First Flare-related Rapid Change of Photospheric Magnetic Field Observed by Solar Dynamics Observatory

Abstract

Photospheric magnetic field not only plays important roles in building up
free energy and triggering solar eruptions, but also has been observed to
change rapidly and permanently responding to the coronal magnetic field
restructuring due to coronal transients. The Helioseismic and Magnetic Imager
instrument (HMI) on board the newly launched Solar Dynamics Observatory (SDO)
produces seeing-free full-disk vector magnetograms at consistently high
resolution and high cadence, which finally makes possible an unambiguous and
comprehensive study of this important back-reaction process. In this study, we
present a near disk-center, GOES-class X2.2 flare occurred at NOAA AR 11158 on
2011 February 15 using the magnetic field measurements made by HMI. We obtained
the first solid evidence of an enhancement in the transverse magnetic field at
the flaring magnetic polarity inversion line (PIL) by a magnitude of 70%. This
rapid and irreversible field evolution is unequivocally associated with the
flare occurrence, with the enhancement area located in between the two
chromospheric flare ribbons. Similar findings have been made for another two
major flare events observed by SDO. These results strongly corroborate our
previous suggestion that the photospheric magnetic field near the PIL must
become more horizontal after eruptions.
In-depth studies will follow to further link the photospheric magnetic field
changes with the dynamics of coronal mass ejections, when full Stokes inversion
is carried out to generate accurate magnetic field vectors.

Full text

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arXiv:1103.0027v1 [astro-ph.SR] 28 Feb 2011
First Flare-related Rapid Change of Photospheric Magnetic Field
Observed by Solar Dynamics Observatory
Shuo Wang1, Chang Liu1, Rui Liu1, Na Deng1, Yang Liu2, and Haimin Wang1
1. Space Weather Research Laboratory, New Jersey Institute of Technology, University
Heights, Newark, NJ 07102-1982, USA; haimin.wang@njit.edu
2. W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA
94305-4085, USA
*in preparation for formal publication*
ABSTRACT
Photospheric magnetic field not only plays important roles in building up
free energy and triggering solar eruptions, but also has been observed to change
rapidly and permanently responding to the coronal magnetic field restructuring
due to coronal transients. The Helioseismic and Magnetic Imager instrument
(HMI) on board the newly launched Solar Dynamics Observatory (SDO) pro-
duces seeing-free full-disk vector magnetograms at consistently high resolution
and high cadence, which finally makes possible an unambiguous and comprehen-
sive study of this important back-reaction process. In this study, we present a
near disk-center, GOES-class X2.2 flare occurred at NOAA AR 11158 on 2011
February 15 using the magnetic field measurements made by HMI. We obtained
the first solid evidence of an enhancement in the transverse magnetic field at the
flaring magnetic polarity inversion line (PIL) by a magnitude of 70%. This rapid
and irreversible field evolution is unequivocally associated with the flare occur-
rence, with the enhancement area located in between the two chromospheric flare
ribbons. Similar findings have been made for another two major flare events ob-
served by SDO. These results strongly corroborate our previous suggestion that
the photospheric magnetic field near the PIL must become more horizontal after
eruptions.
In-depth studies will follow to further link the photospheric magnetic field
changes with the dynamics of coronal mass ejections, when full Stokes inversion
is carried out to generate accurate magnetic field vectors.
Subject headings: Sun: activity — Sun: coronal mass ejections (CMEs) — Sun:
flares — Sun: X-rays, gamma rays — Sun: magnetic topology — Sun: surface
magnetism

