# Homologous flaring activity over a sunspot light bridge in an emerging
active region

Rohan Eugene Louis Udaipur Solar Observatory, Physical Research Laboratory,
Dewali, Badi Road, Udaipur – 313001, India  Julia K. Thalmann University of
Graz, Institute of Physics/IGAM, Universitätsplatz 5, A-8010 Graz, Austria

(Received September 02, 2020; Revised November 20, 2020; Accepted December 14,
2020)

###### Abstract

Sunspot light bridges are known to exhibit a variety of dynamic and persistent
phenomena such as surges, small-scale jets etc. in the chromosphere and
transition region. While it has generally been proposed that magnetic
reconnection is responsible for this small-scale dynamism, persistent flaring
activity lasting several hours from the same spatial location on a sunspot
light bridge, has rarely been reported. We combine observations from the
Atmospheric Imaging Assembly and the Helioseismic Magnetic Imager on board the
Solar Dynamics Observatory to investigate homologous flaring activity over a
small sunspot light bridge in an emerging flux region. The homologous flares
all produced broad, collimated jets, including a B6.4 class flare. The jets
rise at a speed of about 200 km s-1, reach projected heights of about 98 Mm,
and emerge from the same spatial location for nearly 14 hrs, after which they
cease completely. A non-linear force free extrapolation of the photospheric
magnetic field shows a low-lying flux rope connecting the light bridge to a
remote opposite-polarity network. The persistent flares occur as a result of
the rapid horizontal motion of the leading sunspot that causes the relatively
vertical magnetic fields in the adjacent umbra to reconnect with the low-lying
flux rope in the light bridge. Our results indicate that the flaring ceases
once the flux rope has lost sufficient twist through repeated reconnections.

Sunspots (1653) — Solar Magnetic Flux Emergence (2000) — Solar Flares (1496) —
Solar magnetic reconnection (1504) — Solar chromosphere (1479) — Solar corona
(1483)

††journal: ApJL

##  1 Introduction

Sunspots and pores often comprise bright structures called light bridges (LBs)
that divide the umbra into two or several, smaller umbral cores. LBs are
either manifestations of large-scale magneto-convection (Rimmele, 2008; Rouppe
van der Voort et al., 2010; Toriumi et al., 2015) or “field-free” regions of
hot plasma in the “gappy” umbral magnetic field (Parker, 1979; Choudhuri,
1986; Spruit & Scharmer, 2006). As a consequence, LBs are often seen along
fractures where sunspots coalesce or disintegrate (Garcia de La Rosa, 1987;
Schlichenmaier et al., 2010; Louis et al., 2012).

The disruption of the umbral magnetic field by the LB forces the adjacent
umbral magnetic field to form a canopy (Jurčák et al., 2006), which has been
suggested to facilitate magnetic reconnection in the form of several transient
phenomena in the chromosphere and transition region. These include surges
(Roy, 1973; Asai et al., 2001; Robustini et al., 2016; Toriumi et al., 2015),
strong arc-like or extended brightenings (Louis et al., 2008; Shimizu et al.,
2009; Louis et al., 2009), reconnection jets (Louis et al., 2014a; Louis,
2016; Bharti et al., 2017; Tian et al., 2018; Bai et al., 2019), small-scale
transient brightenings which excite chromospheric oscillations in the adjacent
umbra (Song et al., 2017a), chromospheric shocks (Song et al., 2017b; Yang et
al., 2019), and oscillatory surge-like phenomena (Zhang et al., 2017; Hou et
al., 2017).

