, V., and Schwenn, R. (1994), Eruptive prominences as sources of magnetic clouds in the solar wind, Space Sci. Rev., 70, 215–220, 10.1007/BF00777872.
  * Bothmer and Schwenn (1998) Bothmer, V., and Schwenn, R. (1998), The structure and origin of magnetic clouds in the solar wind,  Annales Geophysicae, 16, 1–24, 10.1007/s00585-997-0001-x.
  * Bradley et al. (2020) Bradley, T. J., Cowley, S. W. H., Bunce, E. J., Melin, H., Provan, G., Nichols, J. D., Dougherty, M. K., Roussos, E., Krupp, N., Tao, C., Lamy, L., Pryor, W. R., and Hunt, G. J. (2020), Saturn’s Nightside Dynamics During Cassini’s F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations,  Journal of Geophysical Research: Space Physics, 125(9), e27907, 10.1029/2020JA027907.
  * Branduardi-Raymont et al. (2013) Branduardi-Raymont, G., Ford, P. G., Hansen, K. C., Lamy, L., Masters, A., Cecconi, B., Coates, A. J., Dougherty, M. K., Gladstone, G. R., and Zarka, P. (2013), Search for Saturn’s X-ray aurorae at the arrival of a solar wind shock,  Journal of Geophysical Research: Space Physics, 118(5), 2145–2156, 10.1002/jgra.50112.
  * Brueckner et al. (1995) Brueckner, G. E., Howard, R. A., Koomen, M. J., Korendyke, C. M., Michels, D. J., Moses, J. D., Socker, D. G., Dere, K. P., Lamy, P. L., Llebaria, A., Bout, M. V., Schwenn, R., Simnett, G. M., Bedford, D. K., and Eyles, C. J. (1995), The Large Angle Spectroscopic Coronagraph (LASCO), Sol. Phys., 162, 357–402, 10.1007/BF00733434.
  * Burlaga et al. (1981) Burlaga, L., Sittler, E., Mariani, F., and Schwenn, R. (1981), Magnetic loop behind an interplanetary shock - Voyager, Helios, and IMP 8 observations, J. Geophys. Res., 86, 6673–6684, 10.1029/JA086iA08p06673.
  * Burlaga et al. (1986) Burlaga, L. F., McDonald, F. B., and Schwenn, R. (1986), Formation of a compound stream between 0.85 AU and 6.2 AU and its effects on solar energetic particles and galactic cosmic rays, J. Geophys. Res., 91(A12), 13,331–13,340, 10.1029/JA091iA12p13331.
  * Burlaga et al. (1997) Burlaga, L. F., Ness, N. F., and Belcher, J. W. (1997), Radial evolution of corotating merged interaction regions and flows between ~14AU and ~43AU, J. Geophys. Res., 102(A3), 4661–4672, 10.1029/96JA03629.
  * Burlaga et al. (2001) Burlaga, L. F., Ness, N. F., Richardson, J. D., and Lepping, R. P. (2001), The Bastille day Shock and Merged Interaction Region at 63 au: Voyager 2 Observations, Sol. Phys., 204, 399–411, 10.1023/A:1014269926730.
  * Cane (2000) Cane, H. V. (2000), Coronal Mass Ejections and Forbush Decreases, Space Sci. Rev., 93, 55–77, 10.1023/A:1026532125747.
  * Cane and Richardson (2003) Cane, H. V., and Richardson, I. G. (2003), Interplanetary coronal mass ejections in the near-Earth solar wind during 1996-2002, J. Geophys. Res., 108(A4), 1156, 10.1029/2002JA009817.
  * Cane et al. (1997) Cane, H. V., Richardson, I. G., and Wibberenz, G. (1997), Helios 1 and 2 observations of particle decreases, ejecta, and magnetic clouds, J. Geophys. Res., 102, 7075–7086, 10.1029/97JA00149.
  * Chen (2011) Chen, P. F. (2011), Coronal Mass Ejections: Models and Their Observational Basis,  Living Reviews in Solar Physics, 8, 1, 10.12942/lrsp-2011-1.
  * Clarke et al. (2009) Clarke, J. T., Nichols, J., Gérard, J. C., Grodent, D., Hansen, K. C., Kurth, W., Gladstone, G. R., Duval, J., Wannawichian, S., Bunce, E., Cowley, S. W. H., Crary, F., Dougherty, M., Lamy, L., Mitchell, D., Pryor, W., Retherford, K., Stallard, T., Zieger, B., Zarka, P., and Cecconi, B. (2009), Response of Jupiter’s and Saturn’s auroral activity to the solar wind, J. Geophys. Res., 114(A5), A05210, 10.1029/2008JA013694.
  * Collinson et al. (2015) Collinson, G. A., Grebowsky, J., Sibeck, D. G., Jian, L. K., Boardsen, S., Espley, J., Hartle, D., Zhang, T. L., Barabash, S., and Futaana, Y. (2015), The impact of a slow interplanetary coronal mass ejection on Venus,  Journal of Geophysical Research: Space Physics, 120(5), 3489–3502, 10.1002/2014JA020616.
  * Crary et al. (2005) Crary, F. J., Clarke, J. T., Dougherty, M. K., Hanlon, P. G., Hansen, K. C., Steinberg, J. T., Barraclough, B. L., Coates, A. J., Gérard, J. C., Grodent, D., Kurth, W. S., Mitchell, D. G., Rymer, A. M., and Young, D. T. (2005), Solar wind dynamic pressure and electric field as the main factors controlling Saturn’s aurorae, Nature, 433(7027), 720–722, 10.1038/nature03333.
  * Dasso et al. (2009) Dasso, S., Mandrini, C. H., Schmieder, B., Cremades, H., Cid, C., Cerrato, Y., Saiz, E., Démoulin, P., Zhukov, A. N., Rodriguez, L., Aran, A., Menvielle, M., and Poedts, S. (2009), Linking two consecutive nonmerging magnetic clouds with their solar sources, Journal of Geophysical Research (Space Physics), 114(A2), A02109, 10.1029/2008JA013102.
  * Davies et al. (2020) Davies, E. E., Forsyth, R. J., Good, S. W., and Kilpua, E. K. J. (2020), On the Radial and Longitudinal Variation of a Magnetic Cloud: ACE, Wind, ARTEMIS and Juno Observations, Sol. Phys., 295(11), 157, 10.1007/s11207-020-01714-z.
  * Davies et al. (2021) Davies, E. E., Möstl, C., Owens, M. J., Weiss, A. J., Amerstorfer, T., Hinterreiter, J., Bauer, M., Bailey, R. L., Reiss, M. A., Forsyth, R. J., Horbury, T. S., O’Brien, H., Evans, V., Angelini, V., Heyner, D., Richter, I., Auster, H.-U., Magnes, W., Baumjohann, W., Fischer, D., Barnes, D., Davies, J. A., and Harrison, R. A. (2021), In-Situ Multi-Spacecraft and Remote Imaging Observations of the First CME Detected by Solar Orbiter and BepiColombo, A&A, in press, 10.1051/0004-6361/202040113.
