Spatial Distribution of Magnetic Reconnection Rate in an M6.5 Solar Flare

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Introduction

Magnetic reconnection is an important process in highly conducting plasmas duringwhich free magnetic energy is quickly converted into other forms of energy, such as kinetic energy, thermal energy, and nonthermal particle acceleration. In the standard picture of a solar flare, reconnecting coronal magnetic field links directly to the field swept out by the solar flare ribbons in the lower atmosphere. Therefore, observations of the flare ribbon motions provide an opportunity to determine the magnetic reconnection rate, i.e., E = v_rib B_z (Ref. [1]) 1984), where v_rib is the apparent separation velocity of the flare ribbon, and B_z is the vertical component of the magnetic field at the ribbon location.

The establishment of the above equation has led to remarkable progress in the study of magnetic reconnection in solar flares over the past two decades (Ref. [2]). However, previous studies have mainly focused on either the temporal properties of E or its statistical correlation with coronal mass ejections (CMEs), while relatively few studies have investigated the spatial properties of the reconnection E field. We had an opportunity to determine the spatial distribution of 2D magnetic reconnection rate when an M6.5 flare on 2015 June 22 (SOL2015-06-22T18:23) was studied with the high-resolution data taken by the 1.6 m Goode Solar Telescope (GST; Ref. [3]) at the Big Bear Solar Observatory (BBSO).

Observation and Analysis

Figure 1 shows snapshots of the flare and the pre-flare magnetic field B_z. The field of view (FOV) of the GST covers mainly the eastern part of the active region with positive magnetic polarity and the progression of the eastern ribbon over this field. The high-resolution observation presents the fine structure of the ribbon front in the form of whirls or hooks, which is evident, for example, when the ribbon leaves the narrow light bridge (LB) between the two sunspots.

Figure 1: (a) Snapshot of the eastward-moving ribbon of the flare, taken by GST in Hα. (b) GST longitudinal magnetogram, taken at 17:34:03 UT on the same day, 5 min prior to the flare onset. The blue contour shows the major magnetic polarity inversion line (PIL). The arrow indicates that the ribbon near the sunspot's light bridge moves faster than the other parts.

Figure 2a shows the spatial distribution of E over the region swept by the ribbon motion. The value of E at each point is the product of v_rib (obtained by the Local Correlation Tracking algorithm) and B_z at that point. The E field thus obtained is then compared with the energy deposition rate as indicated by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) hard X-ray (HXR) emission. The HXR image is reconstructed using the CLEAN algorithm (Ref. [3]) with RHESSI's front detectors 4-8 integrated in the time interval of 18:04:46 UT to 18:05:46 UT. Within the FOV of the GST observation, we can see one HXR source (outlined in red contour) in the 25-50 keV energy range located near a sunspot in positive polarity. The other HXR footpoint in negative polarity is outside the FOV of the GST images but can be seen in the HMI magnetogram with an expanded FOV (panel c).

is obtained over 18:04:46 UT to 18:05:46 UT when the HXR map is reconstructed. The strongest E is highlighted by the yellow oval. (c) The longitudinal magnetogram B_z obtained by HMI at 17:36:00 UT, in which the high-resolution GST magnetogram B_z at 17:31:29 UT is embedded, superimposed with contours of 80%, 90% and 97% of maximum HXR intensity. The white box shows the FOV of panel (a).

Although the HXR source within the GST's FOV appears in a location where E is relatively strong, there are other locations with strong E but without strong HXR emission. This may be due to the low dynamic range and low resolution of the HXR maps, as well as finite time differences between the E map and the HXR map. The E map is obtained at different times (17:51:14 UT to 18:29:29 UT), while the HXR map is derived at a 1-minute (18:04:46 UT to 18:05:46 UT) integration. Thus, in Figure 2b, only the |E| distribution over the 1-minute period when the HXR map is reconstructed is plotted. During this short period, the strongest |E| (highlighted with a yellow oval) overlaps with the location of the concentrated HXR source. The average |E| within the yellow oval is about 7.5 V/cm, while the average |E| elsewhere in the ribbon front is 0.04 V/cm.

Summary

To our knowledge, this GST observation is one of the best datasets to study the spatial distribution of the reconnection electric field. It is stronger when the ribbon is close to the PIL and decreases as it deviates from the PIL. The HXR emission is concentrated and appears close to a sunspot. At the same time, the strongest values of |E| and the concentrated HXR source show a spatial correlation. Since the flare ribbon is a projection of the coronal magnetic reconnection on the chromosphere, the |E| structure derived from the ribbon morphology and motion guides us to the dynamics of the reconnecting current sheet in the corona. Ref. [5] has the complete story.

Acknowledgments

The full author list for this Nugget is Ju JING, Bryce CANNON, Qin LI, Nian LIU, Jeongwoo LEE, Wenda CAO, and Haimin WANG.

References

[1] "Numerical simulation of reconnection in an emerging magnetic flux region"

[2] "Dimensionality of solar magnetic reconnection"

[3] "The 1.6 m off-axis New Solar Telescope (NST) in Big Bear"

[4] "The RHESSI Imaging Concept"

[5] "Magnetic Reconnection Rate in the M6.5 Solar Flare on 2015 June 22"