The small-scale jets originating from LBs reported so far exhibited
characteristic plasma temperatures below 0.1 MK (Robustini et al., 2016; Tian
et al., 2018) and the general notion is that the jets occur as a result of
magnetic reconnection between the overlying umbral field with either emerging
bipoles in the LB or with the twisted flux tube comprising the LB (Shimizu et
al., 2009). However this is at odds with most spectropolarimetric observations
with exceptions as seen in Louis (2015). Louis et al. (2015a) reported the
emergence of a small-scale, flat $\Omega$-loop in a LB. However, this loop
only produced a temperature excess of about 700 K in the chromosphere with
close to non-zero velocities, when it encountered the overlying umbral field,
thus ruling out magnetic reconnection arising from the emergence. The
information provided by the photospheric magnetic field does not unambiguously
explain how, and if, magnetic reconnection in the LB can produce the
recurrent, small-scale phenomena seen in the chromosphere and transition
region.

In this article we study the homologous flaring activity in a sunspot LB
during the early emerging phase of active region (AR) NOAA 11515, which
involved the splitting of the AR’s main sunspot. These strong, large-scale,
homologous flares in a LB, which included a B6.4 flare (SOL2012-07-01T01:27),
are observed rarely and we provide explanations for this particular activity
pattern, based on coronal field modeling, for the first time here.

Figure 1: B6.4 flare in the leading sunspot of NOAA AR 11515. Panels a, b, and
c correspond to AIA 94 Å, 171 Å and 1700 Å channels, respectively. Panel d is
a color composite image made from the AIA 304 Å (red), 94 Å (green), and 171 Å
(blue) channels. The black plus symbol indicates the base of the jet on the
LB. Panel e shows the light curve extracted from the mean intensity within the
contour outlining the jet. Black and red curves correspond to AIA 304 Å and 94
Å channels, respectively. The inset shows the AIA light curve around the time
of the B6.4 flare.

Figure 2: Examples of flaring activity on the LB in the leading sunspot on
2012 July 01 seen in the AIA 304 Å images. See accompanying animation.

##  2 Observations and Analysis

###  2.1 Data

We use data products from the Helioseismic and Magnetic Imager (HMI; Schou et
al., 2012) and the Atmospheric Imaging Assembly (AIA; Lemen et al., 2012) on
board the Solar Dynamics Observatory (SDO; Pesnell et al., 2012), to study the
time evolution of NOAA AR 11515 from 2012 June 30, 20:00 UT to 2012 July 2,
00:00 UT. The HMI continuum images and Dopplergrams have a cadence of 45
seconds. The AIA data comprise images in the 1700 Å, 304 Å, 171 Å, and 94 Å
channels with a 5 min cadence. Projection effects are assumed to be negligible
as the heliocentric angle of the AR was about 33∘.

In order to study the three-dimensional (3D) coronal magnetic field in and
around AR 11515, we use hmi.sharp_cea_720s data which provides the Cylindrical
Equal Area (CEA) projected photospheric magnetic field vector within
automatically-identified AR patches (Bobra et al., 2014) with the azimuthal
component of the vector magnetic field being disambiguated (Metcalf, 1994;
Leka et al., 2009). We use the full-resolution CEA data (pixel scale $\approx$
360 km at disk center) at 2012 July 1, 01:36 UT and 17:00 UT as input to a
nonlinear force-free (NLFF) method (see Sect 2.2).

###  2.2 Modeling

In order to perform the NLFF modeling, we apply the method of Wiegelmann et
al. (2012), i.e., we combine the improved optimization scheme of Wiegelmann &
Inhester (2010) and a multi-scale approach (Wiegelmann, 2008) to the
preprocessed vector magnetic field data. The latter is achieved by applying
the preprocessing method of Wiegelmann et al. (2006) to the original vector
magnetic field data. For both, preprocessing and optimization, standard model
parameter choices, as suggested in Wiegelmann et al. (2012), have been used.
We adopted a computational domain of $544\times 336\times
432~{}\mathrm{pixel}^{3}$, with the photospheric magnetic flux on the lower
boundary (at $z=0$) being balanced to within $\sim 10\%$.