  * Davies et al. (2012) Davies, J. A., Harrison, R. A., Perry, C. H., Möstl, C., Lugaz, N., Rollett, T., Davis, C. J., Crothers, S. R., Temmer, M., Eyles, C. J., and Savani, N. P. (2012), A Self-similar Expansion Model for Use in Solar Wind Transient Propagation Studies, ApJ, 750, 23, 10.1088/0004-637X/750/1/23.
  * Davies et al. (2013) Davies, J. A., Perry, C. H., Trines, R. M. G. M., Harrison, R. A., Lugaz, N., Möstl, C., Liu, Y. D., and Steed, K. (2013), Establishing a Stereoscopic Technique for Determining the Kinematic Properties of Solar Wind Transients based on a Generalized Self-similarly Expanding Circular Geometry, ApJ, 777, 167, 10.1088/0004-637X/777/2/167.
  * de Lucas et al. (2011) de Lucas, A., Dal Lago, A., Schwenn, R., and Clúa de Gonzalez, A. L. (2011), Multi-spacecraft observed magnetic clouds as seen by Helios mission,  Journal of Atmospheric and Solar-Terrestrial Physics, 73(11-12), 1361–1371, 10.1016/j.jastp.2011.02.007.
  * DeForest et al. (2013) DeForest, C. E., Howard, T. A., and McComas, D. J. (2013), Tracking Coronal Features from the Low Corona to Earth: A Quantitative Analysis of the 2008 December 12 Coronal Mass Ejection, ApJ, 769, 43, 10.1088/0004-637X/769/1/43.
  * Démoulin and Dasso (2009) Démoulin, P., and Dasso, S. (2009), Causes and consequences of magnetic cloud expansion, A&A, 498(2), 551–566, 10.1051/0004-6361/200810971.
  * Démoulin et al. (1996) Démoulin, P., Priest, E. R., and Lonie, D. P. (1996), Three-dimensional magnetic reconnection without null points 2. Application to twisted flux tubes, J. Geophys. Res., 101, 7631–7646, 10.1029/95JA03558.
  * Desai et al. (2020) Desai, R. T., Zhang, H., Davies, E. E., Stawarz, J. E., Mico-Gomez, J., and Iváñez-Ballesteros, P. (2020), Three-Dimensional Simulations of Solar Wind Preconditioning and the 23 July 2012 Interplanetary Coronal Mass Ejection, Sol. Phys., 295(9), 130, 10.1007/s11207-020-01700-5.
  * Desch (1982) Desch, M. D. (1982), Evidence for solar wind control of solar radio emission, J. Geophys. Res., 87(A6), 4549–4554, 10.1029/JA087iA06p04549.
  * Domingo et al. (1995) Domingo, V., Fleck, B., and Poland, A. I. (1995), The SOHO Mission: an Overview, Sol. Phys., 162, 1–37, 10.1007/BF00733425.
  * Dougherty et al. (2004) Dougherty, M. K., Kellock, S., Southwood, D. J., Balogh, A., Smith, E. J., Tsurutani, B. T., Gerlach, B., Glassmeier, K.-H., Gleim, F., Russell, C. T., Erdos, G., Neubauer, F. M., and Cowley, S. W. H. (2004), The Cassini Magnetic Field Investigation, Space Sci. Rev., 114, 331–383, 10.1007/s11214-004-1432-2.
  * Du et al. (2010) Du, D., Zuo, P. B., and Zhang, X. X. (2010), Interplanetary Coronal Mass Ejections Observed by Ulysses Through Its Three Solar Orbits, Sol. Phys., 262(1), 171–190, 10.1007/s11207-009-9505-8.
  * Dumbović et al. (2020) Dumbović, M., Vršnak, B., Guo, J., Heber, B., Dissauer, K., Carcaboso, F., Temmer, M., Veronig, A., Podladchikova, T., Möstl, C., Amerstorfer, T., and Kirin, A. (2020), Evolution of Coronal Mass Ejections and the Corresponding Forbush Decreases: Modeling vs. Multi-Spacecraft Observations, Sol. Phys., 295(7), 104, 10.1007/s11207-020-01671-7.
  * Dunn et al. (2020) Dunn, W. R., Gray, R., Wibisono, A. D., Lamy, L., Louis, C., Badman, S. V., Branduardi-Raymont, G., Elsner, R., Gladstone, G. R., Ebert, R., Ford, P., Foster, A., Tao, C., Ray, L. C., Yao, Z., Rae, I. J., Bunce, E. J., Rodriguez, P., Jackman, C. M., Nicolaou, G., Clarke, J., Nichols, J., Elliott, H., and Kraft, R. (2020), Comparisons Between Jupiter’s X-ray, UV and Radio Emissions and In-Situ Solar Wind Measurements During 2007,  Journal of Geophysical Research: Space Physics, 125(6), e27222, 10.1029/2019JA027222.
  * Dunn et al. (2021) Dunn, W. R., Ness, J. U., Lamy, L., Tremblay, G. R., Branduardi-Raymont, G., Snios, B., Kraft, R. P., Yao, Z., and Wibisono, A. D. (2021), A Low Signal Detection of X Rays From Uranus,  Journal of Geophysical Research: Space Physics, 126(4), e28739, 10.1029/2020JA028739.
  * Echer (2019) Echer, E. (2019), Solar wind and interplanetary shock parameters near Saturn’s orbit ($\sim$10 AU), Planet. Space Sci., 165, 210–220, 10.1016/j.pss.2018.10.006.
  * Eyles et al. (2009) Eyles, C. J., Harrison, R. A., Davis, C. J., Waltham, N. R., Shaughnessy, B. M., Mapson-Menard, H. C. A., Bewsher, D., Crothers, S. R., Davies, J. A., Simnett, G. M., Howard, R. A., Moses, J. D., Newmark, J. S., Socker, D. G., Halain, J.-P., Defise, J.-M., Mazy, E., and Rochus, P. (2009), The Heliospheric Imagers Onboard the STEREO Mission, Sol. Phys., 254, 387–445, 10.1007/s11207-008-9299-0.
  * Farrugia and Berdichevsky (2004) Farrugia, C., and Berdichevsky, D. (2004), Evolutionary signatures in complex ejecta and their driven shocks,  Annales Geophysicae, 22(10), 3679–3698, 10.5194/angeo-22-3679-2004.
  * Forbes (2000) Forbes, T. G. (2000), A review on the genesis of coronal mass ejections, J. Geophys. Res., 105, 23,153–23,166, 10.1029/2000JA000005.
  * Forbush (1937) Forbush, S. E. (1937), On the Effects in Cosmic-Ray Intensity Observed During the Recent Magnetic Storm, Physical Review, 51, 1108–1109, 10.1103/PhysRev.51.1108.3.
  * Freeland and Handy (1998) Freeland, S. L., and Handy, B. N. (1998), Data Analysis with the SolarSoft System, Sol. Phys., 182, 497–500, 10.1023/A:1005038224881.
