A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints

TL;DR

This study analyzes 172 hard X-ray peaks from 53 solar flares with double-footpoint structures, revealing spectral index similarities, limited asymmetries, and correlations with flare properties, supporting magnetic mirror trap models.

Contribution

It provides a comprehensive statistical analysis of spectral characteristics of solar flare footpoints, highlighting the limited asymmetries and their implications for acceleration models.

Findings

Spectral index differences mostly between 0 and 0.6.

Footpoint asymmetries are more pronounced during peak HXR flux.

Correlations found between HXR flux, GOES flux, and loop length.

Abstract

Using RHESSI data, we have analyzed some 172 hard X-ray peaks during 53 solar flares which exhibited a double-footpoint structure. Fitting both footpoints with power-laws, we find that spectral index differences range mostly between 0 to 0.6, and only rarely go beyond. Asymmetries between footpoints were not observed to be significantly dependent on their mean heliographic position, their relative position with respect to each other, nor their orientation with respect to the solar equator. Assuming a symmetric acceleration process, it is also clear that differences in footpoint spectral indices and footpoint flux ratios can seldom be attributed to a difference in column densities between the two legs of a coronal loop. Our results corroborate better the magnetic mirror trap scenario. Moreover, footpoint asymmetries are more marked during times of peak HXR flux than when averaging over the whole HXR burst, suggesting that the magnetic configuration evolves during individual HXR bursts. We observed also a linear correlation between the peak 50-keV flux and the peak GOES 1-8A channel flux, and that HXR burst duration seem correlated with loop length.