Successful NLFF modeling is expected to deliver a 3D corona-like model
magnetic field with a vanishing Lorentz force and divergence. In order to
quantify the force-free consistency we use the current-weighted angle between
the modeled magnetic field and electric current density (Schrijver et al.,
2006) and find characteristic values of $\simeq 10^{\circ}$. In order to
quantify the divergence-free consistency of our NLFF solutions we compute the
fractional flux metric as introduced by Wheatland et al. (2000) and find
values of $\simeq 1.5\times 10^{-4}$.

Figure 3: Top: Continuum image (left) and vertical magnetic field (right) on
2012 July 01 at 01:12 UT. Middle: Enlarged FOV (indicated by the black box in
panel a) of the LB in the eastern part of the leading sunspot. Panel c
corresponds to the horizontal magnetic field, shown with white arrows for
every second pixel. Panels e–g correspond to the continuum intensity, vertical
component of the magnetic field, and line-of-sight (LOS) velocity at 01:12 UT.
Panels h–j are the same as those on the left but at 17:00 UT. The magenta
contours indicate red-shifts of 550 m s-1 and the black arrows point to a
network flux patch. The thick black circle encloses the LB. The dotted black
contours outline the sunspot boundary, while the orange and black solid
contours denote the smaller and larger umbral cores, respectively.

##  3 Results

###  3.1 Homologous Flaring Activity

During its early phase of emergence and evolution, between July 01 and July
02, the leading sunspot of AR 11515 rapidly split into two, nearly equal,
halves with the front half separating from its rear twin. The M5.6
(SOL2012-07-02T10:43) and C8.3 (SOl2012-07-01T15:41) flare/CME events
associated with the splitting of the sunspot have been studied in detail by
Louis et al. (2014b) and Louis et al. (2015b), respectively. In our work, we
present a detailed analysis of the homologous flaring activity on 2012 July 01
that originated from a small LB located in the leading positive-polarity
sunspot prior to the splitting.

Figure 1a shows the coronal emission in the AR on 2012 July 01 at 01:30 UT,
when a B6.4 flare originated from a thin LB (location marked by a black cross)
that divided the main sunspot into two umbral cores. The related emission is
observed in the form of a rather broad collimated jet, in the absence of
classical flare ribbon emission (for a corresponding observation see, e.g.,
Berger & Berdyugina, 2003). The base of the jet coincides with the location of
the LB. Discrete blobs are seen in the 1700 Å image, at the base and mid-
section of the jet (black arrows). A color composite image (panel d) shows
that the central section in the lower half of the jet comprises plasma at
coronal temperatures while, the upper half exhibits chromospheric/transition
region temperatures.

Fig. 1e shows the normalized mean intensity at characteristic chromospheric
and transition region (AIA 304 Å; $\sim$0.05 MK) as well as coronal (AIA 94 Å;
$\sim$6 MK) temperatures, computed for all pixels within a contour outlining
the maximal extent of the B6.4 flare related emission. There were as many as
14 flaring events between June 30 20:00 UT and July 01 10:00 UT, that were
observationally verified, with the B6.4 flare being the strongest event. The
flare activity in the LB ceased after around 12:00 UT on July 1st.

Figure 2 shows the morphology of selected flares emanating from the LB. From
the time series of observations, an untwisting motion is recognized, with the
plasma descending along similar trajectories during the individual events. The
plasma comprising the jet moves outwards with a maximum projected speed of
about 200 km s-1 with the cool plasma being ejected to a projected height of
about 98 Mm from the base of the jet. These representative values were
estimated from the 304 Å channel, using a space-time map along a parabolic cut
outlining the ejection for Event 9 at 07:16 UT.

The homologous flaring activity described above is seen in all the AIA
channels, including the 94 Å channel. The events are also detected in the GOES
X-ray flux curve, at least for the B6.3 class flare, while the majority of
other events are weaker, B-class flares (comparing the relative intensity in
the 94 Å channel of the B6.3 flare). In comparison, the small-scale
jets/surges reported previously in LBs, are confined to the lower chromosphere
to transition region, where their temperatures do not exceed 0.1 MK, and
clearly lack the intense emission associated with the flares (Figs. 1 and 2).
Additionally, the spatial scales of the flares are far larger than those of
small-scale jets/surges which are typically ejected to 10–15 Mm. These
characteristics make the flares in the LB distinct from the recurrent jets and
surges reported previously.