  * Freiherr von Forstner et al. (2018) Freiherr von Forstner, J. L., Guo, J., Wimmer-Schweingruber, R. F., Hassler, D. M., Temmer, M., Dumbović, M., Jian, L. K., Appel, J. K., Čalogović, J., and Ehresmann, B. (2018), Using Forbush Decreases to Derive the Transit Time of ICMEs Propagating from 1 AU to Mars,  Journal of Geophysical Research: Space Physics, 123(1), 39–56, 10.1002/2017JA024700.
  * Freiherr von Forstner et al. (2020) Freiherr von Forstner, J. L., Guo, J., Wimmer-Schweingruber, R. F., Dumbović, M., Janvier, M., Démoulin, P., Veronig, A., Temmer, M., Papaioannou, A., Dasso, S., Hassler, D. M., and Zeitlin, C. J. (2020), Comparing the Properties of ICME-Induced Forbush Decreases at Earth and Mars,  Journal of Geophysical Research: Space Physics, 125(3), e2019JA027662, 10.1029/2019JA027662.
  * Gazis et al. (2006) Gazis, P. R., Balogh, A., Dalla, S., Decker, R., Heber, B., Horbury, T., Kilchenmann, A., Kota, J., Kucharek, H., and Kunow, H. (2006), ICMEs at High Latitudes and in the Outer Heliosphere. Report of Working Group H, Space Sci. Rev., 123(1-3), 417–451, 10.1007/s11214-006-9023-z.
  * Génot et al. (2021) Génot, V., Budnik, E., Jacquey, C., Bouchemit, M., Renard, B., Dufourg, N., André, N., Cecconi, B., Pitout, F., Lavraud, B., Fedorov, A., Ganfloff, M., Plotnikov, I., Modolo, R., Lormant, N., Mohand, H. S. H., Tao, C., Besson, B., Heulet, D., Boucon, D., Durand, J., Bourrel, N., Brzustowski, Q., Jourdane, N., Hitier, R., Garnier, P., Grison, B., Aunai, N., Jeandet, A., and Cabrolie, F. (2021), Automated Multi-Dataset Analysis (AMDA): An on-line database and analysis tool for heliospheric and planetary plasma data, Planet. Space Sci., 201, 105214, 10.1016/j.pss.2021.105214.
  * Good and Forsyth (2016) Good, S. W., and Forsyth, R. J. (2016), Interplanetary Coronal Mass Ejections Observed by MESSENGER and Venus Express, Sol. Phys., 291, 239–263, 10.1007/s11207-015-0828-3.
  * Good et al. (2018) Good, S. W., Forsyth, R. J., Eastwood, J. P., and Möstl, C. (2018), Correlation of ICME Magnetic Fields at Radially Aligned Spacecraft, Sol. Phys., 293, 52, 10.1007/s11207-018-1264-y.
  * Good et al. (2019) Good, S. W., Kilpua, E. K. J., LaMoury, A. T., Forsyth, R. J., Eastwood, J. P., and Möstl, C. (2019), Self-Similarity of ICME Flux Ropes: Observations by Radially Aligned Spacecraft in the Inner Heliosphere,  Journal of Geophysical Research: Space Physics, 124(7), 4960–4982, 10.1029/2019JA026475.
  * Gopalswamy et al. (2000) Gopalswamy, N., Lara, A., Lepping, R. P., Kaiser, M. L., Berdichevsky, D., and St. Cyr, O. C. (2000), Interplanetary acceleration of coronal mass ejections, Geophys. Res. Lett., 27, 145–148, 10.1029/1999GL003639.
  * Green et al. (2007) Green, L. M., Kliem, B., Török, T., van Driel-Gesztelyi, L., and Attrill, G. D. R. (2007), Transient Coronal Sigmoids and Rotating Erupting Flux Ropes, Sol. Phys., 246, 365–391, 10.1007/s11207-007-9061-z.
  * Green et al. (2018) Green, L. M., Török, T., Vršnak, B., Manchester, W., and Veronig, A. (2018), The Origin, Early Evolution and Predictability of Solar Eruptions, Space Sci. Rev., 214, 46, 10.1007/s11214-017-0462-5.
  * Gurnett et al. (2004) Gurnett, D. A., Kurth, W. S., Kirchner, D. L., Hospodarsky, G. B., Averkamp, T. F., Zarka, P., Lecacheux, A., Manning, R., Roux, A., Canu, P., Cornilleau-Wehrlin, N., Galopeau, P., Meyer, A., Boström, R., Gustafsson, G., Wahlund, J. E., Åhlen, L., Rucker, H. O., Ladreiter, H. P., Macher, W., Woolliscroft, L. J. C., Alleyne, H., Kaiser, M. L., Desch, M. D., Farrell, W. M., Harvey, C. C., Louarn, P., Kellogg, P. J., Goetz, K., and Pedersen, A. (2004), The Cassini Radio and Plasma Wave Investigation, Space Sci. Rev., 114(1-4), 395–463, 10.1007/s11214-004-1434-0.
  * Hanlon et al. (2004) Hanlon, P. G., Dougherty, M. K., Forsyth, R. J., Owens, M. J., Hansen, K. C., Tóth, G., Crary, F. J., and Young, D. T. (2004), On the evolution of the solar wind between 1 and 5 AU at the time of the Cassini Jupiter flyby: Multispacecraft observations of interplanetary coronal mass ejections including the formation of a merged interaction region, J. Geophys. Res., 109(A9), A09S03, 10.1029/2003JA010112.
  * Harrison et al. (2018) Harrison, R. A., Davies, J. A., Barnes, D., Byrne, J. P., Perry, C. H., Bothmer, V., Eastwood, J. P., Gallagher, P. T., Kilpua, E. K. J., and Möstl, C. (2018), CMEs in the Heliosphere: I. A Statistical Analysis of the Observational Properties of CMEs Detected in the Heliosphere from 2007 to 2017 by STEREO/HI-1, Sol. Phys., 293(5), 77, 10.1007/s11207-018-1297-2.
  * Heinemann et al. (2019) Heinemann, S. G., Temmer, M., Farrugia, C. J., Dissauer, K., Kay, C., Wiegelmann, T., Dumbović, M., Veronig, A. M., Podladchikova, T., Hofmeister, S. J., Lugaz, N., and Carcaboso, F. (2019), CME-HSS Interaction and Characteristics Tracked from Sun to Earth, Sol. Phys., 294(9), 121, 10.1007/s11207-019-1515-6.
  * Hess and Demmelmair (1937) Hess, V. F., and Demmelmair, A. (1937), World-wide Effect in Cosmic Ray Intensity, as Observed during a Recent Magnetic Storm, Nature, 140, 316–317, 10.1038/140316a0.
  * Howard et al. (2008) Howard, R. A., Moses, J. D., Vourlidas, A., Newmark, J. S., Socker, D. G., Plunkett, S. P., Korendyke, C. M., Cook, J. W., Hurley, A., Davila, J. M., Thompson, W. T., St Cyr, O. C., Mentzell, E., Mehalick, K., Lemen, J. R., Wuelser, J. P., Duncan, D. W., Tarbell, T. D., Wolfson, C. J., Moore, A., Harrison, R. A., Waltham, N. R., Lang, J., Davis, C. J., Eyles, C. J., Mapson-Menard, H., Simnett, G. M., Halain, J. P., Defise, J. M., Mazy, E., Rochus, P., Mercier, R., Ravet, M. F., Delmotte, F., Auchere, F., Delaboudiniere, J. P., Bothmer, V., Deutsch, W., Wang, D., Rich, N., Cooper, S., Stephens, V., Maahs, G., Baugh, R., McMullin, D., and Carter, T. (2008), Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), Space Sci. Rev., 136, 67–115, 10.1007/s11214-008-9341-4.
