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.

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Part of Special Issue "Spatio-temporal analysis and multipoint measurements in space"Abstract. By means of a simulation model, the earlier predicted nonlinear kinetic structure, a Forced Kinetic Current Sheet (FKCS), with extremely anisotropic ion distributions, is shown to appear as a result of a fast nonlinear process of transition from a previously existing equilibrium. This occurs under triggering action of a weak MHD disturbance that is applied at the boundary of the simulation box. In the FKCS, current is carried by initially cold ions which are brought into the CS by convection from both sides, and accelerated inside the CS. The process then appears to be spontaneously self-sustained, as a MHD disturbance of a rarefaction wave type propagates over the background plasma outside the CS. Comparable to the Alfvénic discontinuity in MHD, transformation of electromagnetic energy into the energy of plasma flows occurs at the FKCS. But unlike the MHD case, "free" energy is produced here: dissipation should occur later, through particle interaction with turbulent waves generated by unstable ion distribution being formed by the FKCS action. In this way, an effect of magnetic field "annihilation" appears, required for fast magnetic reconnection. Application of the theory to observations at the magnetopause and in the magnetotail is considered.