Hard X-ray Emission During Flares and Photospheric Field Changes

TL;DR

This study investigates the relationship between photospheric magnetic field changes and hard X-ray emissions during X-class solar flares, revealing temporal and spatial correlations and differences that shed light on flare dynamics.

Contribution

It provides new insights into the timing and spatial relationship between magnetic field changes and HXR emissions during solar flares, highlighting the sequence of events and their implications.

Findings

Field changes occur mainly near the PIL in strong magnetic regions.

Most field changes start before or around the HXR onset.

Field changes often follow the HXR footpoint movement and peak earlier in strong flares.

Abstract

We study the correlation between abrupt permanent changes of magnetic field during X-class flares observed by the GONG and HMI instruments, and the hard X-ray (HXR) emission observed by RHESSI, to relate the photospheric field changes to the coronal restructuring and investigate the origin of the field changes. We find that spatially the early RHESSI emission corresponds well to locations of the strong field changes. The field changes occur predominantly in the regions of strong magnetic field near the polarity inversion line (PIL). The later RHESSI emission does not correspond to significant field changes as the flare footpoints are moving away from the PIL. Most of the field changes start before or around the start time of the detectable HXR signal, and they end at about the same time or later than the detectable HXR flare emission. Some of the field changes propagate with speed close to that of the HXR footpoint at a later phase of the flare. The propagation of the field changes often takes place after the strongest peak in the HXR signal when the footpoints start moving away from the PIL, i.e. the field changes follow the same trajectory as the HXR footpoint, but at an earlier time. Thus, the field changes and HXR emission are spatio-temporally related but not co-spatial nor simultaneous. We also find that in the strongest X-class flares the amplitudes of the field changes peak a few minutes earlier than the peak of the HXR signal. We briefly discuss this observed time delay in terms of the formation of current sheets during eruptions.