Hard X-ray emission at the footpoints of solar flares Original Research Article

We analyse X-ray flare observations made by the Soft X-ray Telescope and Hard X-ray Telescope on the Yohkoh satellite during period 1999–2001. For the strongest flares, when HXT imaging with high time cadence (0.5–1 s) is possible, clear changes in the asymmetry of the observed footpoints fluxes are revealed. We present four examples of flares which illustrate such asymmetrical time variations the best. Observed asymmetries are usually interpreted as due to a difference in magnetic field strength or field divergence at the footpoints, however this interpretation should lead to the existence of strong magnetic field oscillations or movements of the footpoints’ locations in the presence of strong magnetic gradients. Our analysis indicates rather that either differences in the electron injection or electron distribution in each loop’s leg is the source of the observed asymmetry variations. We discuss possible mechanisms which may be responsible for such effect.