  * Howard et al. (2017) Howard, T. A., DeForest, C. E., Schneck, U. G., and Alden, C. R. (2017), Challenging Some Contemporary Views of Coronal Mass Ejections. II. The Case for Absent Filaments, ApJ, 834(1), 86, 10.3847/1538-4357/834/1/86.
  * Huttunen et al. (2005) Huttunen, K. E. J., Schwenn, R., Bothmer, V., and Koskinen, H. E. J. (2005), Properties and geoeffectiveness of magnetic clouds in the rising, maximum and early declining phases of solar cycle 23,  Annales Geophysicae, 23, 625–641, 10.5194/angeo-23-625-2005.
  * Isavnin et al. (2014) Isavnin, A., Vourlidas, A., and Kilpua, E. K. J. (2014), Three-Dimensional Evolution of Flux-Rope CMEs and Its Relation to the Local Orientation of the Heliospheric Current Sheet, Sol. Phys., 289, 2141–2156, 10.1007/s11207-013-0468-4.
  * Jackman et al. (2008) Jackman, C. M., Forsyth, R. J., and Dougherty, M. K. (2008), The overall configuration of the interplanetary magnetic field upstream of Saturn as revealed by Cassini observations, J. Geophys. Res., 113(A8), A08114, 10.1029/2008JA013083.
  * Janvier et al. (2014a) Janvier, M., Démoulin, P., and Dasso, S. (2014a), In situ properties of small and large flux ropes in the solar wind,  Journal of Geophysical Research: Space Physics, 119(9), 7088–7107, 10.1002/2014JA020218.
  * Janvier et al. (2014b) Janvier, M., Aulanier, G., Bommier, V., Schmieder, B., Démoulin, P., and Pariat, E. (2014b), Electric Currents in Flare Ribbons: Observations and Three-dimensional Standard Model, ApJ, 788(1), 60, 10.1088/0004-637X/788/1/60.
  * Janvier et al. (2019) Janvier, M., Winslow, R. M., Good, S., Bonhomme, E., Démoulin, P., Dasso, S., Möstl, C., Lugaz, N., Amerstorfer, T., Soubrié, E., and Boakes, P. D. (2019), Generic Magnetic Field Intensity Profiles of Interplanetary Coronal Mass Ejections at Mercury, Venus, and Earth From Superposed Epoch Analyses,  Journal of Geophysical Research: Space Physics, 124(2), 812–836, 10.1029/2018JA025949.
  * Jian et al. (2006) Jian, L., Russell, C. T., Luhmann, J. G., and Skoug, R. M. (2006), Properties of Interplanetary Coronal Mass Ejections at One AU During 1995 – 2004, Sol. Phys., 239, 393–436, 10.1007/s11207-006-0133-2.
  * Jian et al. (2008a) Jian, L. K., Russell, C. T., Luhmann, J. G., Skoug, R. M., and Steinberg, J. T. (2008a), Stream Interactions and Interplanetary Coronal Mass Ejections at 5.3 AU near the Solar Ecliptic Plane, Sol. Phys., 250(2), 375–402, 10.1007/s11207-008-9204-x.
  * Jian et al. (2008b) Jian, L. K., Russell, C. T., Luhmann, J. G., Skoug, R. M., and Steinberg, J. T. (2008b), Stream Interactions and Interplanetary Coronal Mass Ejections at 0.72 AU, Sol. Phys., 249(1), 85–101, 10.1007/s11207-008-9161-4.
  * Jian et al. (2018) Jian, L. K., Russell, C. T., Luhmann, J. G., and Galvin, A. B. (2018), STEREO Observations of Interplanetary Coronal Mass Ejections in 2007-2016, ApJ, 855, 114, 10.3847/1538-4357/aab189.
  * Kaiser et al. (1984) Kaiser, M. L., Desch, M. D., Kurth, W. S., Lecacheux, A., Genova, F., Pedersen, B. M., and Evans, D. R. (1984), Saturn as a radio source, in Saturn, edited by T. Gehrels and M. S. Matthews, pp. 378–415, University of Arizona Press.
  * Kaiser et al. (2008) Kaiser, M. L., Kucera, T. A., Davila, J. M., St. Cyr, O. C., Guhathakurta, M., and Christian, E. (2008), The STEREO Mission: An Introduction, Space Sci. Rev., 136, 5–16, 10.1007/s11214-007-9277-0.
  * Kay et al. (2015) Kay, C., Opher, M., and Evans, R. M. (2015), Global Trends of CME Deflections Based on CME and Solar Parameters, ApJ, 805, 168, 10.1088/0004-637X/805/2/168.
  * Kilpua et al. (2017) Kilpua, E., Koskinen, H. E. J., and Pulkkinen, T. I. (2017), Coronal mass ejections and their sheath regions in interplanetary space,  Living Reviews in Solar Physics, 14, 5, 10.1007/s41116-017-0009-6.
  * Kilpua et al. (2009) Kilpua, E. K. J., Liewer, P. C., Farrugia, C., Luhmann, J. G., Möstl, C., Li, Y., Liu, Y., Lynch, B. J., Russell, C. T., Vourlidas, A., Acuna, M. H., Galvin, A. B., Larson, D., and Sauvaud, J. A. (2009), Multispacecraft Observations of Magnetic Clouds and Their Solar Origins between 19 and 23 May 2007, Sol. Phys., 254, 325–344, 10.1007/s11207-008-9300-y.
  * Kilpua et al. (2011) Kilpua, E. K. J., Jian, L. K., Li, Y., Luhmann, J. G., and Russell, C. T. (2011), Multipoint ICME encounters: Pre-STEREO and STEREO observations,  Journal of Atmospheric and Solar-Terrestrial Physics, 73, 1228–1241, 10.1016/j.jastp.2010.10.012.
  * Kilpua et al. (2019) Kilpua, E. K. J., Good, S. W., Palmerio, E., Asvestari, E., Lumme, E., Ala-Lahti, M., Kalliokoski, M. M. H., Morosan, D. E., Pomoell, J., Price, D. J., Magdalenić, J., Poedts, S., and Futaana, Y. (2019), Multipoint Observations of the June 2012 Interacting Interplanetary Flux Ropes,  Frontiers in Astronomy and Space Sciences, 6, 50, 10.3389/fspas.2019.00050.
  * Klimchuk (2001) Klimchuk, J. A. (2001), Theory of Coronal Mass Ejections, Washington DC American Geophysical Union Geophysical Monograph Series, 125, 143, 10.1029/GM125p0143.
