# Formation of Isolated Radio Type II Bursts at Low Frequencies

Silja Pohjolainen  Nasrin Talebpour Sheshvan  Tuorla Observatory, Department
of Physics and Astronomy, University of Turku, Turku, Finland  Space Research
Laboratory, Department of Physics and Astronomy, University of Turku, Turku,
Finland

###### keywords:

Corona, Radio Emission; Coronal mass ejections, Interplanetary; Radio bursts,
Type II, Dynamic Spectrum

##  1 Introduction

Introduction

The first discovery of solar radio type II bursts dates back to 1947, by
Payne-Scott, Yabsley, and Bolton (1947). Type II bursts appear as continuous
and slowly drifting bursts, from high to low frequencies, in the radio dynamic
spectra (Wild and McCready, 1950). The frequency drift was then found to
display the decreasing electron density in the solar atmosphere, and the
discovery of harmonics was taken as evidence that there was a common source
producing oscillations at a fundamental plasma frequency and its second
harmonic, see for example the historic review by Pick and Vilmer (2008). The
disturbance producing type II emission was later identified as a MHD shock
(Uchida, 1960).

Type II bursts can be categorized into metric (m), decameter hectometric (DH),
and kilometric (km) bursts, based on the wavelengths they are observed. DH
waves cover the frequency range of 30 MHz-300 kHz, but as radio emission below
$\sim$25 MHz is blocked by the Earth atmosphere the whole wavelength range
cannot be observed with ground-based instruments. The existing space
instruments have instrumental upper limits at 16 MHz (STEREO) and 14 MHz
(Wind), and so it is customary to define DH and interplanetary (IP) type II
bursts as those that occur at frequencies below these space instrument upper
limits.

Previous studies have shown that DH and km type II bursts are produced by
coronal mass ejection (CME) driven shocks (Sheeley et al., 1985; Cane,
Sheeley, and Howard, 1987; Gopalswamy et al., 2001). CMEs are large-scale
plasma structures ejected from the solar chromosphere and corona, and these
transients propagate out to the IP space. In shock theory, a shock can form as
a bow shock at the leading front of the CME, as a driven shock near the CME
front (even well-separated from the driver after launch), or as a driven
(expansion) shock near the CME flanks, see for example the reviews by Warmuth
(2007) and Vrsnak and Cliver (2008).

A bow shock is formed when the speed of a coronal transient exceeds the local
magnetosonic speed, which in the solar corona is well represented by the
Alfvén speed. Modelling the magnetic field structure of an active region
produced a local Alfvén speed minimum at a distance of 1.2 - 1.8 solar radii
R⊙, and a maximum of 740 km s-1 at a distance of 3.8 R⊙ (Mann et al., 2003).
The model by Warmuth and Mann (2005) found a local speed minimum at the
coronal base, 300 km s-1, and a local maximum at a distance of 3.5 R⊙, 1100 km
s-1. For example Evans et al. (2008) have compared the Alfvén speed values
obtained for different types of regions (open field regions, streamers, active
regions) with the different models and techniques. Due to the high
magnetosonic speed, most metric type II bursts do not continue into the outer
corona. And as the magnetosonic speed decreases at distances larger than 3.5 -
3.8 R⊙, DH type bursts are more likely to form there. This distance
corresponds to a frequency of 8-12 MHz in most coronal density models, see for
example the comparison between heights and frequencies in Pohjolainen et al.
(2007).

The characteristics of CMEs associated with DH type II bursts have been well-
studied and reported (Gopalswamy, 2000; Gopalswamy et al., 2001, 2002; Sharma
et al., 2008; Pappa Kalaivani et al., 2010), and also the differences between
radio-loud and radio-quiet CMEs have been investigated (Gopalswamy et al.,
2010). Basically, CMEs associated with DH type II bursts have been found to be
faster and wider. In addition, Vasanth and Umapathy (2013) found that in the
DH type II burst-associated events, CME properties like speed, width, and
shock speed were higher in limb events than in disk events, suggesting that
projection effects play a part in the measurements.

Also DH type II bursts have been studied statistically, by their start and end
frequencies, their duration, burst source location, and emission bandwidth.
Sharma and Mittal (2017) concluded that the average duration of bursts that
started between 16 and 1 MHz was considerably less than those that started
below 1 MHz. In their analysis of type II burst end frequencies, Vasanth et
al. (2015) concluded that the lower the emission end frequency, the higher the
possibility of having a strong geomagnetic storm near Earth. Shanmugaraju et
al. (2018) divided the type II bursts into groups by their end frequency: (A)
higher than 1 MHz, (B) intermediate, between 1 MHz and 100 kHz, and (C) lower
than 100 kHz. Their analysis indicated that there was a correlation between
the observed CME height and the estimated type II burst height for groups B
and C, but no correlation for group A. They suggested that the type II shock
could occur at the flanks of the CME in group A, and at the CME leading front
in groups B and C. The statistical results obtained by Aguilar-Rodriguez et
al. (2005) showed that the average bandwidth-to-frequency ratio was $\sim$0.3
in the DH type II bursts observed with the Wind/WAVES radio instruments,
irrespective of their spectral domain. The ratio distribution, however,
covered a large interval of 0.05 - 0.8, indicating that in individual bursts
the emission bandwidth can be quite different.

The DH type II burst appearances have also been used to classify events.
Pohjolainen, Allawi, and Valtonen (2013) selected wide-band and diffuse IP
type II bursts, to find out where they originated. Most of the analysed bursts
(and shocks) could be considered to be due to plasma emission, formed at or
just above the CME leading edge. However, for some radio events the source was
unclear and could have been due to other mechanisms, like Bastian (2007) had
earlier suggested.

DH type II bursts are typically observed in the radio dynamic spectra mixed
with other strong emissions like type III bursts (fast electron beams), but
they can also appear quite suddenly and without other close-in-time radio
features. Some of these well-separated, isolated DH type II bursts can be
easily explained by their source location: If the flare-CME launch happens on
the far side of the Sun, radio emission would be observed only when the radio
emission source gets high enough in the solar atmosphere, where the emission
is no longer blocked by the solar disk or by the dense solar atmosphere, i.e.,
where the emission frequency exceeds the local plasma frequency. Observation
of accelerated electron beams would then depend on the direction of the
magnetic field lines. For type II bursts, propagation direction and wideness
of the shock front could also play a part.

Our study presents analysis of selected DH type II radio bursts that were
observed well-separated from other radio emission features. With statistical
analysis of the burst, flare, and CME characteristics we try to determine the
reasons for them to appear isolated in the radio dynamic spectra, and discuss
their appearances and source origins. Some of the most interesting events are
analysed in more detail.

##  2 Data Analysis

We have selected radio type II bursts that appear to start at a low frequency,
at decameter-hectometer (DH) wavelengths, and which can be identified as a
separate structure from any other emission feature visible in the radio
spectrum at that time. The type II bursts were searched using the Wind/Plasma
and Radio Waves (WAVES) Type II lists at https://solar-
radio.gsfc.nasa.gov/wind/data˙products.html and the Type II list at
https://cdaw.gsfc.nasa.gov/CME˙list/radio/waves˙type2.html, including the
years 1998-2016.

Coronagraph images and CME leading front heights were obtained from the LASCO
CME Catalog, maintained at the CDAW Data Center. This catalogue contains data
from both the Solar and Heliospheric Observatory (SOHO) and Solar Terrestrial
Relations Observatory (STEREO) missions, with white-light coronagraph images
and EUV images at several different wavelengths. Solar flare data were
obtained from the NOAA’s Satellite and Information Service (NESDIS) website.
Information on radio type II bursts at meter wavelengths were obtained from
the NOAA radio spectral listings (up to year 2011), and from individual
spectral plots from different radio observatories (mainly from the Radio Solar
Telescope Network, RSTN, at https://www.ngdc.noaa.gov/stp/space-weather/solar-
data/solar-features/solar-radio/rstn-spectral/).

