Energetic electron acceleration during multi-island coalescence

The mechanisms for the production of relativistic electrons associated with the coalescence/reconnection of multiple magnetic islands are investigated using two-dimensional particle-in-cell simulations for the case where the initial island half width L is comparable to the ion inertia length. Configurations without and with a uniform magnetic guide field are considered. Significant energization occurs only when the number of islands is reduced to 2 or 3 with wavelength satisfying kxL≲0.2. The energization proceeds in two distinct stages. In the first stage, a small number of electrons are accelerated to relativistic energies at the X-line by the inductive electric field, corresponding to perpendicular acceleration in the absence of the guide field and parallel/anti-parallel acceleration with a guide field. The second stage is associated with the final coalescence into one large island and produces a considerably larger number of relativistic electrons. With a guide field, this stage is dominated by the formation of elongated density cavities along one pair of separatrices and continued direct acceleration at the X-line. Without the guide field, the direct X-line acceleration becomes unimportant, and the acceleration is localized in the flux pile-up regions and results from the curvature drift interacting with the localized inductive electric field. Typically, some 15%–20% of the decrease in magnetic field energy is transferred to the electrons, with a few percent appearing in relativistic (E∕mec2>0.3) electrons.