RHESSI investigation of solar flare footpoints

Footpoints of several strong solar flares have been investigated using images obtained by Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The altitude of hard X-ray sources in various energy bands has been measured. Some observational characteristics of the energy-height relation have been obtained and presented in the form of histograms. Using them we were able to conclude that during impulsive phase the region in which electrons are stopped is rather narrow with the thickness in the range 1500-4000 km. Moreover, it is shown that all electrons producing photons with energies exceeding 20-30 keV are stopped at similar altitudes. For the flare of 3 August 2002 we analysed the temporal evolution of the energy-height relation. It has been shown that changes in this relation can be interpreted as a manifestation of simultaneous upflows and downflows of the transition region and chromosphere. Obtained values of velocities are between −50 and 150 km/s.