Spatio-temporal dynamics of sources of hard X-ray pulsations in solar   flares
  We present systematic analysis of spatio-temporal evolution of sources of hard X-ray (HXR) pulsations in solar flares. We concentrate on disk flares whose impulsive phase are accompanied by a series of more than three peaks (pulsations) of HXR emission detected in the RHESSI 50-100 keV channel with 4-second cadence. 29 such flares observed from February 2002 to June 2015 with time differences between successive peaks of 8-270 s are studied. The main observational result is that sources of HXR pulsations in all flares are not stationary, they demonstrate apparent displacements from pulsation to pulsation. The flares can be subdivided into two groups depending on character of dynamics of HXR sources. The group-1 consists of 16 flares (55%) with systematic dynamics of HXR sources from pulsation to pulsation with respect to a magnetic polarity inversion line (MPIL), which has simple extended trace on the photosphere. The group-2 consists of 13 flares (45%) with more chaotic displacements of HXR sources with respect to an MPIL having more complicated structure. Based on the observations we conclude that the mechanism of flare HXR pulsations is related to successive triggering of energy release in different magnetic loops. Group-1 flare regions consist of loops stacked into magnetic arcades extended along MPILs. Group-2 flare regions have more complicated magnetic structures and loops are arranged more chaotically. We also found that at least 14 (88%) group-1 flares and 11 (85%) group-2 flares are accompanied by coronal mass ejections, i.e. the majority of flares studied are eruptive events. This gives an indication that eruptive processes play important role in generation of HXR pulsations. We suggest that an erupting flux rope can act as a trigger of energy release. Its successive interaction with different loops can lead to apparent motion of HXR sources and to a series of HXR pulsations. 
S. A. Kuznetsov, I. V. Zimovets, A. S. Morgachev, A. B. Struminsky
astro-ph.SR