A statistical survey of hard X-ray spectral characteristics of solar flares with two footpoints

Using RHESSI data, we have analyzed 172 hard X-ray (HXR) peaks during 53 solar flares that exhibited a double-footpoint structure. Fitting both footpoints with power laws, we find that spectral index differences Delta gamma 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. By 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 also observed a linear correlation between the peak 50-keV flux and the peak GOES 1-8 angstrom channel flux and that HXR burst duration seem correlated with loop length.