  * Korreck et al. (2020) Korreck, K. E., Szabo, A., Nieves Chinchilla, T., Lavraud, B., Luhmann, J., Niembro, T., Higginson, A., Alzate, N., Wallace, S., Paulson, K., Rouillard, A., Kouloumvakos, A., Poirier, N., Kasper, J. C., Case, A. W., Stevens, M. L., Bale, S. D., Pulupa, M., Whittlesey, P., Livi, R., Goetz, K., Larson, D., Malaspina, D. M., Morgan, H., Narock, A. A., Schwadron, N. A., Bonnell, J., Harvey, P., and Wygant, J. (2020), Source and Propagation of a Streamer Blowout Coronal Mass Ejection Observed by the Parker Solar Probe, ApJS, 246(2), 69, 10.3847/1538-4365/ab6ff9.
  * Krall and St. Cyr (2006) Krall, J., and St. Cyr, O. C. (2006), Flux-Rope Coronal Mass Ejection Geometry and Its Relation to Observed Morphology, ApJ, 652(2), 1740–1746, 10.1086/508337.
  * Krimigis et al. (2004) Krimigis, S. M., Mitchell, D. G., Hamilton, D. C., Livi, S., Dandouras, J., Jaskulek, S., Armstrong, T. P., Boldt, J. D., Cheng, A. F., Gloeckler, G., Hayes, J. R., Hsieh, K. C., Ip, W.-H., Keath, E. P., Kirsch, E., Krupp, N., Lanzerotti, L. J., Lundgren, R., Mauk, B. H., McEntire, R. W., Roelof, E. C., Schlemm, C. E., Tossman, B. E., Wilken, B., and Williams, D. J. (2004), Magnetosphere Imaging Instrument (MIMI) on the Cassini Mission to Saturn/Titan, Space Sci. Rev., 114, 233–329, 10.1007/s11214-004-1410-8.
  * Kurth et al. (2005) Kurth, W. S., Gurnett, D. A., Clarke, J. T., Zarka, P., Desch, M. D., Kaiser, M. L., Cecconi, B., Lecacheux, A., Farrell, W. M., Galopeau, P., Gérard, J. C., Grodent, D., Prangé, R., Dougherty, M. K., and Crary, F. J. (2005), An Earth-like correspondence between Saturn’s auroral features and radio emission, Nature, 433(7027), 722–725, 10.1038/nature03334.
  * Kurth et al. (2016) Kurth, W. S., Hospodarsky, G. B., Gurnett, D. A., Lamy, L., Dougherty, M. K., Nichols, J., Bunce, E. J., Pryor, W., Baines, K., Stallard, T., Melin, H., and Crary, F. J. (2016), Saturn kilometric radiation intensities during the Saturn auroral campaign of 2013, Icarus, 263, 2–9, 10.1016/j.icarus.2015.01.003.
  * Lamy (2017) Lamy, L. (2017), The Saturnian kilometric radiation before the Cassini Grand Finale, in Planetary Radio Emissions VIII, edited by G. Fischer, G. Mann, M. Panchenko, and P. Zarka, pp. 171–190, 10.1553/PRE8s171.
  * Lamy (2020) Lamy, L. (2020), Auroral emissions from Uranus and Neptune,  Philosophical Transactions of the Royal Society of London Series A, 378(2187), 20190481, 10.1098/rsta.2019.0481.
  * Lamy et al. (2008) Lamy, L., Zarka, P., Cecconi, B., Prangé, R., Kurth, W. S., and Gurnett, D. A. (2008), Saturn kilometric radiation: Average and statistical properties, J. Geophys. Res., 113(A7), A07201, 10.1029/2007JA012900.
  * Lamy et al. (2009) Lamy, L., Cecconi, B., Prangé, R., Zarka, P., Nichols, J. D., and Clarke, J. T. (2009), An auroral oval at the footprint of Saturn’s kilometric radio sources, colocated with the UV aurorae, J. Geophys. Res., 114(A10), A10212, 10.1029/2009JA014401.
  * Lamy et al. (2012) Lamy, L., Prangé, R., Hansen, K. C., Clarke, J. T., Zarka, P., Cecconi, B., Aboudarham, J., André, N., Branduardi-Raymont, G., Gladstone, R., Barthélémy, M., Achilleos, N., Guio, P., Dougherty, M. K., Melin, H., Cowley, S. W. H., Stallard, T. S., Nichols, J. D., and Ballester, G. (2012), Earth-based detection of Uranus’ aurorae, Geophys. Res. Lett., 39(7), L07105, 10.1029/2012GL051312.
  * Lamy et al. (2017) Lamy, L., Prangé, R., Hansen, K. C., Tao, C., Cowley, S. W. H., Stallard, T. S., Melin, H., Achilleos, N., Guio, P., Badman, S. V., Kim, T., and Pogorelov, N. (2017), The aurorae of Uranus past equinox,  Journal of Geophysical Research: Space Physics, 122(4), 3997–4008, 10.1002/2017JA023918.
  * Lamy et al. (2018) Lamy, L., Prangé, R., Tao, C., Kim, T., Badman, S. V., Zarka, P., Cecconi, B., Kurth, W. S., Pryor, W., Bunce, E., and Radioti, A. (2018), Saturn’s Northern Aurorae at Solstice From HST Observations Coordinated With Cassini’s Grand Finale, Geophys. Res. Lett., 45(18), 9353–9362, 10.1029/2018GL078211.
  * Lario et al. (2020) Lario, D., Balmaceda, L., Alzate, N., Mays, M. L., Richardson, I. G., Allen, R. C., Florido-Llinas, M., Nieves-Chinchilla, T., Koval, A., Lugaz, N., Jian, L. K., Arge, C. N., Macneice, P. J., Odstrcil, D., Morgan, H., Szabo, A., Desai, M. I., Whittlesey, P. L., Stevens, M. L., Ho, G. C., and Luhmann, J. G. (2020), The Streamer Blowout Origin of a Flux Rope and Energetic Particle Event Observed by Parker Solar Probe at 0.5 au, ApJ, 897(2), 134, 10.3847/1538-4357/ab9942.
  * Lee et al. (2017) Lee, C. O., Hara, T., Halekas, J. S., Thiemann, E., Chamberlin, P., Eparvier, F., Lillis, R. J., Larson, D. E., Dunn, P. A., and Espley, J. R. (2017), MAVEN observations of the solar cycle 24 space weather conditions at Mars,  Journal of Geophysical Research: Space Physics, 122(3), 2768–2794, 10.1002/2016JA023495.
  * Lemen et al. (2012) Lemen, J. R., Title, A. M., Akin, D. J., Boerner, P. F., Chou, C., Drake, J. F., Duncan, D. W., Edwards, C. G., Friedlaender, F. M., Heyman, G. F., Hurlburt, N. E., Katz, N. L., Kushner, G. D., Levay, M., Lindgren, R. W., Mathur, D. P., McFeaters, E. L., Mitchell, S., Rehse, R. A., Schrijver, C. J., Springer, L. A., Stern, R. A., Tarbell, T. D., Wuelser, J.-P., Wolfson, C. J., Yanari, C., Bookbinder, J. A., Cheimets, P. N., Caldwell, D., Deluca, E. E., Gates, R., Golub, L., Park, S., Podgorski, W. A., Bush, R. I., Scherrer, P. H., Gummin, M. A., Smith, P., Auker, G., Jerram, P., Pool, P., Soufli, R., Windt, D. L., Beardsley, S., Clapp, M., Lang, J., and Waltham, N. (2012), The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO), Sol. Phys., 275, 17–40, 10.1007/s11207-011-9776-8.