For the type II burst height estimates we used the ’hybrid’ atmospheric
density model by Vrsnak, Magdalenic, and Zlobec (2004). As the plasma
frequency $f_{p}$ depends only on the plasma density $n_{e}$ ($f_{p}$ = 9000
$\sqrt{n_{e}}$, when $f$ is in Hz and $n_{e}$ is in cm-3), the type II
emission frequency can be converted to source height if we know how the
density changes in the corona and interplanetary space, see for example
Pohjolainen et al. (2007). Only fundamental emission lanes were used for the
calculations. As at DH wavelengths fundamental emission should be stronger
than the harmonic (Lengyel-Frey, Stone, and Bougeret, 1985), we assumed
fundamental emission in events that showed only one emission lane.

###  2.1 Selection Criteria and Visual Appearances

Our selection criteria for the DH type II bursts was that the emission lane
(or lanes, if both fundamental and harmonic lanes were visible) must appear
below 16 or 14 MHz (instrumental upper limits), must be separated from any
other radio emission feature, and must show a frequency drift in order to
verify it really is a type II burst. With these criteria we excluded events
with separate multiple emission lanes (fundamental-harmonic bursts accepted
but mixed multiple bursts excluded), and also excluded lanes where the start
of the type II burst was superposed or mixed with any other type of burst
activity. For example intense type III bursts usually make it impossible to
identify the actual type II start time and frequency. For data analysis
purposes we also demanded that adequate CME data was available for each event.
We found 26 such events from the years 1998-2016, see Table table1 in the
Appendix.

Figure 1.: Examples of isolated type II bursts at DH wavelengths. a) A short-
duration type II burst with one single lane appears at 10:25 UT at 10 MHz
start frequency on 20 April 1998. b) A short-duration type II burst with
fundamental (F) and harmonic (H) emission lane pair at 5 and 10 MHz appears at
20:57 UT on 4 July 2013. The spectrum also shows earlier type III bursts
(fast-drift bursts that are formed by propagating electron beams). c) A long-
duration type II burst with one single lane appears at 16:22 UT with a low-
frequency start at 700 kHz on 1 July 2015. d) A strong type II burst with F
and H emission lane pair appears at 4 and 8 MHz at 10:00 UT on 25 October
2000. examples

A visual inspection of the spectral events showed different appearances. Some
of the bursts were very short (duration 5-8 min) and some were long-duration
events that were visible for hours. Some showed narrow-band emission
($\Delta$f/f $\leq$ 0.1) and some were very wide-band ($\Delta$f/f $>$ 0.3).
Some showed only one emission lane and some showed two emission lanes, where
the second lane appeared at twice the frequency of the first. One lane events
were interpreted to be due to fundamental plasma emission (F), and two-lane
events due to fundamental and harmonic (F+H) plasma emission. Examples of the
burst appearances are shown in Figure examples. The type II burst duration,
emission band, and F and F+H appearance are listed in Tables table1 and
table2, together with other burst characteristics and associated features. The
radio spectra of all events are shown in the Appendix, in Figures
spectra19980420 - spectra20160815.

###  2.2 Correlation Analysis

The DH type II bursts were associated with 9 GOES M-class flares, 7 C-class
flares, 3 eruptive prominences (EP), and 7 had their origin behind the limb,
on the far side of the Sun, so that flare classification for them was not
available.

All the DH type II bursts were associated with a close-in-time CME. CME speeds
were determined at the time of the DH type II burst appearance, using the
second order fits to the CME leading front heights listed in the LASCO CME
Catalog. The CME speeds ranged from 480 to 1800 km s-1. Of the CMEs, 9 were
accelerating, 14 were decelerating, and 3 had a constant speed.

Figure 2.: Flare/CME source longitude on the solar disk compared with the
projected CME speed (top left), the CME height at the time of the type II
burst start (top right), the type II burst start frequency (bottom left), and
the type II burst duration(bottom right). Behind-the-limb events are given as
-100 (behind the eastern limb) and +100 (behind the western limb). F and F+H
lane bursts are plotted with different symbols. longitude

In Figure longitude we compare the source longitude (i.e., location of the
active region from where the eruption originated) with projected CME speed,
CME height at the type II start time, type II start frequency, and type II
burst duration. Figure longitude shows that near the disk center, inside
E30-W42 longitudes there are no isolated DH type II bursts. Within the
longitudes E50-E30 and W42-W50 all the isolated type II bursts are single (F)
lane events.

The highest CME speeds are observed when the flare/CME source longitudes are
$\gtrsim$ 90 degrees. It may be that the observed CME speeds for the on-the-
disk events suffer from projection effects and look to be less than the real
speeds, as was suggested by Vasanth and Umapathy (2013). Comparison between
the CME height at the time of the type II burst start and the source longitude
gives a similar result as the comparison to the CME speed.

The type II start frequency does not seem to have a clear correlation with the
source longitude, but the highest start frequencies are in bursts that that
have their source origin near the solar limb.

The comparison between the type II emission duration and the source longitude
shows that the type II events that originate near or behind the solar limb
have the longest durations. Our sample of events shows quite an opposite
tendency compared to that of Mittal and Verma (2017), who found that the
durations of the DH type II radio bursts located at the solar disc center are
longer than the durations of the bursts located at the solar limb. However,
their sample of 426 DH type II bursts and our sample of 26 bursts are not
directly comparable, so ours could be a selection effect.

Figure 3.: Type II burst duration compared with the type II burst start
frequency (top left) and the CME height at the time of type II start (top
right). Projected CME speed at the time of type II burst start compared with
the type II burst start frequency (bottom left) and the type II burst duration
(bottom right). duration-speed

Figure duration-speed shows the type II burst start frequency compared to the
type II burst duration. This plot is in accordance with the finding of Sharma
and Mittal (2017), that bursts starting at lower frequencies have longer
durations. We also compared the projected CME leading front height at the time
of type II start with the type II burst duration, and this shows a tendency
that the higher the CME is at the type II burst start, the longer the type II
burst duration.

The DH type II burst start frequency does not look to have any clear
correlation with the projected CME speed. When the projected CME speed is
compared to the type II burst duration, we see that the highest speed CMEs do
not produce long-duration DH type II bursts.

Figure 4.: CME angular width near the time of type II start compared with the
flare/CME source longitude (left) and the type II start frequency (right).
width

We then compared the CME angular width with the source longitude and the type
II burst start frequency, in Figure width. The CME Catalog lists the
associated CMEs as full halos (10/26), partial halos (13/26), and with widths
94 - 101 degrees (3/26). We measured the CME widths near the time of the type
II emission start, which gave more narrow widths that ranged from 40 to 230
degrees. The smallest CME angular widths were in the events that occurred at
or behind the solar limb. The closer to the disk center the event originated,
the larger the angular width. This could be a simple projection effect.

A large CME angular width looks to correlate with a low type II start
frequency (i.e., high formation height). However, a narrow CME width does not
look to imply a high type II formation height, as some of the very narrow CMEs
were associated with bursts that had a very low start frequency. This may rule
out the assumption that type II burst formation requires a certain angular
width for the CME.

