Abstract We carry out a detailed analysis of the X3.5 solar flare that occurred on 2002 July 20, which is the strongest partially limb-occulted flare ever observed by the RHESSI satellite. The main results are: (1) during the main impulsive phase that lasts ~ 10 minutes, the motion of the thermal sources follows a U-shaped trajectory. Non-thermal sources move in a similar way, but in a series of larger zigzags. We further show that the non-thermal sources are actually leading the contraction motion. (2) During the main impulsive phase, X-ray sources at different energies continuously form a loop-like configuration, with the highest energy source (up to ~ 100 keV) and the lowest energy source (down to ~ 10 keV) being located at two ends. The entire loop-like configuration moves in a U-shaped trajectory, while keeping the order of descending energy from highest to lowest during motion. Two non-thermal hard X-ray sources with different energies are spatially well separated in the distribution. The unusual complexities of the X-ray emissions in the tenuous solar corona challenge interpretations using bremsstrahlung in a simple magnetic configuration.

Keywords Sun: activity — Sun: X-ray — Sun: flares

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