  * Lepping et al. (1995) Lepping, R. P., Acũna, M. H., Burlaga, L. F., Farrell, W. M., Slavin, J. A., Schatten, K. H., Mariani, F., Ness, N. F., Neubauer, F. M., Whang, Y. C., Byrnes, J. B., Kennon, R. S., Panetta, P. V., Scheifele, J., and Worley, E. M. (1995), The Wind Magnetic Field Investigation, Space Sci. Rev., 71, 207–229, 10.1007/BF00751330.
  * Lepri and Zurbuchen (2004) Lepri, S. T., and Zurbuchen, T. H. (2004), Iron charge state distributions as an indicator of hot ICMEs: Possible sources and temporal and spatial variations during solar maximum, J. Geophys. Res., 109(A1), A01112, 10.1029/2003JA009954.
  * Li et al. (2014) Li, Y., Luhmann, J. G., Lynch, B. J., and Kilpua, E. K. J. (2014), Magnetic clouds and origins in STEREO era,  Journal of Geophysical Research: Space Physics, 119(5), 3237–3246, 10.1002/2013JA019538.
  * Li et al. (2018) Li, Y., Luhmann, J. G., and Lynch, B. J. (2018), Magnetic Clouds: Solar Cycle Dependence, Sources, and Geomagnetic Impacts, Sol. Phys., 293, 135, 10.1007/s11207-018-1356-8.
  * Liu et al. (2009) Liu, R., Alexander, D., and Gilbert, H. R. (2009), Asymmetric Eruptive Filaments, ApJ, 691(2), 1079–1091, 10.1088/0004-637X/691/2/1079.
  * Liu et al. (2005) Liu, Y., Richardson, J. D., and Belcher, J. W. (2005), A statistical study of the properties of interplanetary coronal mass ejections from 0.3 to 5.4 AU, Planet. Space Sci., 53(1-3), 3–17, 10.1016/j.pss.2004.09.023.
  * Liu et al. (2014) Liu, Y. D., Luhmann, J. G., Kajdič, P., Kilpua, E. K. J., Lugaz, N., Nitta, N. V., Möstl, C., Lavraud, B., Bale, S. D., Farrugia, C. J., and Galvin, A. B. (2014), Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections,  Nature Communications, 5, 3481, 10.1038/ncomms4481.
  * Liu et al. (2019) Liu, Y. D., Zhao, X., Hu, H., Vourlidas, A., and Zhu, B. (2019), A Comparative Study of 2017 July and 2012 July Complex Eruptions: Are Solar Superstorms “Perfect Storms” in Nature?, ApJS, 241(2), 15, 10.3847/1538-4365/ab0649.
  * Lugaz and Farrugia (2014) Lugaz, N., and Farrugia, C. J. (2014), A new class of complex ejecta resulting from the interaction of two CMEs and its expected geoeffectiveness, Geophys. Res. Lett., 41(3), 769–776, 10.1002/2013GL058789.
  * Lugaz et al. (2013) Lugaz, N., Farrugia, C. J., Manchester, I., W. B., and Schwadron, N. (2013), The Interaction of Two Coronal Mass Ejections: Influence of Relative Orientation, ApJ, 778(1), 20, 10.1088/0004-637X/778/1/20.
  * Lugaz et al. (2017a) Lugaz, N., Temmer, M., Wang, Y., and Farrugia, C. J. (2017a), The Interaction of Successive Coronal Mass Ejections: A Review, Sol. Phys., 292(4), 64, 10.1007/s11207-017-1091-6.
  * Lugaz et al. (2017b) Lugaz, N., Farrugia, C. J., Winslow, R. M., Small, C. R., Manion, T., and Savani, N. P. (2017b), Importance of CME Radial Expansion on the Ability of Slow CMEs to Drive Shocks, ApJ, 848(2), 75, 10.3847/1538-4357/aa8ef9.
  * Lugaz et al. (2020) Lugaz, N., Winslow, R. M., and Farrugia, C. J. (2020), Evolution of a Long-Duration Coronal Mass Ejection and Its Sheath Region Between Mercury and Earth on 9-14 July 2013,  Journal of Geophysical Research: Space Physics, 125(1), e2019JA027213, 10.1029/2019JA027213.
  * Luhmann et al. (2020) Luhmann, J. G., Gopalswamy, N., Jian, L. K., and Lugaz, N. (2020), ICME Evolution in the Inner Heliosphere, Sol. Phys., 295(4), 61, 10.1007/s11207-020-01624-0.
  * Lynch et al. (2003) Lynch, B. J., Zurbuchen, T. H., Fisk, L. A., and Antiochos, S. K. (2003), Internal structure of magnetic clouds: Plasma and composition, J. Geophys. Res., 108, 1239, 10.1029/2002JA009591.
  * Lynch et al. (2009) Lynch, B. J., Antiochos, S. K., Li, Y., Luhmann, J. G., and DeVore, C. R. (2009), Rotation of Coronal Mass Ejections during Eruption, ApJ, 697, 1918–1927, 10.1088/0004-637X/697/2/1918.
  * Lynch et al. (2010) Lynch, B. J., Li, Y., Thernisien, A. F. R., Robbrecht, E., Fisher, G. H., Luhmann, J. G., and Vourlidas, A. (2010), Sun to 1 AU propagation and evolution of a slow streamer-blowout coronal mass ejection, J. Geophys. Res., 115, A07106, 10.1029/2009JA015099.
  * Manchester et al. (2017) Manchester, W., Kilpua, E. K. J., Liu, Y. D., Lugaz, N., Riley, P., Török, T., and Vršnak, B. (2017), The Physical Processes of CME/ICME Evolution, Space Sci. Rev., 10.1007/s11214-017-0394-0.
  * Manchester et al. (2004) Manchester, W. B., Gombosi, T. I., Roussev, I., Ridley, A., de Zeeuw, D. L., Sokolov, I. V., Powell, K. G., and Tóth, G. (2004), Modeling a space weather event from the Sun to the Earth: CME generation and interplanetary propagation, J. Geophys. Res., 109(A2), A02107, 10.1029/2003JA010150.
  * Martin (1998) Martin, S. F. (1998), Filament Chirality: A Link Between Fine-Scale and Global Patterns (Review), in IAU Colloq. 167: New Perspectives on Solar Prominences,  Astronomical Society of the Pacific Conference Series, vol. 150, edited by D. F. Webb, B. Schmieder, and D. M. Rust, p. 419.
  * Martin (2003) Martin, S. F. (2003), Signs of helicity in solar prominences and related features,  Advances in Space Research, 32, 1883–1893, 10.1016/S0273-1177(03)90622-3.