###  3.2 Photospheric Magnetic Field Configuration

Figure 3 shows the photospheric structure of AR 11515 at 01:12 UT (top row).
The field strength in the umbral cores on either side of the LB is about 2 kG
while the average and minimum field strength in the LB is about 1.2 kG and 1.0
kG, respectively. The field inclination is about 60–70∘ in the LB and about
80∘ in the penumbra. However at HMI’s spatial resolution we do not see any
indication of the field changing polarity either in the LB or in the penumbral
filament extending into the LB (panel d).

The bottom panels of Fig. 3 show the leading sunspot at 2 different instances
of time on 2012 July 01, before the B6.4 flare at 01:12 UT (panels e–g) and
after the jet/flaring activity ceased at 17:00 UT (panels h–j). Evidently, by
17:00 UT, the LB has become more diffuse and the penetration of the highly
inclined field, associated with the LB, into the umbra has receded
significantly. The area and hence the magnetic flux of the smaller umbral core
east of the LB (orange contour), also reduces by 75%, from 1.5$\times 10^{20}$
Mx to 3.6$\times 10^{19}$ Mx. In comparison, the flux of the larger umbral
core only reduces by about 5%.

The LB as well as the associated penumbral sector harbor red-shifts of up to
550 m s-1 at 01:12 UT, which become considerably smaller at 17:00 UT. A
comparison of Figs. 3f and 3i show signatures of flux cancellation at the edge
of the moat, north of the main sunspot possibly due to the relative motion of
the leading sunspot in a north-westerly direction.

Figure 4:  NLFF modeling of the coronal magnetic field of AR 11515 on 2012
July 1 at $\sim$01:36 UT. Shown is a sub-domain of the original NLFF model
volume, covering the connectivity of the observed LB located within the
positive-polarity leading sunspot (see dashed outline centered around
$(x,y)=(28,21)$ Mm) and the moat area to the north-east of the AR (see dashed
outline centered around $(x,y)=(15,38)$ Mm). Field lines rooted with at least
one of their footpoints within the LB or moat area and with an inherent twist
of $\gtrsim-0.35$ turns are shown, and color-coded according to the total
current along them. The color-coded background shows the coronal emission in
the 304 Å band at 01:35:30 UT. Black (gray) contours outline the NLFF lower
boundary vertical magnetic field at 1000 (-200) G, respectively.

###  3.3 Coronal Magnetic Field Configuration

The NLFF modeling of the 3D coronal magnetic field on July 1 at 01:36 UT is
shown in Fig. 4. A system of low-lying twisted magnetic field (likely
comprising a magnetic flux rope, and so-called hereafter) connects the LB
(outlined by the dashed rectangle centered around $(x,y)=(28,21)$ Mm) to the
negative-polarity moat area (see dashed outline centered around
$(x,y)=(15,38)$ Mm and black arrow in Fig. 3f). The particular field lines
shown have been selected based on their inherent twist, with a twist of more
than $0.35$ turns (see Sect. 3.4), and which are rooted with one of their
footpoints within the LB or moat area. The field lines extending beyond the
FOV suggest the presence of bald batches, i.e., locations where the field
overlying a polarity inversion line is parallel to the photosphere (see, e.g.,
Titov et al., 1993). The apexes of the twisted field lines reach coronal
heights up to $\sim 5$ Mm (Fig. 4b).

Visual comparison to the AIA 304Å image, taken near-simultaneously (color
coded background in Fig. 4a), evidences the realistic approximation of the
solar corona by the employed NLFF modeling. We find that the direction of the
jet feature coincides spatially between the LB and moat areas and the
extension of the jet feature to the north-east of the AR is also well
recovered by the field lines associated with the bald patches.

