Theory of a thin one‐dimensional current sheet in collisionless space plasma

Abstract

It is widely believed that evolution of a current sheet in collisionless space plasma often results in fast magnetic field merging, for example, during substorm onsets. The current sheet structure on the merging sites should exhibit a considerable change, in order for the field energy transformation into the energy of particles to become possible. The specific current sheet structure is the quasi‐one‐dimensional kinetic forced current sheet. A full analytical theory of such a sheet has been constructed for the typical case when the plasma parameter in the background plasma is small, β ≪ 1, and bulk motions of the plasma are sub‐alfvenic, MA ≪ 1. Theoretical consideration of that equilibrium state is based on the existence of a specific adiabatic invariant, corresponding to ion oscillations about the sheet central plane in their “Speiser” orbits. The theory describes the sheet structure self‐consistently, identifying its dependence on the features of the ion distribution function. Detailed structure of the emerging equilibrium sheet has been numerically studied. For a certain current sheet profile, while outside the sheet the calculated ion distribution function is close to a pair of interpenetrating shifted Maxwellians, inside the sheet it is also highly anisotropic but quite different, involving all velocities from 0 up to 2vA. Dependence of the structure scale length on the parameters of the problem corresponds to estimates obtained earlier. This scale length also agrees with observational estimates of the current sheet thickness in the magnetotail during its extreme thinning near substorm onset.