Variations of the hard X-ray footpoint asymmetry in a solar flare

We analysed hard and soft X-ray observations from the SXT and HXT instruments on the Yohkoh satellite of an X5.3 class flare of 25 August 2001. The most prominent features of this flare are: a fast (450 km s(-1)), hard (up to 90 keV) and compact X-ray moving feature, two clear footpoint sources, and loop-top emission between them. The Yohkoh/HXT observations with high time cadences (0.5-1 s) revealed clear, quasi-periodic changes in the asymmetry of the observed footpoint fluxes. Observed asymmetry is usually interpreted as an effect of the difference in magnetic field strength or the field divergence at the footpoints. Such a traditional interpretation should lead in our case to the existence of strong magnetic field oscillations or movements of the footpoints' locations in the presence of a strong magnetic gradient. Our analysis indicates instead that differences in the injection conditions in each loop's leg are a source of the observed asymmetry variations. We also observed that the emission for each footpoint had a different spectrum, and its evolution was also different, but generally with the spectrum hardening at highest energies. We argue that these effects are connected with the mechanism of energy release and/or electron distribution.