We also compared the projected CME speed with the GOES soft X-ray maximum
flux. Of the 26 events three were eruptive prominences without an observed
X-ray flare and seven were far side events where GOES intensity could not be
measured. The correlation plot (not shown here) suggested that the more
intense the flare flux, the higher the CME speed.

Furthermore, we checked if these events showed any metric type II emission, to
see if a propagating shock existed already at lower coronal heights. Due to
the atmospheric cut-off in ground-based radio observations (that are possible
down to 30-25 MHz) and the instrumental limit of space observations (start
frequency 16-14 MHz), type II emission lanes that start at meter waves cannot
be followed directly to DH waves, because of the spectral gap in observations.
Of the analysed events, 10 out of 26 were associated with earlier metric type
II emission, see Table table2. It is obvious that far side events and eruptive
prominences lack metric emission. An association to flare intensity and CME
speed was also noted: in our data set metric type II bursts required an
M-class GOES flare and/or a very high speed CME (1100-1800 km s-1).

Our events were also compared with the solar energetic particle (SEP) event
catalogue by Paassilta et al. (2017) that covers the years in our list. A
match was found in five events. In three events (20 April 1998, 25 October
2000, 17 July 2012) the SEP injection time, determined with the velocity
dispersion analysis (VDA) method, matched well with the time of the DH type II
burst appearance. In two events (22 August 2005, 1 July 2015) the SEP
injection time was earlier, but could be explained with an earlier metric type
II burst. Due to the small number of SEP events, no further correlation
analysis was performed. We note, however, that considering the spectral
structures of type II bursts may help to improve the SEP forecasting accuracy
(Iwai et al., 2020).

##  3 Events with Large Height Difference between the CME Leading Front and
the Type II Burst Source

Heightsep

To evaluate where our DH type II bursts could be formed, we calculated the
height difference between the CME leading front and the type II burst for each
event. These are listed in Table table1. Figure corr2 shows the height
difference versus source longitude, and the value is positive when the CME
front is located higher than the type II burst source.

Figure 5.: Flare/CME source longitude on the solar disk compared with the
height difference between the CME leading front and the type II burst source
at the time of the type II burst start (positive when the CME is higher than
the type II burst). Solid lines mark the $\pm$2 R⊙ error margin for calculated
heights (mainly from density models) and dashed lines indicate the values that
we concluded to be within acceptable height error in individual events. corr2

Errors in type II burst height estimation can be quite large, as the
calculated heights depend on the chosen atmospheric density model and on the
selected frequency within the emission lane taken for calculation (for
example, center or leading edge of the lane, affected also by the wideness of
the emission lane). In the frequency range of 10 - 1 MHz, the height
difference that comes from using different (but generally accepted) density
models is $\sim$2 R⊙, see for example the height values in Table 1 in
Pohjolainen et al. (2007).

Equally, the CME leading front height can be uncertain, especially if the
front consists of a bright flux rope and a diffuse faint region ahead of it.
The diffuse region has often been identified as the CME-driven shock front,
see Schmidt et al. (2016a) and references therein. Usually, the shock is more
visible toward the flank regions of the CME, and the shock’s stand off
distance from the CME’s leading edge is not more than 1-2 R⊙ in this height
range (Schmidt et al., 2016b), and often much less (Lee et al., 2017).

Of our 26 events, 16 could be considered to be a match between the type II
burst height and the CME leading front height, within the possible $\pm$2 R⊙
error. Similar heights suggest that the type II burst was most probably
created by a CME bow shock.

Of the remaining ten events, five had an intermediate height difference of
2.6-3.4 R⊙. These events showed relatively wide emission lanes. Wide-band
emission is tricky in a sense that the frequency selected for height
calculation (lower border, lane center, or some other point inside the lane)
will have a large effect on the height. The spectral plots of these events
show how the frequency drift of the type II burst follows that of the CME
quite well, but due to the wideness of the emission lane the heights
calculated from the lane center do not always match with the CME heights.
Hence, some other frequency point within the emission lane could still give a
good match. We also noted that in many of these events the type II height at
the start of the burst was different from the CME leading front height, but
later on the the heights became similar. Based on our analysis, we conclude
that also the five type II bursts with intermediate height differences can
reasonably well be associated with the CME leading fronts.

The height difference between the CME leading front and the type II shock was
observed to be very large, 5.2-7.2 R⊙, in five events. On 27 March 2012, the
type II burst source was located 5.2 R⊙ higher than the leading front of the
CME. In the other four events, the type II burst sources were located 5.9–7.2
R⊙ lower than leading front of the CME. We now examine the possible reasons
for these large height differences.

###  3.1 Event on 27 March 2012

event27032012

On 27 March 2012 the DH type II burst source was located 5.2 R⊙ higher than
the leading front of the main CME, which was propagating toward the west at
speed 700 km s-1 at the time when the type II burst appeared (dashed line
marked CME WEST in Figure newspectra20120327, here the CME heights have been
converted to frequencies). The CME heights were also verified from the
STEREO-A/COR1 observations, and they were not higher than what was observed by
SOHO/LASCO. The active region (AR) was most probably NOAA AR 11440, which had
rotated to $\sim$W100, as it was last observed from Earth on 26 March at
location S25W91.

Figure 6.: Event on 27 March 2012, source origin at SW90b (on the far side).
The type II burst source is located 5.2 R⊙ higher than the leading front of
the main CME (CME WEST, dashed white line). There are earlier faint and narrow
CMEs that propagate toward west (CME WEST, dash-dotted white line) and toward
east (CME EAST, dashed white line). STEREO-A dynamic spectrum shows how radio
type III emission lanes cross the features that were interpreted as type II
emission. Some parts of these type III bursts are visible in the Wind/WAVES
and STEREO-B/WAVES spectrum, but only at frequencies below 1 MHz. The
positions of both STEREO spacecraft on 27 March 2012 are also shown.
Wind/WAVES was located in L1, along the Sun-Earth line. newspectra20120327

On closer inspection, there were two earlier-launched faint and narrow CMEs
which were catalogued as very poor events, and did not show up in the
coronagraph images at the time of the type II burst appearance. The first one
propagated toward the west (dash-dotted line marked CME WEST, the heights have
been extrapolated onward from the last observed height of 4.5 R⊙ at 19:00 UT),
and the second one toward the east (marked CME EAST) in Figure
newspectra20120327. The heights of CME EAST are near the heights of the type
II burst harmonic emission. However, as STEREO-B/WAVES did not record a type
II burst at all, and only one type III burst was visible below 700 kHz, it
does not look probable that the CME EAST could have created the burst recorded
by Wind/WAVES and STEREO-A/WAVES.

STEREO-A/WAVES dynamic spectrum shows that the radio enhancements, which we
interpreted as type II F+H lanes, were located along type III burst emission
lanes that were not visible in the Wind/WAVES spectrum. The radio dynamic
spectrum at 180-25 MHz from RSTN Sagamore Hill (Earth view, same as for Wind)
does not show any metric type III or type II radio emission within the time
range.

We therefore conclude that the association of the type II burst to the
westward propagating fast (main) CME is questionable. It is possible that the
type II-like structures were not a type II burst, but flare-accelerated
electrons propagating through the dense (earlier) CME structures, i.e., they
were type III burst enhancements when the beams travelled through local
turbulent plasma. Fundamental-harmonic structures can occur also in type III
bursts, as a significant number of type III bursts at meter and decameter
waves have been found to show harmonics, see for example Robinson and Cairns
(1998) and references therein.