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1.INTRODUCTION
Almost two decades ago, we discovered rapid and permanent changes of vector mag-
netic fields associated with flares (Wang 1992; Wang et al. 1994). Specifically, the transverse
field near the flaring magnetic polarity inversion line (PIL) is found to enhance impulsively
and substantially, which is also often accompanied by an increase of magnetic shear. Sim-
ilar trend of photospheric field evolution associated with flares and coronal mass ejections
(CMEs) has continued to be observed later on in many observations (Wang et al. 2002,
2004b, 2005; Liu et al. 2005; Wang et al. 2007; Jing et al. 2008), and shows some agreement
as well when compared to recent model predictions (Li et al. 2011). Nevertheless, such a
study is unavoidably hampered by the obvious limitations of ground-based observation (e.g.,
seeing variation and the limited number of observing spectrum position), most probably be-
cause of which mixed results were also reported (Ambastha et al. 1993; Hagyard et al. 1999;
Chen et al. 1994; Li et al. 2000a,b).
On the other hand, flare-related variations in the line-of-sight (LOS) component of pho-
tospheric magnetic field have been clearly recognized (e.g., Wang et al. 2002; Spirock et al.
2002; Yurchyshyn et al. 2004; Sudol & Harvey 2005; Wang 2006; Wang & Liu 2010; Petrie & Sudol
2010). However, it is noted that the changes of LOS field are qualitative in nature without
providing an exact understanding of the coronal field restructuring (Hudson 2011).
It is notable that systematic vector measurements has been made available with the He-
lioseismic and Magnetic Imager (HMI) instrument (Schou et al. 2010) on board the newly
launched Solar Dynamics Observatory (SDO). Its unprecedented observing capabilities give
a favorable opportunity to finally resolve any uncertainties regarding the evolution of pho-
tospheric magnetic field in relation to flares/CMEs.
In this study, we investigate a near disk-center X2.2 flare on 2011 February 15, where
we found the first solid evidence of the enhancement in the transverse field at the flaring
PIL using the seeing-free and consistent HMI data. We will discuss the implications of such
a change under the context of the recently developed theory.
2.OBSERVATIONS AND DATA REDUCTION
The βγδ region NOAA 11158 lies close to the disk center (S21◦, W21◦) when the 2011
February 15 X2.2 flare started at 01:44 UT and peaked at 01:56 UT in GOES soft X-ray
flux. As a preliminary approach in handling the HMI data, we used the signal of linear
polarization as a proxy of the transverse magnetic field strength Bt. We normalized the
data in a way to keep the ?Bt? value of a control region (i.e., an α-spot in the active region)

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-260
-190
-250
-240
-230
-220
-210
-200
-190
Y (arcsecs)
HMI Bt
01:36:00 UT
a

180200220240260
X (arcsecs)
-260
-250
-240
-230
-220
-210
-200
Y (arcsecs)
HMI Bt
04:00:00 UT

0300600
Gauss
900 1200
b













HMI Bz 01:36:00 UT
RHESSI 50-100 keV 01:55:18 UT

c
180 200220 240260
X (arcsecs)








Hinode Ca II H 01:55:18 UT
RHESSI 50-100 keV 01:55:18 UT
d
Fig. 1.— Pre- (a) and post-flare (b) Btmap obtained by HMI clearly showing the enhance-
ment of transverse field at the PIL (white line) in a region enclosed by the white bordered
line, which is located directly between the HXR footpoints (green bordered contours) and
the portions of the brightest chromospheric ribbons (c and d). The CLEAN RHESSI image
is reconstructed for the peak of 50–100 keV emissions from 01:55:00 to 01:55:36 UT obtaining
a total count of 1.7 × 104. The contour levels are 50%, 60%, 70%, 80%, and 90% of the
maximum flux.
constant, so that any observed change in the flaring region could be meaningfully reduced.
The resulted Btmap has a spatial resolution of 0.5′′and a cadence of 12 minutes.
The evolution of the flare hard X-ray (HXR) emission was entirely registered by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; Lin et al. 2002). As a
quick assessment of the location of high energy particles bombardment, a CLEAN image
(Hurford et al. 2002) was reconstructed using the front segment of detectors 3–8 and natural
weighting (giving an FWHM resolution of ∼14′′), covering the peak of the 50–100 keV
emissions from 01:55:00 to 01:55:36 UT. As a reference, we also present a cotemporal Ca ii
H image taken by the Solar Optical Telescope (SOT; Tsuneta et al. 2008) on board Hinode
(Kosugi et al. 2007), which shows the chromospheric flare ribbons.

