With the aim of studying the relationship between the relative motions of the loop-top （LT） source and footpoints （FPs） during the rising phase of solar flares, we give a detailed analysis of the X7.1 class flare that occurred on 2005 January 20. The flare was clearly observed by RHESSI, showing a distinct X-ray flaring loop with a bright LT source and two well-defined hard X-ray （HXR） FPs. In particular, we correct the projection effect for the positions of the FPs and magnetic polarity inversion line. We find that： （1） The LT source showed an obvious U-shaped trajectory. The source of the higher energy LT shows a faster downward/upward speed. （2） The evolution of FPs was temporally correlated with that of the LT source. The converging/separating motion of FPs corresponds to the downward/upward motion of the LT source. （3） The initial flare shear of this event is found to be nearly 50 degrees, and it has a fluctuating decrease throughout the contraction phase as well as the expansion phase. （4） Four peaks of the time profile of the unshearing rate are found to be temporally correlated with peaks in the HXR emission flux. This flare supports the overall contraction pic- ture of flares： a descending motion of the LT source, in addition to converging and unshearing motion of FPs. All results indicate that the magnetic field was very highly sheared before the onset of the flare.
Tuan-Hui ZhouJun-Feng WangDong LiQi-Wu SongVictor MelnikovHai-Sheng Ji
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. Nonthermal 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.
Jin-Hua ShenVictor MelnikovTuan-Hui ZhouHai-Sheng Ji
