The Coalescence of Magnetic Islands in a Large Aspect Ratio Current Sheet

The formation of secondary islands and their coalescence play an important role in dissipating energy during magnetic reconnection. Recently, we have studied magnetic reconnection initiated by the tearing instability using a hybrid simulation with Lorentz force ions and fluid electrons. For current sheets with small aspect ratios, only a single island is present. For current sheets with large aspect ratios, multiple islands form and eventually coalesce into one large elongated island. We have observed ion pressure anisotropy near the $X$ point, which may prevent the full contraction of the islands and hence keep them from breaking into smaller islands. In addition, we find that a larger fraction of the dissipated magnetic energy is converted into the ion kinetic energy as the aspect ratio increases. Asymptotically, the ratio reaches slightly over $50\%$. Ion heating is identified by the ion energy spectra inside the island region which exhibits a slightly larger tail than the Maxwell distribution function. We believe that the bipolar in-plane electric field associated with the Hall term is related to the ion acceleration. Diagnostics from tracer particles are presented to illustrate how and where the ions are accelerated.