Whistler-mediated magnetic reconnection in large systems: Magnetic flux pileup and the formation of thin current sheets

Abstract

We compute numerical solutions of the resistive Hall MHD equations corresponding to pairwise magnetic island coalescence. The simulation results can be organized according to the relative sizes of three length scales: the electron dissipation length, ℓe; the ion inertial length, d j; and the island wavelength, λ. We identify three qualitatively distinct regimes of magnetic island coalescence: (1) the resistive MHD limit, di ≲ ℓe ≪ λ; (2) the "whistler-mediated" limit, ℓe ≪ di ≪ λ; and (3) the "whistler-driven" limit, ℓe ≪ λ ≲ di. In the resistive MHD limit, magnetic flux piles up outside thin current sheets between the islands. The upstream Alfvén speed increases with increasing Lundquist number, and the reconnection rate is insensitive to the Lundquist number. In the whistler-driven limit, the electron and ion bulk flows decouple on the island wavelength scale. Magnetic flux pileup does not occur, and the coalescence proceeds on a whistler timescale that is much shorter than the Alfvén time. In the whistler-mediated limit, electron and ion bulk flows decouple in spatially localized "ion inertial sheets" around the island separatrices. Flux pileup is reduced, and the upstream Alfvén speed approaches a nearly constant value as the Lundquist number is increased. The maximum reconnection rate in the whistler-mediated limit is comparable to that observed in the resitive MHD limit over the Lundquist number range 500 < Sλ < 10000.