###  3.2 Event on 11 December 2001

event11122001

The DH type II burst on 11 December 2001 showed relatively weak emission but
had a clear start at 12:45 UT, with fundamental and harmonic emission lanes
(Figure newspectra20011211). After 14:00 UT there is uncertainty if and how
the lane continues, narrow or wide, as we see enhanced emission at a much
wider frequency range. However, some of these enhancements could be due to
type III bursts.

Figure 7.: Event on 11 December 2001, source location at SW90b (on the far
side). The leading front of the main CME is located 5.9 R⊙ higher than the
type II burst source. However, there is a later CME propagating westward, in
the wake of the main CME. The LASCO C2 difference image (top left) shows the
northern edge of the main CME and the leading front of the later CME. The CME
propagation heights, converted to frequencies, are indicated with the dashed
white lines in the radio dynamic spectrum (top right). The height-time plot
(bottom) shows the height evolution of these two CME fronts (crosses and
stars) and the calculated type II burst heights from the fundamental emission
lane (triangles). newspectra20011211

The main CME was located 5.9 R⊙ higher than the type II burst source at 12:45
UT, but a second bright structure had appeared inside the main CME at 11:30
UT. It propagated to the same westward direction, at speed 500 km s-1. The
height evolution of the main CME and the later, inner CME structure are shown
in Figure newspectra20011211, indicated with white dashed lines in the dynamic
spectrum. The type II burst shows a curved structure, and a similar change is
observed in the height evolution of the later CME structure (height-time plot
in Figure newspectra20011211). The height difference between this later CME
front and the type II burst is $\sim$2 R⊙, which falls inside the error margin
in height estimation.

We therefore conclude that the DH type II burst was created by the later CME
structure, and that the curved type II structures were due to density changes
along the CME propagation path, caused by the earlier (main) CME.

Figure 8.: On 22 December 2002, the burst source origin is at N23W42. The
leading front of the CME (dashed white line, bottom left) is located 6.7 R⊙
higher than the type II burst source. On 31 December 2004, the burst source
origin is at N04E46. The leading front of the CME is located 7.2 R⊙ higher
than the type II burst source (bottom right). The DH type III bursts along the
CME propagation paths were ’cut’,i.e., their emission did not cross the whole
spectral range. Both events showed also metric type II burst activity at 140 -
25 MHz, observed by the RSTN Learmonth station. lear-wind

###  3.3 Events on 22 December 2002 and 31 December 2004

events22122002and31122004

The DH type II burst events on 22 December 2002 and 31 December 2004 look very
similar (Figure lear-wind). Both were on-the-disk events that showed EUV
dimmings. The associated GOES flares were class M1.1 and M4.2, and the
flare/CME source locations were N23W42 and N04E46, respectively. The CME speed
at the type II start was 1100 km s-1 in both events. The durations of the type
II emission were 30 and 29 minutes, the type II formation heights were 4.5 and
4.8 R⊙, and the respective CME heights were 11.2 and 12.0 R⊙. The height
differences were 6.7 and 7.2 R⊙, respectively.

In both events there was strong type II burst activity at metric wavelengths
(Figure lear-wind, top panels), starting near 140 MHz and continuing down to
25 MHz (observational limit for ground-based observations, RSTN Learmonth). In
both cases the metric type II emission was band-split: On 22 December 2002 at
start the fundamental frequency was split to 34 and 39 MHz and the harmonic to
68 and 78 MHz, and on 30 December 2004 split lanes can be identified to start
at 80 and 100 MHz. The 30 December 2004 spectrum is more complicated, for
example at 22:25 UT a separate lane is observed to start at 30 MHz but due to
the observational limit it cannot be followed below 25 MHz. We calculated the
radio source heights using the lower split-lanes.

Figure 9.: Height-time plots for the 22 December 2002 and the 30-31 December
2004 events. The heights of the CME leading fronts (crosses) can be compared
with the heights of the metric and DH type II bursts
(triangles).events2002and2004heights

The height-time plots in Figure events2002and2004heights show that the metric
type II burst heights are in line with the CME heights, but the later DH type
II burst sources are at much lower heights. The coronagraph images in Figures
spectra20021222 and spectra20041231 in the Appendix show streamers near the
sides of the CMEs, and it is possible that the type II bursts were caused by
shocks near the CME flanks, that were due to CME-streamer interaction at low
heights.

Type III bursts were observed within the full frequency range in both events
near the CME launch times, but later on the type III bursts showed ’cuts’,
i.e., the bursts were visible only at frequencies below the CME propagation
heights, below $\sim$1 MHz. This suggests that either the type III electron
beams were accelerated near the CME fronts or the CMEs blocked their
visibility. Unfortunately these events occurred in the pre-STEREO era, so we
cannot verify visibility toward other directions.

###  3.4 Event on 1 February 2005

event01022005

In the 1 February 2005 event the CME originated from behind the east limb, and
no X-ray flare was observed in the Earth view. The short, only five minute
duration DH type II burst showed both fundamental and harmonic emission
(Figure newspectra20050201). The leading front of the CME was faint and
diffuse, and denser structures were located below it, at lower heights.

Figure 10.: Event on 1 February 2005, source location at NE90b (on the far
side). The leading front of the CME (white dashed line in the radio spectrum)
is located 6.5 R⊙ higher than the type II burst source. A lower, sideways
propagating front is also visible near the south-east streamer in the LASCO C2
image (left). newspectra20050201

The CORIMP map (Figure spectra20050201 in the Appendix) also shows a lower
front in the eastern flank of the CME (yellow lines). This lower front made a
sudden south-westward movement in between the LASCO images at 11:30 UT and
11:54 UT, which matches with the short lifetime of the type II burst at
11:45-11:50 UT. It is most probable that the DH type II burst was a short-
duration shock, created by the fast sideways movement within the CME.

##  4 Observations from Different Viewing Angles

stereocomp

In 12 cases the solar event could be observed from at least two different
viewing angles, with the instruments onboard Wind, STEREO-A, and STEREO-B
satellites. These events, with the DH type II burst start times and
frequencies, are listed in Table table3 in the Appendix. In five events the
radio emission start time and start frequency were different when observed
from a different viewing angle. The start time delays were 5 min, 7 min, 8
min, 26 min, and 83 min, in the order of time delay. In one event the start
time differed 2 min, but as the start frequency was the same in both, we
concluded that there was no real delay in the type II start. We now describe
the delayed events in more detail.

5 July 2012. The 5 min delay in the DH type II start looks to be due to band-
splitting of the fundamental emission lane. The lower band-split lane start is
observed at the same time with both Wind and STEREO-A instruments at 22:57 UT
at $\sim$1.4 MHz, but STEREO-A records emission from the upper band-split lane
earlier, at 22:52 UT at 2.2 MHz. The STEREO-A/WAVES dynamic spectrum also
shows emission patches that can be interpreted as harmonic emission. Therefore
the difference between the observations is most probably due to sensitivity,
that Wind/WAVES did not record the less intense emission from the upper split
band. Moreover, a frequency drift from 2.2 MHz to 1.4 MHz within 5 min would
require a burst driver speed of 3000 km s-1, which also suggests that the
emission structures did not belong to the same emission lane.

22 October 2012. The type II burst became visible 26 minutes earlier in the
STEREO-B spectrum, compared to Wind and STEREO-A. By STEREO-B the burst
started as a narrow lane at 01:24 UT at 1.4 MHz, but became much wider after
01:45 UT. The type II burst emission is very faint in the STEREO-A spectrum,
but the start time is the same as in the Wind spectrum, at 01:50 UT. The
beginning of the DH type II burst in the Wind spectrum is visible only
partially below 1 MHz (Figure spectra20121022 in the Appendix), and therefore
we conclude that this could also be a sensitivity issue between the Wind/WAVES
RAD1 and RAD2 receivers.

