Particle Acceleration Mediated by Magnetic Islands: Observations and Simulations

Magnetic reconnection is the underlying process that impulsively release an enormous amount of magnetic energy in solar and stellar flares, and substorms in the Earth's magnetosphere. Studies of energy release during solar flares, in particular, indicate that up to 50 percent of the released energy is carried by accelerated 20-100 keV suprathermal electrons. How so many electrons can gain so much energy during reconnection has been a long-standing question. In this study (to appear in Nature Physics 2007), we use multi-spacecraft measurements of energetic electrons from the Earth's magnetotail to demonstrate that the presence of energetic electrons is linked to the dynamics of magnetic islands. Specifically, we show that the peaks of energetic electron fluxes occur at sites of compressed density within islands. The strong density sub-structures within islands suggest that the islands may have undergone coalescence, which presents a potential new mechanism for electron acceleration during reconnection. We will compare these observations with Hall MHD and PIC simulations to assess the particle acceleration efficiency of this mechanism.