###  3.4 Time Evolution of Magnetic Twist

We perform an analysis of the inherent magnetic twist (thus self-helicity) of
the system of twisted field lines shown in Fig. 4, at 01:36 UT and 17:00 UT.
We consider field lines that are rooted within the LB or moat area (dashed
outlines in Fig. 4) with at least one of their footpoints, and which emerge
from locations where $|B_{z}|>100$ G, at the NLFF lower boundary. Based on
these criteria, we compute 167 (104) field lines at 01:36 UT (17:00 UT).

Figure 5:  Force-free $\alpha$ and twist parameter $T_{\overline{\alpha}}$
within NOAA 11515 at (a) 01:36 UT and (b) 17:00 UT on 2012 July 01. Dashed
outlines to the north-east ($x,y=15,38$ Mm) and south-west ($x,y=28,21$ Mm)
outline the moat area and LB, respectively from which field lines were
computed. Footpoint locations of all considered field lines adhering to
positive (negative) $T_{\overline{\alpha}}$ are marked by gray triangles
(squares). The color-coded background shows the force-free parameter $\alpha$,
scaled to $\pm 1~{}\rm{Mm}^{-1}$. Black (gray) contours outline $B_{z}$ of
$1000$ $(-200)$ G. The insets show the field lines’ twist as a function of
$\alpha$ (top right corners) and as a function of the field lines’ lengths
(bottom right corners).

Subsequently, we estimate the twist for all computed field lines as
$T_{\overline{\alpha}}=\overline{\alpha}L/4\pi$, where $\overline{\alpha}$ is
the mean value of $\alpha$ at both footpoints and $L$ is the arc-length of
each considered field line (e.g., Leamon et al., 2003). We restrict ourselves
to estimate $T_{\overline{\alpha}}$ for all field lines that are rooted at
locations for which $|\overline{\alpha}|\geq 0.1$ Mm-1. We set the threshold
on $\alpha$ in order to avoid that the twist estimation is not dominated by
the many existing small-scale field lines. This reduces the number of analyzed
field lines to 74 (16) at 01:36 UT (17:00 UT). The footpoints of these
remaining field lines are indicated by gray triangles (positive twist) and
squares (negative twist) in Fig. 5. The force-free parameter
$\alpha=\mu_{0}J_{z}/B_{z}$, where $J_{z}$ is the vertical component of the
electric current density, is displayed as a color-coded background in Fig. 5.

We find that at 01:36 UT the majority of the considered field lines rooted in
the LB and moat areas, are linked to locations of negative $\alpha$, and
consequently adhere to a negative twist (see inset at top right corner of Fig.
5a). The distribution of $T_{\overline{\alpha}}$ as a function of $L$ (bottom
right inset), shows that the negative twist is characteristic for both,
shorter and longer field lines. From the considered field lines we find a
median value of $\langle T_{\overline{\alpha}}\rangle=-0.3\pm 0.1$.

At 17:00 UT the situation is clearly different. The LB appears less
pronounced, while the moat region moved towards the solar west quite
considerably. Still, most of the considered field lines are rooted in
locations of predominantly negative $\alpha$, resulting in a median value of
$\langle T_{\overline{\alpha}}\rangle=-0.2\pm 0.1$. Note that the number of
considered field lines is greatly reduced, by a factor of nearly five, as many
more of the field lines rooted in the selected sub-fields correspond to a
value of $|\overline{\alpha}|<0.1$ Mm-1, based on the criterion described
above.

These results suggest that the flux rope connecting the LB and moat area lost
a considerable amount of twist, as the number of field lines with a twist
$\gtrsim 0.1$ at 01:36 UT, reduced significantly at 17:00 UT, likely due to
the repeated reconnection events.

