  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.
