Energetics of semirelativistic electron-ion magnetic reconnection in 2D and 3D simulations*

With particle-in-cell simulation, we investigate how energy is partitioned among electrons, ions, and electromagnetic fields as magnetic energy dissipates via magnetic reconnection (MR) and other mechanisms in thin current sheets, in collisionless semirelativistic plasma (i.e., subrelativistic ions and relativistic electrons). We evolve current sheets in 2D and 3D with varying guide magnetic field, thus altering mechanism strengths to discern their roles in energy partition. We find that, in 2D MR without guide field, ions gain more energy than electrons; but electrons gain more than ions when (in a different 2D projection) the drift-kink instability (DKI) operates but MR (tearing) does not. In 3D the electron/ion gains meet halfway as both MR and DKI act. Increasing guide field quenches DKI; it slows MR and diminishes the total magnetic energy released (in 2D and 3D). Ion gains drop accordingly, while electron energization is buoyed by another mechanism: as reconnection-generated flux ropes move and merge, their motional electric field grows and eventually energizes electrons preferentially. Thus guide field boosts the electron/ion energy ratio. While 2D effects are important, capturing all dissipation channels--especially the flux-rope kink instability, which in 3D releases additional magnetic energy--requires full 3D simulation.