##  4 Discussion

The homologous flaring activity in the sunspot LB lasts for at least 14 hrs
with the strongest event being a B6.4 flare (see Fig. 2). In the present case,
only the 3D NLFF reconstruction of the coronal magnetic field reveal the
importance of temporal changes within specific photospheric areas of the AR,
such as the moat region located to the north-east of the main sunspot (see
Fig. 4). In particular, it is connected by a low-lying magnetic flux rope to
the LB, spatially associated to the observed flare events.

The chief driver for the recurrent flares originating from the LB is the
horizontal motion of the leading sunspot in a north-westerly direction.
Consequently the more vertically oriented magnetic fields in the adjacent
sunspot umbra to the east of the LB, may reconnect with the highly inclined
fields at the leg of the flux rope producing the observed collimated jet
emission (top panel of Fig. 4). During the several subsequent, homologous
reconnection events the flux rope’s twist gets liberated (Fig. 5), resulting
in a weakly twisted/sheared field arcade no longer conducive to magnetic
reconnection. Additionally, flux cancellation at the remote leg of the flux
rope in the moat area could facilitate reconnection in the rope, and the
flaring ceases as the footpoint diffuses. The reduction of photospheric
magnetic flux at/close to the regions where the eruptions ensue is generally
consistent with observations and MHD simulations (Forbes et al., 2006, and
references therein).

The LOS velocity in the LB and its surroundings is dominantly red-shifted
(Fig. 3g). If the homologous nature of the flares were the result of small-
scale loops emerging in the LB and reconnecting with the overlying umbral
field (Tian et al., 2018), one would expect blue-shifts either in localized
patches or along the body of the LB (Louis et al., 2020). With HMI’s native
spatial resolution of $\sim$320 km, we do not find evidence for opposite-
polarity fields above uncertainties within the LB, though we do not exclude
the possibility of small-scale velocity and magnetic inhomogeneities having
been present (Louis et al., 2009, 2015a; Louis, 2015).

It has been known that LBs can be associated with the large-scale magnetic
configuration of an AR. Observations by Berger & Berdyugina (2003) describe a
set of flare ribbons extending along the LB as a result of magnetic
reconnection in the corona. However, the C2.0 flare was the only event that
occurred in the AR on that day. Guo et al. (2010) reported recurrent surges in
H$\alpha$ from a LB which triggered a filament eruption and an associated M2.5
flare. However, the lack of vector magnetic field data does not account for
the persistent nature of the surges or the topological association of the
filament to the LB. The homologous flaring activity in a sunspot LB, described
in this article, is observed unambiguously in all AIA channels and is strongly
supported by our models of the coronal magnetic field, which, to the best of
our knowledge, has not been reported previously.

##  5 Conclusions

The presence of small-scale chromospheric and coronal transients such as
surges, jets, etc. in sunspot LBs has generally been attributed to magnetic
reconnection but the processes, rendering these transients homologous, are
poorly understood. Homologous flares in LBs can be considered extremely unique
and rare, as they are an irrefutable evidence of magnetic reconnection
occurring within the strong field domain of sunspots. In this article, we
describe a series of intermittent flares in a sunspot LB. The leading sunspot
exhibits a rapid horizontal motion consequently splitting into two, nearly
equal halves. Our coronal magnetic field models show a low-lying twisted flux
rope connecting the LB to the moat area of the sunspot. The proper motion of
the sunspot causes the umbral magnetic fields to reconnect with the low-lying
flux rope, producing the flares, with the collimated jets perfectly aligned
along the flux rope’s axis. The repeated reconnections render a considerable
loss of twist in the flux rope which is substantiated by our extrapolations.
The combined effect of the above renders the flux rope to return to a more
relaxed state whereby the flaring activity ceases. The uniqueness of the
observations and the coronal magnetic field models provide new insight into
the recurrent nature of flares in a LB.

SDO data are courtesy of the NASA/ SDO AIA and HMI science teams. They are
provided by the Joint Science Operations Center – Science Data Processing at
Stanford University. J. K. T. was supported by the Austrian Science Fund
(FWF): P31413-N27. We thank the referee for providing useful comments.

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