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22:0000:0002:0004:0006:00
Start Time (14-Feb-11 22:00:00)
600
700
800
900
1000
1100
1200
Bt (Gauss)
50
100
150
200
RHESSI 50–100 keV count rate (s-1 det-1)

10-6
10-5
10-4
GOES 1.6–12.4 keV flux (Watts m-2)
Fig. 2.— Temporal evolution of ?Bt? of the region enclosed by the white bordered line in
Fig. 1, in comparison with the light curves of RHESSI HXR count rate in the 50–100 keV
energy range (red) and GOES flux in 1–8˚ A (blue).
3.RESULTS
From the pre- and post-flare Btmaps presented in Figure 1ab, a pronounced enhance-
ment in the transverse field strength can be seen in a region at the main flaring PIL (enclosed
by the white bordered line). The location of this region is between the two closely spaced
chromospheric ribbons, the portions of which are cospatial with the footpoints of the flare
HXR emissions in the 50–100 keV (Fig. 1cd).
In Figure 2, we plot the temporal evolution of ?Bt? of this region spanning ∼8 hrs
centering on the occurrence of the flare. It is evident that the transverse magnetic field
increases by ∼70% from ∼650 G to ∼1100 G, the transition time of which is cotemporal with
the peaking of the flare HXR emissions representing the abrupt release of the accumulated
free magnetic energy.
These observations strongly endorse our previous studies, in which similar changes in
the transverse field were observed using ground-based data with the influence of seeing (e.g.,
Wang et al. 2005; Liu et al. 2005).

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4. DISCUSSION
The seeing-free and consistent SDO/HMI magnetic field observation with unprecedented
high spatial and temporal resolution has produced, for the first time, unambiguous and solid
evidence of the rapid, flare-related enhancement in the transverse field at the flaring PIL.
Analysis on another two major flares observed by SDO is currently underway, in which we
have obtained very similar results. These provide strong support to our previous observations
(see Wang & Liu 2010, for a review), based on which we have suggested that photospheric
magnetic fields respond to coronal field restructuring (namely, an inward collapse after the
eruption of CMEs) by tilting toward the surface (i.e., toward a more horizontal state) near
the PIL. This view is well in line with the recent theoretical development (Hudson et al.
2008; Fisher et al. 2010), where the back reaction on the solar surface and interior resulting
from the coronal field evolution required to release energy is quantitatively assessed in terms
of the downward Lorentz force integrated over the region where the field change is significant.
In the following, we discuss a few points related to observations and modeling work.
1. The endeavor of studying the flare-related photospheric magnetic field changes has been
plagued by the problem that the magnetic field measurement could be affected by flare
emissions (Qiu & Gary 2003; Patterson & Zirin 1981). Here we only concentrate on the
magnetic fields at the PIL in between the flare ribbons, where both the flare emission
is insignificant and the weak-field approximation is usually acceptable. We anticipate
that full Stokes inversion will minimize such effects so that a fully reliable analysis of
field evolution can be carried out.
2. Using the magnetic vectors derived from inversion, we will examine the following as-
pects of magnetic field properties: (1) magnetic shear. It has been demonstrated
that increase of magnetic shear and of transverse field strength are interrelated (e.g.,
Wang et al. 2002). (2) Lorentz force. A correlation between the downward Lorentz
force computed from changes in the vector magnetograms and the dynamics of the
upward propelled CMEs has been theoretically suggested (Fisher et al. 2010).
3. The changes of transverse magnetic field can be directly visualized by the evolution of
the sunspot penumbral structure. Similar to our previous analysis (Wang et al. 2004a;
Deng et al. 2005; Liu et al. 2005; Chen et al. 2007), the darkening of penumbrae in
the studied region with enhancement of transverse field is quite discernible. However,
the decay of penumbrae in the outside boundary of sunspots, which we surmised to
be associated with the inward collapse of active region magnetic field, is less obvious
in the present event, probably because the source active region is still in the growing