Relating photospheric magnetic field changes and hard X-ray emission during flares

Motivation. Significant, abrupt and permanent photospheric magnetic field changes during major X-class and M-class flares have been reported by many authors (e.g., Kosovichev & Zharkova 2001, ApJ 550, 105; Wang et al. 2002, ApJ 576, 497; Sudol & Harvey 2005, ApJ 635, 647; Petrie & Sudol 2010, ApJ 724, 1218; Wang et al. 2012, ApJ Letters 745, 17, Petrie 2013, Solar Phys. 287, 415). No numerical model has yet reproduced these observations and physically related the field changes at the forced photosphere with the flare that occurs in the nearly force-free corona.

The hard X-ray (HXR) emission in solar flares generally comes from the footpoints of newly reconnected magnetic loops in the chromosphere. Also, the HXR footpoint motions away from the neutral line are consistent with the reconnection occurring at different heights of arcade magnetic fields with a displacement speed roughly proportional to the rate of reconnection (e.g., Asai et al. 2004, ApJ 611, 557; Temmer et al., 2007, ApJ 654, 665).

We investigate the spatial and temporal correlation between abrupt permanent changes of magnetic field during five X-class flares observed by GONG and one X-class flare observed by HMI, and the location of HXR emission observed by RHESSI.

Our purpose is to explore the likelihood that the photospheric field changes and the chromospheric HXR sources are generally caused by a common physical process in the corona during a flare, and to provide new observational constraints for models of solar flares.

Data Analysis. •  GONG full-disk LOS magnetograms have 1-min cadence, 2.5-arcsec pixel size, 3 G / pixel noise level. The

images are remapped to heliographic coordinates by azimuthal equidistant projection. •  We use HMI 12-minute vector magnetograms in the form of Active Region Patches (HARPs), maps of Br, Bθ

and Bφ with pixel size 0°.03 in heliographic coordinates derived by cylindrical equal-area (CEA) projection. •  For the GONG data the abrupt field changes were modeled by step-like arctan functions (Sudol & Harvey

2005) fitted pixel-by-pixel to 4hr time series of images. The HMI vector field changes are derived by differencing pre- and post-flare vector magnetograms.

•  The RHESSI images were synthesized using the CLEAN algorithm. The cadence used was 20 to 30 seconds in an energy range from 30 to 300 keV.

Date, class, time and location of analyzed flares.

This work utilizes GONG data obtained by the NSO Integrated Synoptic Program (NISP), managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísica de Canarias, and Cerro Tololo Interamerican Observatory. The HMI data are provided by NASA/SDO and the HMI Science Team. G. Petrie, O. Burtseva and A. Pevtsov are partially supported by NASA grant NNX14AE05G. J. C. Martinez-Oliveros was supported by NASA grant NNX11AP05G for RHESSI.

Spatial correlation of the field changes and HXR sources.

The HXR sources display a complex motion pattern, where the flare footpoints move along the neutral line in the impulsive phase, and later move away from and perpendicular to the neutral line (Krucker et al., 2003, ApJ 595, 103; Temmer et al., 2007, ApJ 654, 665).

The RHESSI HXR footpoint contours near the emission peak time and temporal evolution of the source centroids positions during the five flares observed by GONG are shown in Figures 1 and 2, and during the flare observed by HMI are shown in Figure 3. The early RHESSI emission corresponds well to locations of the strong field changes. The strong field changes occur predominantly in the regions of strong magnetic field near the PIL. The later RHESSI emission doesn’t correspond to significant field changes as the footprint is moving away from the PIL. The amplitudes of these field changes are smaller.

Figure 1. GONG magnetogram of the flaring active region (left) and the amplitudes of the step-like field changes (right) for the two weaker flares in our analysis: the X1.0 and X2.6 flares. RHESSI 30-70 keV and 50-300 keV HXR footpoint contours for the two flares, respectively, at levels from 35 to 95% of the maximum counts at around the emission peak time are superposed in light blue. The temporal evolution of the source centroids is shown with plus symbols, which increasing size represents progressing times during flares. HXR sources are labeled by 'S' letter with a number.