Figure 11.: Comparison between STEREO-B, Wind, and STEREO-A observations on 4
July 2013. Longer arrows point to the first fundamental-harmonic structures
observed by Wind and shorter arrows to the later emission structures observed
also by STEREO-B. Type III bursts are labelled a-d, to compare their
visibility with the different instruments. The burst source region was located
at S14E62. Coronagraph and EUV difference images from the three different
viewing angles are also shown, with the PFSS magnetic field extrapolation map
(Earth view). Cartoon insert shows the positions of the three satellites, Wind
in Earth view. comp20130704

4 July 2013. The time delay in the type II burst start was 8 min, as Wind
observed the burst at 20:57 UT at 5 MHz and STEREO-B at 21:05 UT at 4 MHz
(Figure comp20130704). There were several type III bursts that preceded the
type II burst, the closest-in-time are listed as a, b, c in the spectral
plots. We note that the bursts b and c have a lower start frequency in the
STEREO-B/WAVES spectrum, 8 MHz and 2.5 MHz, compared to the Wind/WAVES
spectrum where the bursts are already visible at 14 MHz. This suggests that
the emission of the type III bursts and the type II burst were occulted at low
coronal heights in the STEREO-B view.

Figure 12.: SOHO/LASCO C2-SDO/AIA composite image (left) of the CME and its
source region at N20W85 on 2 October 2013. The STEREO-A/COR1 coronagraph image
(right) shows the same structure. The CME leading front is marked with a
dashed line in both images. The LASCO C2 image shows a streamer that crosses
the CME (in projection), but it is uncertain weather the streamer is located
in front or behind of the CME bubble. comp20131002

2 October 2013. STEREO-A observed the type II burst starting at 20:39 UT at 12
MHz and Wind later at 20:46 UT at 10 MHz. With the hybrid atmospheric density
model a frequency drift from 12 to 10 MHz in 7 min would indicate a burst
driver speed of $\sim$460 km s-1. The CME speed at that time was $\sim$600 km
s-1, so the type II burst speed would be quite realistic.

We find there is a streamer visible in the SOHO/LASCO image, located across
the (projected) CME loop (Figure comp20131002). If the streamer location was
such that it lay in between Wind and the type II shock, then STEREO-A would
have been able to observe the type II directly, and Wind only through the
streamer. Modeled streamer densities (Decraemer, Zhukov, and Van Doorsselaere,
2019) are very near to the type II density values at these heights.

It is therefore possible that the DH type II burst on 2 October 2013 became
visible only when the type II burst emission frequency got above the plasma
frequency of the streamer, when the streamer density and the critical plasma
frequency decreased with height.

Figure 13.:  Wind-SOHO and STEREO-B observations on 2 April 2014. The active
region associated with this event was located behind the north-eastern solar
limb. Arrows point to the later CME structure (unlisted as a CME) that
propagated inside the main CME. The fragments of the main CME are visible in
the LASCO C3 difference image, north-east of the solar disk. STEREO-A
observations are available only after 20:39 UT, and the recorded emission is
similar to the STEREO-B observations. Wind/WAVES recorded the start of the
type II burst 83 minutes before STEREO-B/WAVES. comp20140402

2 April 2014. This event showed the longest gap in the starting time of the
type II burst, 83 min, as Wind recorded the burst start at 18:49 UT at 2.4 MHz
and STEREO-B at 20:12 UT at 1.4 MHz. The type II burst was formed near a time
when a new bright front appeared inside the main CME. The height difference
between this new CME front and the type II burst was large only near the
beginning of the radio burst (Figure spectra20140402, Appendix), as after
19:40 UT the heights were a good match.

The curved structures in the Wind/WAVES spectrum, near the burst start,
resemble those reported in Pohjolainen, Pomoell, and Vainio (2008). They found
that the emission fragmentation was due to a shock passing through regions
with highly variable densities, inside and outside high-density CME loops.
Earlier launched CME loops on the shock propagation path would therefore be a
good explanation for the emission curvature.

The type II burst was observed by STEREO-B only after 20:12 UT, i.e., the
curved structures were not observed (Figure comp20140402). STEREO-B was
located 164 degrees back from Earth orbit, almost directly on the opposite
side of the Sun, so it should have had a good view to this new bright CME
front. On the other hand, the earlier CME fragments were propagating toward
north-east, i.e., toward STEREO-B, so occultation was possible.

##  5 Coronal Conditions for Type II Burst Formation

conditions

A large fraction of interplanetary shocks has been found to be radio quiet
(Gopalswamy et al., 2010), and so even the appearance of a DH type II burst
requires some specific coronal conditions. The simplest scenario would be that
a shock is formed when the speed of a coronal transient exceeds the local
magnetosonic speed, i.e., Alfvén speed in the solar corona, and this leads to
type II radio emission at some location along the shock front.

Figure 14.: Type II burst formation height compared with the projected CME
speed. The dotted line shows the local Alfvén speed, following the velocity
curve presented in Kim et al. (2012). F and F+H lane bursts are plotted with
different symbols and the three prominence eruption events are encircled.
alfvenspeed

Figure alfvenspeed shows the calculated DH type II burst formation heights
compared with the projected CME speeds. The dotted line in the plot shows how
the Alfvén speed rises with distance from the Sun above an active region, and
reaches a local maximum of $\sim$800 km s-1 near $\sim$4 R⊙ as in Kim et al.
(2012). Even a higher value for the Alfvén speed, 1100 km s-1 at 3.5 R⊙ as in
Warmuth and Mann (2005), would not change the distribution of our events in
most cases (below or above the Alfvén speed).

In six events the CME speed looks to be less than the assumed Alfvén speed.
All six type II bursts were estimated to be due to CME bow shocks, as the type
II heights matched well with the CME leading front heights, within the
reasonable height calculation error. The height difference was 0.1 - 0.5 R⊙ in
the four events that were well below the Alfvén speed peak value, and 1.0 and
1.2 R⊙ in the two events that were closer to the Alfvén speed value (the CME
speeds in these two events were 700 and 720 km s-1). The only F lane event
among the six was a prominence eruption.

The four F+H lane type II bursts that had their CME speeds well below the
modelled Alfvén speeds, had their source origin in similar locations on the
solar disk, at longitudes E62, W63, W65, and W85. The CME speeds can be
affected by projection effects in on-the-disk events, but a CME speed
difference of 100 - 250 km s-1 to the Alfvén speed cannot be explained with
projection effects only. As these events were determined to be bow shock
events, we conclude that the Alfvén speed had to be much lower in these
coronal regions. This would be possible, for example, above some active
regions and streamer regions (Evans et al., 2008). But, for example Su et al.
(2016) suggested that the generation of shocks, and type II bursts, may
require larger values of compression ratio and Alfvén Mach number, rather than
simply a higher speed of the disturbance.

Already Roberts (1959) reported to have observed fundamental and harmonic
emission in 60% of all type II bursts. The intensity of the fundamental
emission band has been observed to increase relative to the harmonic, as the
burst evolves with heliocentric distance (Lengyel-Frey and Stone, 1989). The
fundamental and harmonic frequencies are assumed to be products of three-wave
interaction processes of beam-excited Langmuir waves. Langmuir waves would
first have to scatter off sound waves at a sufficient rate to create a
suitable population of counter-propagating Langmuir waves, and if this process
was too slow, harmonic emission would not be able to attain intensities
comparable to the fundamental plasma emission process (Ganse et al., 2012a).