  * Marubashi and Lepping (2007) Marubashi, K., and Lepping, R. P. (2007), Long-duration magnetic clouds: a comparison of analyses using torus- and cylinder-shaped flux rope models,  Annales Geophysicae, 25(11), 2453–2477, 10.5194/angeo-25-2453-2007.
  * Marubashi et al. (2015) Marubashi, K., Akiyama, S., Yashiro, S., Gopalswamy, N., Cho, K.-S., and Park, Y.-D. (2015), Geometrical Relationship Between Interplanetary Flux Ropes and Their Solar Sources, Sol. Phys., 290, 1371–1397, 10.1007/s11207-015-0681-4.
  * Matson et al. (2002) Matson, D. L., Spilker, L. J., and Lebreton, J.-P. (2002), The Cassini/Huygens Mission to the Saturnian System, Space Sci. Rev., 104(1), 1–58, 10.1023/A:1023609211620.
  * Möstl and Davies (2013) Möstl, C., and Davies, J. A. (2013), Speeds and Arrival Times of Solar Transients Approximated by Self-similar Expanding Circular Fronts, Sol. Phys., 285, 411–423, 10.1007/s11207-012-9978-8.
  * Möstl et al. (2008) Möstl, C., Miklenic, C., Farrugia, C. J., Temmer, M., Veronig, A., Galvin, A. B., Vršnak, B., and Biernat, H. K. (2008), Two-spacecraft reconstruction of a magnetic cloud and comparison to its solar source,  Annales Geophysicae, 26, 3139–3152, 10.5194/angeo-26-3139-2008.
  * Möstl et al. (2009a) Möstl, C., Farrugia, C. J., Miklenic, C., Temmer, M., Galvin, A. B., Luhmann, J. G., Kilpua, E. K. J., Leitner, M., Nieves-Chinchilla, T., Veronig, A., and Biernat, H. K. (2009a), Multispacecraft recovery of a magnetic cloud and its origin from magnetic reconnection on the Sun, J. Geophys. Res., 114, A04102, 10.1029/2008JA013657.
  * Möstl et al. (2009b) Möstl, C., Farrugia, C. J., Temmer, M., Miklenic, C., Veronig, A. M., Galvin, A. B., Leitner, M., and Biernat, H. K. (2009b), Linking Remote Imagery of a Coronal Mass Ejection to Its In Situ Signatures at 1 AU, ApJ, 705(2), L180–L185, 10.1088/0004-637X/705/2/L180.
  * Möstl et al. (2015) Möstl, C., Rollett, T., Frahm, R. A., Liu, Y. D., Long, D. M., Colaninno, R. C., Reiss, M. A., Temmer, M., Farrugia, C. J., Posner, A., Dumbović, M., Janvier, M., Démoulin, P., Boakes, P., Devos, A., Kraaikamp, E., Mays, M. L., and Vršnak, B. (2015), Strong coronal channelling and interplanetary evolution of a solar storm up to Earth and Mars,  Nature Communications, 6, 7135, 10.1038/ncomms8135.
  * Möstl et al. (2017) Möstl, C., Isavnin, A., Boakes, P. D., Kilpua, E. K. J., Davies, J. A., Harrison, R. A., Barnes, D., Krupar, V., Eastwood, J. P., Good, S. W., Forsyth, R. J., Bothmer, V., Reiss, M. A., Amerstorfer, T., Winslow, R. M., Anderson, B. J., Philpott, L. C., Rodriguez, L., Rouillard, A. P., Gallagher, P., Nieves-Chinchilla, T., and Zhang, T. L. (2017), Modeling observations of solar coronal mass ejections with heliospheric imagers verified with the Heliophysics System Observatory,  Space Weather, 15, 955–970, 10.1002/2017SW001614.
  * Müller et al. (2017) Müller, D., Nicula, B., Felix, S., Verstringe, F., Bourgoignie, B., Csillaghy, A., Berghmans, D., Jiggens, P., García-Ortiz, J. P., Ireland, J., Zahniy, S., and Fleck, B. (2017), JHelioviewer. Time-dependent 3D visualisation of solar and heliospheric data, A&A, 606, A10, 10.1051/0004-6361/201730893.
  * Mulligan et al. (1998) Mulligan, T., Russell, C. T., and Luhmann, J. G. (1998), Solar cycle evolution of the structure of magnetic clouds in the inner heliosphere, Geophys. Res. Lett., 25, 2959–2962, 10.1029/98GL01302.
  * Nieves-Chinchilla and Viñas (2008) Nieves-Chinchilla, T., and Viñas, A. F. (2008), Solar wind electron distribution functions inside magnetic clouds, J. Geophys. Res., 113(A2), A02105, 10.1029/2007JA012703.
  * Nieves-Chinchilla et al. (2005) Nieves-Chinchilla, T., Hidalgo, M. A., and Sequeiros, J. (2005), Magnetic Clouds Observed at 1 Au During the Period 2000-2003, Sol. Phys., 232(1-2), 105–126, 10.1007/s11207-005-1593-5.
  * Nieves-Chinchilla et al. (2011) Nieves-Chinchilla, T., Gómez-Herrero, R., Viñas, A. F., Malandraki, O., Dresing, N., Hidalgo, M. A., Opitz, A., Sauvaud, J. A., Lavraud, B., and Davila, J. M. (2011), Analysis and study of the in situ observation of the June 1st 2008 CME by STEREO,  Journal of Atmospheric and Solar-Terrestrial Physics, 73(11-12), 1348–1360, 10.1016/j.jastp.2010.09.026.
  * Nieves-Chinchilla et al. (2012) Nieves-Chinchilla, T., Colaninno, R., Vourlidas, A., Szabo, A., Lepping, R. P., Boardsen, S. A., Anderson, B. J., and Korth, H. (2012), Remote and in situ observations of an unusual Earth-directed coronal mass ejection from multiple viewpoints, J. Geophys. Res., 117(A6), A06106, 10.1029/2011JA017243.
  * Nieves-Chinchilla et al. (2018a) Nieves-Chinchilla, T., Vourlidas, A., Raymond, J. C., Linton, M. G., Al-haddad, N., Savani, N. P., Szabo, A., and Hidalgo, M. A. (2018a), Understanding the Internal Magnetic Field Configurations of ICMEs Using More than 20 Years of Wind Observations, Sol. Phys., 293, 25, 10.1007/s11207-018-1247-z.
  * Nieves-Chinchilla et al. (2018b) Nieves-Chinchilla, T., Linton, M. G., Hidalgo, M. A., and Vourlidas, A. (2018b), Elliptic-cylindrical Analytical Flux Rope Model for Magnetic Clouds, ApJ, 861(2), 139, 10.3847/1538-4357/aac951.
  * Nieves-Chinchilla et al. (2019) Nieves-Chinchilla, T., Jian, L. K., Balmaceda, L., Vourlidas, A., dos Santos, L. F. G., and Szabo, A. (2019), Unraveling the Internal Magnetic Field Structure of the Earth-directed Interplanetary Coronal Mass Ejections During 1995 - 2015, Sol. Phys., 294(7), 89, 10.1007/s11207-019-1477-8.
