Particle orbits during the development of plasmoids

Single‐particle orbits are numerically followed in a time‐dependent electric and magnetic field obtained from a two‐dimensional magnetohydrodynamic simulation of bursty reconnection (Scholer and Roth, 1987). During the reconnection simulation the reconnection rate at the original X point decreases, a long current sheet develops, and reconnection sets in again at a new pair of X points. This leads to the development of a plasmoid. Particles injected with parameters appropriate for the plasma sheet at the time of the secondary tearing close to the new X points drift large distances parallel to the X line and gain high energies (up to ∼300 keV). These particles are subsequently trapped on closed field lines in the developing plasmoid. Since the X line has a limited extent, the acceleration mechanism leads to large dawn‐dusk asymmetries. The resulting distribution function within the plasmoid at various cross‐tail distances is computed. The distribution function is close to an exponential in velocity with a higher‐energy cutoff; with increasing distance toward the duskside the e ‐folding velocity increases, and the high‐energy cutoff shifts to larger energies.