Our correlation analysis showed that harmonic emission was present only in
type II bursts that originated at longitudes larger than $\pm$60 degrees. Our
small sample of events, where six events originated inside the longitude range
of E60 - W60 and showed one emission band, may contain selection effects. The
overall ratio, 14 events with F band and 12 events with F+H band looks to be
in agreement with previous studies.

##  6 Results and Discussion

We have analysed 26 well-separated, isolated DH type II radio bursts and
compared their characteristics with active region, flare and CME data.
Association to metric type II emission and SEP (proton) events were also
listed. First, we looked at the type II burst appearances and found bursts
that showed a single emission lane (14 events, emission at the fundamental
plasma frequency), and bursts with two lanes (12 events, fundamental and
harmonic emission).

From the 26 events 8 had their source origin on the far side of the Sun, and 7
were limb events (longitudes 85 - 90 degrees east or west). Of the remaining
11 events, 6 were located near the limb and 5 were located nearer to the disk
center. No isolated DH type II bursts were observed inside the longitude range
E30 - W40. At longitudes E30-50 and W40-50 all the isolated type II bursts
were single lane bursts (no harmonic emission).

The fastest CMEs associated with the DH type II bursts were observed in events
that originated at or behind the solar limb. This is understandable, as height
measurements of on-the-disk CMEs may suffer from projection effects. A similar
result was obtained by Vasanth and Umapathy (2013). The associated CMEs had
variable widths, in the range of 40 - 230 degrees near the time of the DH type
II burst start. The narrowest CMEs were in events that occurred at or behind
the solar limb. The closer to the disk center the event originated, the larger
the CME angular width. The largest angular widths were associated with low-
frequency start of emission (i.e., high formation height of the type II
burst), although low-frequency onset of emission occurred also in events where
the CME had very narrow width.

Of the 26 events, 16 were found to have a good match between the DH type II
burst height and the CME leading front height, within the possible $\pm$2 R⊙
error. In other five events we could also make a match to the CME leading
front if the whole radio emission lane and later times along the type II lane
were considered for the height calculation. These type II bursts could be due
to CME bow shocks, located near the CME nose. We note that in several cases
the radio burst height was closer to the bright front structure of the CME,
and not to the diffuse region above it (if it existed) that is often taken as
the CME height. Shock regions and CME structures are discussed, for example,
in Kwon and Vourlidas (2018), Song et al. (2019), and Mei et al. (2020), and
references therein.

Only five events showed a large height difference between the DH type II shock
and the CME leading front, and these events were analysed in detail. For the
DH type II burst that was located much higher than the CME leading front we
found a possible explanation from two earlier CMEs that had comparable heights
with the type II burst. This event was observed also with STEREO-A/WAVES, and
the spectral features there suggest that the type II-like bursts could also
have been radio enhancements along type III burst lanes, formed by electron
beams passing through the earlier-launched CME material.

For the four DH type II bursts that showed heights much lower than the CME
leading front we found the following explanations. The two DH type II bursts
with source origins at longitudes W42 and E46 and emission only at the
fundamental frequency, a CME flank shock looks to be the most probable
explanation. In one event a later-launched CME structure, inside the main CME,
was visible and the curvature of the type II lane matched with the height
evolution of the later CME. And for one event, a fast sideways movement of a
lower CME structure, creating a short-duration shock, may have been the cause
for the type II burst.

Our sample of isolated DH type bursts where most bursts look to be due to
shocks near the CME leading front, contradicts earlier findings. DH type II
bursts, using for example radio triangulation method, have in most studies
been found to be located near the CME flank regions, and only in few cases
have been located close to the CME nose, see Jebaraj et al. (2020) and
references therein. A statistical study of 153 interplanetary type II radio
bursts observed by the two STEREO spacecraft in 2008-2014 (Krupar et al.,
2019) also suggested that interplanetary type II bursts are more likely to
have a source region situated closer to the CME flanks than the CME leading
edge. The radio bursts located near the CME flanks have also been noted to be
enhanced by shock-streamer interaction (Shen et al., 2013).

The results of Kahler, Ling, and Gopalswamy (2019) suggested that fast and
narrow CMEs can create mostly bow shocks, and fast and wide CMEs predominately
expansion shocks. Also Vourlidas and Ontiveros (2009) have suggested that the
bow shock morphology is associated almost exclusively with narrow CMEs.
Kahler, Ling, and Gopalswamy (2019) further concluded that no CMEs with widths
less than 60∘ are associated with metric type II bursts. Our study confirms
this finding, but we point out that CMEs with widths less than 60∘ can still
be associated with DH type II bursts.

Of our 26 DH type II burst events, 10 were associated with earlier metric type
II bursts. All 10 metric bursts died out before reaching the DH instrumental
limit of 16/14 MHz, and the later-appearing DH bursts were isolated. We note
that the metric type II bursts were formed only with the fastest CMEs, with a
CME velocity range of 1100 - 1800 km s-1. It has been suggested earlier that
metric type II radio bursts are formed by a distinct coronal shock and only
produce radio emission in the low corona, see, e.g., Reiner et al. (2000) and
Magara et al. (2000), but opposite arguments have also been made, see
Gopalswamy et al. (1999) and references therein.

In three of our events the start time of the DH type II burst matched well
with the solar energetic particle (SEP) release time for protons in the 55-80
MeV range. In two SEP-associated events earlier metric type II emission was
observed, but 21 of the isolated DH type II bursts were not associated with
listed proton events.

Comparison between the Wind and STEREO observations, available for 12 events,
showed that the DH type II burst start was not always observed toward all
directions. There was a delay in the starting times in five events. In two
events only Wind recorded the DH type II burst, and both were eruptive
prominences with source longitude at E30. Of the five events that showed time
delays, two seemed to suffer from sensitivity issues in the radio spectral
data. Less intense emission near the DH type II start could therefore go
unnoticed. In the other three events, occultation by a dense streamer or
earlier-launched CME fragments looks possible. Radio emission from the DH type
II burst would become visible only when the type II burst frequency exceeds
the plasma frequency of the occulting structure, i.e., when the density of the
occulting structure is less than in the type II shock region.

In Section conditions we discussed the required conditions in the corona for
the development of a shock wave. It is well known that not all coronal shocks
produce DH type II bursts. In most of our analysed events, the CME speed
looked to be well above the local Alfvén speed, and therefore type II emission
would be possible. Some of the DH type II bursts had very short duration,
which indicates that either the emission ended or it became unobservable
(invisible). In general, the DH type II burst duration was found to depend on
the type II burst start frequency, i.e., the lower the start frequency the
longer the duration.

Type II bursts have a quite narrow emission band (narrow range of densities),
and therefore the emission region cannot be spread out along a large area of
the (curved) shock front. The small-scale type II emission region is thus
expected to have special properties, setting it apart from the rest of the
shock (Ganse et al., 2012b). A specific local region would also provide the
conditions for the enhancement of emission near the plasma frequency harmonics
(Annenkov, Volchok, and Timofeev, 2020).

##  7 Conclusions

We conclude that isolated DH type II bursts can be narrow-band or wide-band,
can have short or long duration, and can show only fundamental or both
fundamental and harmonic emission. The bursts typically have their source
origin some distance away from the central meridian, at longitudes 30-90
degrees east or west, but only some of the bursts originate from the far side
of the Sun.

Almost all of the analysed 26 isolated DH type II bursts could be associated
with a shock near the leading front of the main CME. In only two events a
shock near the CME flanks was estimated to be the cause of the DH type II
burst, and in three events the radio bursts could be explained by earlier or
later CME structures. Our analysis suggests that for the majority of isolated
DH type II bursts a CME bow shock is more probable than a CME flank shock.