  * Nieves-Chinchilla et al. (2020) Nieves-Chinchilla, T., Szabo, A., Korreck, K. E., Alzate, N., Balmaceda, L. A., Lavraud, B., Paulson, K., Narock, A. A., Wallace, S., Jian, L. K., Luhmann, J. G., Morgan, H., Higginson, A., Arge, C. N., Bale, S. D., Case, A. W., Wit, T. D. d., Giacalone, J., Goetz, K., Harvey, P. R., Jones-Melosky, S. I., Kasper, J. C., Larson, D. E., Livi, R., McComas, D. J., MacDowall, R. J., Malaspina, D. M., Pulupa, M., Raouafi, N. E., Schwadron, N., Stevens, M. L., and Whittlesey, P. L. (2020), Analysis of the Internal Structure of the Streamer Blowout Observed by the Parker Solar Probe During the First Solar Encounter, ApJS, 246(2), 63, 10.3847/1538-4365/ab61f5.
  * Odstrcil (2003) Odstrcil, D. (2003), Modeling 3-D solar wind structure,  Advances in Space Research, 32, 497–506, 10.1016/S0273-1177(03)00332-6.
  * Odstrcil et al. (2004) Odstrcil, D., Riley, P., and Zhao, X. P. (2004), Numerical simulation of the 12 May 1997 interplanetary CME event, J. Geophys. Res., 109(A2), A02116, 10.1029/2003JA010135.
  * Ogilvie and Desch (1997) Ogilvie, K. W., and Desch, M. D. (1997), The wind spacecraft and its early scientific results,  Advances in Space Research, 20, 559–568, 10.1016/S0273-1177(97)00439-0.
  * Ogilvie et al. (1995) Ogilvie, K. W., Chornay, D. J., Fritzenreiter, R. J., Hunsaker, F., Keller, J., Lobell, J., Miller, G., Scudder, J. D., Sittler, E. C., Jr., Torbert, R. B., Bodet, D., Needell, G., Lazarus, A. J., Steinberg, J. T., Tappan, J. H., Mavretic, A., and Gergin, E. (1995), SWE, A Comprehensive Plasma Instrument for the Wind Spacecraft, Space Sci. Rev., 71, 55–77, 10.1007/BF00751326.
  * Owens (2008) Owens, M. J. (2008), Combining remote and in situ observations of coronal mass ejections to better constrain magnetic cloud reconstruction, J. Geophys. Res., 113(A12), A12102, 10.1029/2008JA013589.
  * Owens (2018) Owens, M. J. (2018), Solar Wind and Heavy Ion Properties of Interplanetary Coronal Mass Ejections, Sol. Phys., 293(8), 122, 10.1007/s11207-018-1343-0.
  * Owens et al. (2017) Owens, M. J., Lockwood, M., and Barnard, L. A. (2017), Coronal mass ejections are not coherent magnetohydrodynamic structures,  Scientific Reports, 7, 4152, 10.1038/s41598-017-04546-3.
  * Palmaerts et al. (2018) Palmaerts, B., Radioti, A., Grodent, D., Yao, Z. H., Bradley, T. J., Roussos, E., Lamy, L., Bunce, E. J., Cowley, S. W. H., Krupp, N., Kurth, W. S., Gérard, J. C., and Pryor, W. R. (2018), Auroral Storm and Polar Arcs at Saturn—Final Cassini/UVIS Auroral Observations, Geophys. Res. Lett., 45(14), 6832–6842, 10.1029/2018GL078094.
  * Palmerio et al. (2017) Palmerio, E., Kilpua, E. K. J., James, A. W., Green, L. M., Pomoell, J., Isavnin, A., and Valori, G. (2017), Determining the Intrinsic CME Flux Rope Type Using Remote-sensing Solar Disk Observations, Sol. Phys., 292(2), 39, 10.1007/s11207-017-1063-x.
  * Palmerio et al. (2018) Palmerio, E., Kilpua, E. K. J., Möstl, C., Bothmer, V., James, A. W., Green, L. M., Isavnin, A., Davies, J. A., and Harrison, R. A. (2018), Coronal Magnetic Structure of Earthbound CMEs and In Situ Comparison,  Space Weather, 16(5), 442–460, 10.1002/2017SW001767.
  * Palmerio et al. (2019) Palmerio, E., Scolini, C., Barnes, D., Magdalenić, J., West, M. J., Zhukov, A. N., Rodriguez, L., Mierla, M., Good, S. W., Morosan, D. E., Kilpua, E. K. J., Pomoell, J., and Poedts, S. (2019), Multipoint Study of Successive Coronal Mass Ejections Driving Moderate Disturbances at 1 au, ApJ, 878(1), 37, 10.3847/1538-4357/ab1850.
  * Palmerio et al. (2021) Palmerio, E., Kilpua, E. K. J., Witasse, O., Barnes, D., Sánchez-Cano, B., Weiss, A. J., Nieves-Chinchilla, T., Möstl, C., Jian, L. K., Mierla, M., Zhukov, A. N., Guo, J., Rodriguez, L., Lowrance, P. J., Isavnin, A., Turc, L., Futaana, Y., and Holmström, M. (2021), CME Magnetic Structure and IMF Preconditioning Affecting SEP Transport,  Space Weather, 19(4), e2020SW002654, 10.1029/2020SW002654.
  * Panasenco and Martin (2008) Panasenco, O., and Martin, S. F. (2008), Topological Analyses of Symmetric Eruptive Prominences, in Subsurface and Atmospheric Influences on Solar Activity, Astronomical Society of the Pacific Conference Series, vol. 383, edited by R. Howe, R. W. Komm, K. S. Balasubramaniam, and G. J. D. Petrie, p. 243.
  * Papaioannou et al. (2020) Papaioannou, A., Belov, A., Abunina, M., Eroshenko, E., Abunin, A., Anastasiadis, A., Patsourakos, S., and Mavromichalaki, H. (2020), Interplanetary Coronal Mass Ejections as the Driver of Non-recurrent Forbush Decreases, ApJ, 890(2), 101, 10.3847/1538-4357/ab6bd1.
  * Patsourakos et al. (2010a) Patsourakos, S., Vourlidas, A., and Kliem, B. (2010a), Toward understanding the early stages of an impulsively accelerated coronal mass ejection. SECCHI observations, A&A, 522, A100, 10.1051/0004-6361/200913599.
  * Patsourakos et al. (2010b) Patsourakos, S., Vourlidas, A., and Stenborg, G. (2010b), The Genesis of an Impulsive Coronal Mass Ejection Observed at Ultra-high Cadence by AIA on SDO, ApJ, 724(2), L188–L193, 10.1088/2041-8205/724/2/L188.
  * Pesnell et al. (2012) Pesnell, W. D., Thompson, B. J., and Chamberlin, P. C. (2012), The Solar Dynamics Observatory (SDO), Sol. Phys., 275, 3–15, 10.1007/s11207-011-9841-3.
  * Prangé et al. (2004) Prangé, R., Pallier, L., Hansen, K. C., Howard, R., Vourlidas, A., Courtin, R., and Parkinson, C. (2004), An interplanetary shock traced by planetary auroral storms from the Sun to Saturn, Nature, 432(7013), 78–81, 10.1038/nature02986.