C

Abstract

Based on our recent MHD simulations, a conception of the successive merging of plasmoids and fragmentation in the current sheet in the standard flare model is presented. Then, using a 2.5-dimensional electromagnetic particle-in-cell model with free boundary conditions, these processes are modeled on the kinetic level of plasma description. We recognize the plasmoids that mutually interacted and finally merged into one large plasmoid. Between interacting plasmoids, additional plasmoids and current sheets on smaller spatial scales were formed, congruent with the fragmentation found in MHD simulations. During interactions (merging–coalescences) between the plasmoids, the electrons were very efficiently accelerated and heated. We find that after a series of such merging processes, the electrons in some regions reached the energies necessary for emission in the hard X-ray range. Considering these energetic electrons and assuming a plasma density of 109–1010 cm−3 and a source volume equal to the 2007 December 31 flare, we compute the X-ray spectra as produced by the bremsstrahlung emission process. Comparing these spectra with observations, we think that these processes can explain the observed above-the-loop-top hard X-ray sources. Furthermore, we show that the process of fragmentation between two merging plasmoids can generate narrow-band dm-spikes. Formulae for schematic fractal reconnection structures are derived.