We also analyzed the radio spectral data from different viewing angles in 12
events, provided by the STEREO and Wind spacecraft observations. We compared
the spectral features of the DH type II bursts, and their timing. In five
events a time delay in the DH type II start was evident, and only one of the
five was a far side event. In three events occultation by dense matter between
the source and the observer could have been the cause for the delay in the
emission onset, and in two events spectral sensitivity could have affected the
observed starting time.

Why do some DH type II bursts appear isolated, without other radio emission
signatures, and why do many of them disappear from the spectrum as suddenly as
they appear? Our findings suggest that isolated DH type II bursts could be a
special subgroup within DH type II bursts, where the radio emission requires
particular coronal conditions to form, and the bursts can die out rapidly if
these conditions change.

#### Acknowledgments

The authors want to thank all the individuals who have contributed in creating
and updating the various solar event catalogues. The authors also acknowledge
the useful comments and suggestions from the anonymous referee, on how to
improve the paper. The CME catalogue is generated and maintained at the CDAW
Data Center by NASA and the Catholic University of America in cooperation with
the Naval Research Laboratory. The Wind/WAVES radio type II burst catalog was
initially prepared by Michael L. Kaiser and it is maintained at the Goddard
Space Flight Center. SOHO is a project of international cooperation between
ESA and NASA. STEREO is part of the NASA Solar Terrestrial Probes (STP)
Program.

Disclosure of Potential Conflicts of Interest The authors declare that they
have no conflicts of interest.

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##  Appendix A Event images

For each event we show a coronagraph-EUV difference image (SOHO/LASCO -
SOHO/EIT), a Wind/WAVES radio dynamic spectrum that shows the DH type II
burst, and a CORIMP map or a coronagraph image (LASCO or STEREO) that shows
the streamer locations. In the radio dynamic spectra the white vertical lines
indicate the time of the coronagraph image, and white arrows point to the type
II burst structures (one arrow for a single F lane event and two arrows
labelled F and H if a harmonic lane is visible). The white dashed lines in the
radio spectra indicate CME heights converted to frequencies, with the hybrid
plasma density model by Vrsnak, Magdalenic, and Zlobec (2004). The LASCO C2-C3
image from the CORIMP catalog (or the coronagraph image) is from a time close
to the type II start or during the type II burst. In the CORIMP images the
yellow lines detail the detected CME structure, and the red points correspond
to the outer features that are used for tracking the CME heights (see details
in the CORIMP catalog at http://alshamess.ifa.hawaii.edu/CORIMP/). EP stands
for eruptive prominence.

Figure 15.: 20 April 1998, source location S22W90.spectra19980420

Figure 16.: 25 October 2000, source location N09W63.spectra20001025

Figure 17.: 3 September 2001, source location S23E90.spectra20010903

Figure 18.: 11 December 2001, source location SW90b. The DH type II burst was
estimated to be located $\sim$6 R⊙ lower than the CME leading front, and this
event is analysed in more detail in Section event11122001.spectra20011211

Figure 19.: 22 March 2002, source location S09W90.spectra20020322

Figure 20.: 9 July 2002, source location W90b.spectra20020709

Figure 21.: 22 December 2002, source location N23W42. The DH type II burst
source was estimated to be located $\sim$7 R⊙ lower than the CME leading
front, and this event is analysed in more detail in Section
events22122002and31122004.spectra20021222

Figure 22.: 29 July 2004, source location N00W90.spectra20040729

Figure 23.: 31 December 2004, source location N04E46. The DH type II burst
source was estimated to be located $\sim$7 R⊙ lower then the CME leading
front, and this event is analysed in more detail in Section
events22122002and31122004.spectra20041231

Figure 24.: 1 February 2005, source location NE90b. The DH type II burst was
estimated to be located $\sim$6 R⊙ lower than the CME leading front, and this
event is analysed in more detail in Section event01022005.spectra20050201

Figure 25.: 22 August 2005, source location S11W54.spectra20050822

Figure 26.: 18 August 2010, source location N18W88.spectra20100818

Figure 27.: 27 March 2012, source location SW90b. The leading front of the CME
was located $\sim$5 R⊙ lower than the DH type II burst source, and this event
is analysed in more detail in Section event27032012.spectra20120327

Figure 28.: 5 July 2012, source location S12W46. This event is discussed in
more detail in Section stereocomp (delay in DH type II burst start, STEREO
observations). spectra20120705

Figure 29.: 17 July 2012, source location S28W65.spectra20120717

Figure 30.: 22 October 2012, source location S10E76. This event is discussed
in more detail in Section stereocomp (delay in DH type II burst start, STEREO
observations). spectra20121022

Figure 31.: 16 January 2013, source location S33W64.spectra20130116

Figure 32.: 4 July 2013, source location S14E62. This event is discussed in
more detail in Section stereocomp (delay in DH type II burst start, STEREO
observations). spectra20130704

Figure 33.: 2 October 2013, source location N20W85. This event is discussed in
more detail in Section stereocomp (delay in DH type II burst start, STEREO
observations). spectra20131002

Figure 34.: 18 February 2014, source location SE30 (EP).spectra20140218

Figure 35.: 25 March 2014, source location S23W90b (EP).spectra20140325

Figure 36.: 2 April 2014, source location NE90b. The unlisted CME front was
propagating inside an earlier CME. This later front propagation is indicated
with the dashed line in the dynamic spectrum. The earlier, listed CME front
was last observed at height 28 R⊙ at 16:54 UT, which corresponds to $\sim$0.2
MHz frequency. This event is discussed in more detail in Section stereocomp
(delay in DH type II burst start, STEREO observations). spectra20140402

Figure 37.: 30 July 2014, source location N10E30 (EP).spectra20140730

Figure 38.: 6 March 2015, source location S20E87. The CME was propagating in
the wake of an earlier CME, and both CMEs are listed in the CME
Catalog.spectra20150306