Gordon Petrie1, Olga Burtseva1, Juan Carlos Martinez Oliveros2 and Alexei Pevtsov1

1National Solar Observatory, Tucson, AZ 2SSL - UC Berkeley, Berkeley, CA

Date GOES Class

GOES Times (UT) Location NOAA Number

Instrument

start peak end

2003 Oct 29 2003 Nov 2 2004 Aug 13 2005 Jan 15 2006 Dec 6 2011 Sep 6

X10.0 X8.3 X1.0 X2.6 X6.5 X2.1

20:37 17:03 18:07 22:25 18:29 22:12

20:49 17:25 18:12 23:02 18:47 22:20

21:01 17:39 18:15 23:31 19:00 22:24

S15W02 S14W56 S13W23 N15W05 S05E64 N13W18

10486 10486 10656 10720 10930 11283

GONG GONG GONG GONG GONG HMI

Figure 2. The same as Figure 1, but for the three stronger flares in our analysis: the X6.5, X8.3, and X10.0 flares. RHESSI 50-300 keV, 50-300 keV and 50-100 keV HXR footpoint contours for the three flares, respectively, superposed in light blue.

Figure 3. HMI vector magnetic field of the flaring active region (left) and the vector magnetic field changes (right) during the X2.1 flare. From top to bottom: the vertical field component, the horizontal field component and the field tilt angle.

Figure 4. Total flux change as a function of time during each of the five flares observed by GONG. Black solid, dark-grey dashed and light-grey dash-dotted lines correspond to the start, midpoint and end times, respectively. Total HXR flux is over plotted in red.

Figure 5. Midpoint time of the field change as a function of the HXR signal time. Dashed lines creating a rectangle in each panel indicate HXR peak time for the flare on both axes.

Figure 6. Cartoon showing the formation of a current sheet at an X-type coronal magnetic null point above a flaring loop system. The initial location of the reconnection site, indicated by "X” (left), divides into two reconnection sites moving vertically apart as the current sheet forms (right), producing a change in the surface field tilt.

Temporal correlation: tracking of the field changes in magnetograms. We sum the magnetic field inside the footpoint's contours at each footpoint's location at each moment of time. Thus we have temporal profiles of the flux at each location of the HXR sources. The midpoint times of the field changes, defined as a maximum gradient of the step, as a function of HXR time are shown in Figure 5. Most of the field changes occurred earlier than the HXR signal was detected in the region. Some of the field changes propagate with speed close to that of the HXR footpoint at a later phase of the flare, often after the strongest peak in the HXR signal when footpoint start moving away from the PIL. Thus, the field changes follow the same trajectory as the HXR footpoint, but at an earlier time.

Temporal correlation: analysis of the step-function fit parameters maps. Figure 4 shows the total flux change and HXR signal as a function of time. Amplitudes of the field changes for all qualifying pixels from the c-map were integrated over one-minute interval for each of the start, midpoint and end time from the corresponding fit parameters maps. The three strongest flares (right column) show a peak in total flux change ~ 1.5, 3 and 4 minutes earlier than the peak in the HXR signal. The field changes start earlier than the detectable HXR signal and end simultaneously with the HXR signal or a few minutes later. The two weaker flares (left column) do not seem to have a strong peak in the flux change at a certain time and the field changes do not show clear temporal relation to the HXR signal.

Discussion. The HXR emission and the field-change effects do not seem likely to share exactly the same coronal causes, though they must be related in some way. Because of the several-minute time delays between the HXR peaks and the largest flux changes in some of the flares, and the sub-minute coronal travel times, a simple causal link between the HXR and flux changes does not seem to be possible. We suggest that major coronal magnetic field changes occur before most of the particle acceleration and HXR emission. For example, the formation of a current sheet at a magnetic X-point above a flaring loop system could produce major magnetic changes before the major particle acceleration phase, as shown in Figure 6. The field changes might represent the current sheet formation phase, when the arcade field below the X-type neutral point collapses into a vertical sheet with Y-shaped structure at each end, and the low-lying flare loop legs form below the sheet. This explanation is consistent with flux rope eruption scenarios (e.g., Lin & Forbes 2000, JGR 105, 2375).