Spatio-temporal Dynamics of Sources of Hard X-Ray Pulsations in Solar Flares

We present a systematic analysis of the spatio-temporal evolution of sources of hard X-ray (HXR) pulsations in solar flares. We concentrate on disk flares whose impulsive phases are accompanied by a series of more than three successive peaks (pulsations) of HXR emission detected in the RHESSI 50 – 100 keV energy channel with a four-second time cadence. Twenty-nine such flares observed from February 2002 to June 2015 with characteristic time differences between successive peaks P≈8–270s$P \approx8\,\mbox{--}\,270~\mbox{s}$ are studied. The main observational result of the analysis is that sources of HXR pulsations in all flares are not stationary, they demonstrate apparent movements or displacements in the parent active regions from pulsation to pulsation. The flares can be subdivided into two main groups depending on the character of the dynamics of the HXR sources. Group 1 consists of 16 flares (55%$55~\%$) that show systematic dynamics of the HXR sources from pulsation to pulsation with respect to a magnetic polarity inversion line (MPIL), which has a simple extended trace on the photosphere. Group 2 consists of 13 flares (45%$45~\%$) that show more chaotic displacements of the HXR sources with respect to an MPIL with a more complex structure, and sometimes several MPILs are present in the parent active regions of such flares. Based on the observations, we conclude that the mechanism of the flare HXR pulsations (at least with time differences of the considered range) is related to successive triggering of the flare energy release process in different magnetic loops (or bundles of loops) of the parent active regions. Group 1 flare regions consist of loops stacked into magnetic arcades that are extended along MPILs. Group 2 flare regions have more complex magnetic structures, and the loops are arranged more chaotically and randomly there. We also found that at least 14 (88%$88~\%$) group 1 flares and 11 (85%$85~\%$) group 2 flares are accompanied by coronal mass ejections (CMEs), i.e. the absolute majority of the flares we studied are eruptive events. This gives a strong indication that eruptive processes play an important role in the generation of HXR pulsations in flares. We suggest that an erupting flux rope can act as a trigger of the flare energy release. Its successive interaction with different loops of a parent active region can lead to apparent motion of HXR sources and to a series of HXR pulsations. However, the exact mechanism responsible for generating the pulsations remains unclear and requires a more detailed investigation.