Figure 39.: 1 July 2015, source location W90b.spectra20150701

Figure 40.: 15 August 2016, source location E90b.spectra20160815 Table 1.: Selected DH type II bursts, a total of 26 events. Flare/prominence location, CME type (H-halo, PH-partial halo, - not halo), and comparison between the type II height and the simultaneous CME height ($\Delta$h) are listed.table1 Date | Type | Dur. | F- | F- | CME |  $\Delta$h | Flare/ | CME
---|---|---|---|---|---|---|---|---
| II |  | lane | height | height | CME-F | prom. | type
| start |  |  |  |  |  | loc. |
yyyy-mm-dd | UT | min. | MHz | R⊙ | R⊙ | R⊙ |  |
1998-04-20 | 10:25 | 10 | 10 | 3.2 | 5.0 | 1.8 | S22W90 | PH
2000-10-25 | 10:00 | 120+ | 4 | 5.0 | 4.5 | -0.5 | N09W63 | H
2001-09-03 | 18:48 | 12 | 12 | 3.0 | 4.1 | 1.1 | S23E90 | PH
2001-12-11 | 12:45 | 120 | 2 | 7.1 | 13.0 | 5.9 | SW90b | PH
2002-03-22 | 11:30 | 70 | 4 | 5.0 | 7.0 | 2.0 | S09W90 | H
2002-07-09 | 19:46 | 54 | 1 | 10.4 | 7.3 | -3.1 | W90b | H
2002-12-22 | 04:20 | 30 | 5 | 4.5 | 11.2 | 6.7 | N23W42 | PH
2004-07-29 | 13:30 | 300+ | 0.7 | 12.6 | 10.0 | -2.6 | N00W90 | H
2004-12-31 | 00:19 | 29 | 4.2 | 4.8 | 12.0 | 7.2 | N04E46 | H
2005-02-01 | 11:45 | 5 | 4 | 5.0 | 11.5 | 6.5 | NE90b | H
2005-08-22 | 02:00 | 95 | 3 | 5.7 | 7.0 | 1.3 | S11W54 | H
2010-08-18 | 06:01 | 11 | 7 | 3.8 | 4.8 | 1.0 | N18W88 | PH
2012-03-27 | 22:10 | 20 | 1.5 | 8.2 | 3.0 | -5.2 | SW90b | -
2012-07-05 | 22:57 | 60 | 1.4 | 8.3 | 9.0 | 0.7 | S12W46 | -
2012-07-17 | 14:40 | 8 | 6 | 4.1 | 4.0 | -0.1 | S28W65 | PH
2012-10-22 | 01:50 | 500+ | 1 | 10.4 | 13.3 | 2.9 | S10E76 | PH
2013-01-16 | 22:00 | 180 | 0.6 | 14.0 | 13.5 | -0.5 | S33W64 | PH
2013-07-04 | 20:57 | 18 | 5 | 4.5 | 4.9 | 0.4 | S14E62 | PH
2013-10-02 | 20:46 | 24 | 10 | 3.2 | 2.8 | -0.4 | N20W85 | PH
2014-02-18 | 02:16 | 35 | 2 | 7.1 | 6.2 | -0.9 | SE30 | H
2014-03-25 | 07:52 | 83 | 1.7 | 7.7 | 9.6 | 1.9 | S23W90b | PH
2014-04-02 | 18:49 | 230+ | 2.4 | 6.4 | 3.8 | -2.6 | NE90b | H
2014-07-30 | 07:44 | 16 | 6.3 | 4.0 | 5.2 | 1.2 | N10E30 | PH
2015-03-06 | 08:10 | 140 | 2.0 | 7.1 | 7.2 | 0.1 | S20E87 | PH
2015-07-01 | 16:22 | 260 | 0.7 | 12.6 | 16.0 | 3.4 | W90b | H
2016-08-15 | 18:21 | 7 | 5.0 | 4.5 | 5.5 | 1.0 | E90b | -

Table 2.: Selected DH type II bursts and associated events.table2

Date | AR | GOES | CME | CME | CME | Metric | DH | DH
---|---|---|---|---|---|---|---|---
|  | class | first | speed1 | width2 | type | band | lane
|  |  | obs. |  |  | II |  |
yyyy-mm-dd |  |  | UT | km s-1 | deg |  |  $\Delta$f/f |
1998-04-20 | 8194 | M1.4 | 10:07 | 1700 ac. | 120 | yes | 0.04 | F
2000-10-25 | 9199 | C4.0 | 08:26 | 650 ac. | 120 | - | 0.30 | F+H
2001-09-03 | 9607 | M2.5 | 18:35 | 1500 de. | 70 | yes | 0.06 | F
2001-12-11 | - | - | 09:543 | 900 ac. | 120 | - | 0.25 | F+H
2002-03-22 | 9866 | M1.6 | 11:06 | 1800 de. | 110 | yes | 0.12 | F+H
2002-07-09 | - | - | 19:31 | 900 de. | 200 | - | 0.20 | F
2002-12-22 | 10223 | M1.1 | 03:30 | 1100 de. | 120 | yes | 0.12 | F
2004-07-29 | 10652 | C2.1 | 12:06 | 1100 ac. | 100 | - | 0.36 | F
2004-12-31 | 10715 | M4.2 | 22:30 | 1100 co. | 130 | yes | 0.07 | F
2005-02-01 | - | - | 11:06 | 1500 de. | 100 | yes | 0.10 | F+H
2005-08-22 | 10798 | M2.6 | 01:31 | 1250 de. | 180 | yes | 0.10 | F
2010-08-18 | 11099 | C4.5 | 05:48 | 1550 de. | 90 | yes | 0.14 | F+H
2012-03-27 | - | - | 22:00 | 700 co. | 45 | - | 0.20 | F+H
2012-07-05 | 11515 | M1.6 | 22:00 | 1150 de. | 60 | yes | 0.21 | F
2012-07-17 | 11520 | C9.9 | 13:48 | 600 ac. | 80 | - | 0.18 | F+H
2012-10-22 | 11598 | M1.3 | 20:57 | 480 ac. | 180 | - | 0.20 | F
2013-01-16 | 11650 | C2.2 | 19:00 | 650 de. | 180 | - | 0.33 | F+H
2013-07-04 | 11787 | C6.8 | 20:12 | 550 de. | 85 | - | 0.10 | F+H
2013-10-02 | 11850 | C1.2 | 20:36 | 580 ac. | 70 | - | 0.05 | F+H
2014-02-18 | EP | - | 01:25 | 850 de. | 230 | - | 0.10 | F
2014-03-25 | EP | - | 05:36 | 650 ac. | 80 | - | 0.09 | F
2014-04-02 | - | - | 13:364 | 870 de. | 50 | - | 0.12 | F
2014-07-30 | EP? | - | 07:00 | 700 co. | 100 | - | 0.05 | F
2015-03-06 | 12297 | M1.5 | 07:125 | 760 ac. | 120 | - | 0.30 | F+H
2015-07-01 | - | - | 14:36 | 1400 de. | 200 | yes | 0.21 | F
2016-08-15 | - | - | 17:24 | 720 de. | 40 | - | 0.10 | F+H

1CME speed at the time of DH type II burst start, from second order fit, with
acceleration/deceleration/constant speed information (CDAW CME Catalog).
2CME angular width at the time of type II burst start.
3Later CME front observed at 11:30 UT, inside the listed CME, speed 500 km s-1
and accelerating.
4Long duration X-ray flare, bright propagating CME front appears at 18:36
within the listed CME, height and speed are for the new feature.
5Propagating in the wake of an earlier CME.

Table 3.: DH type II bursts observed with Wind and STEREO (total of 12 events).table3 Date | STEREO-B | Wind | STEREO-A | Source | Comment
---|---|---|---|---|---
| UT / MHz | UT / MHz | UT / MHz |  |
2010-08-18 | - | 06:01 / 7.0 | 06:01 / 7.0 | N18W88 | No delay
2012-03-27 | - | 22:06 / 1.5 | 22:06 / 1.5 | SW90b | No delay
2012-07-05 | - | 22:57 / 1.4 | 22:52 / 2.2 | S12W46 | A first, 5 m
2012-07-17 | - | 14:40 / 6.0 | 14:40 / 6.0 | S28W65 | No delay
2012-10-22 | 01:24 / 1.4 | 01:50 / 1.0 | 01:50 / 0.9 | S10E76 | B first, 26 m
2013-07-04 | 21:05 / 4.0 | 20:57 / 5.0 | - | S14E62 |  Wind first, 8 m
2013-10-02 | - | 20:46 / 10.0 | 20:39 / 12.0 | N20W85 | A first, 7 m
2014-02-18 | - | 02:16 / 2.0 | - | SE30 |  Wind only, EP
2014-03-25 | 07:54 / 1.7 | 07:52 / 1.7 | 07:54 / 1.7 | S23W90b | No delay, EP
2014-04-02 | 20:12 / 1.4 | 18:49 / 2.4 | no observ. | NE90b |  Wind first, 83 m
2014-07-30 | - | 07:44 / 6.3 | - | N10E30 |  Wind only, EP?
2016-08-15 | no observ. | 18:21 / 5.0 | 18:21 / 5.0 | E90b | No delay