Internal Shocks in the Magnetic Reconnection Jet in Solar Flares: Multiple Fast Shocks Created by the Secondary Tearing Instability

Space solar missions such as {\it Yohkoh} and {\it RHESSI} observe the hardX- and gamma-ray emission from energetic electrons in impulsive solar flares.Their energization mechanism, however, is unknown. In this paper, we suggestthat the internal shocks are created in the reconnection jet and that they arepossible sites of particle acceleration. We examine how magnetic reconnectioncreates the multiple shocks by performing two-dimensional resistivemagnetohydrodynamic simulations. In this paper, we use a very small grid toresolve the diffusion region. As a result, we find that the current sheetbecomes thin due to the tearing instability, and it collapses to a Sweet-Parkersheet. The thin sheet becomes unstable to the secondary tearing instability.Fast reconnection starts by the onset of anomalous resistivity immediatelyafter the secondary tearing instability. During the bursty, time-dependentmagnetic reconnection, the secondary tearing instability continues in thediffusion region where the anomalous resistivity is enhanced. As a result, manyweak shocks are created in the reconnection jet. This situation producesturbulent reconnection. We suggest that multiple fast shocks are created in thejet and that the energetic electrons can be accelerated by these shocks.