Quasi-adiabatic dynamics of ions in a bifurcated current sheet

Abstract

The study is devoted to ion dynamics in bifurcated current sheets with a two-peak current-density distribution observed in the Earth's magnetotail and solar wind. The ion motion is described by a Hamiltonian system with two degrees of freedom. The presence of a small parameter κ characterizing the ratio between the amplitudes of the normal and tangential magnetic field components allows one to separate variables into fast and slow ones and introduce the quasi-adiabatic invariant of motion I_z. Conservation of this invariant makes it possible to analytically describe the dynamics of charged particles. Deviations of the particle dynamics from the quasi-adiabatic one, which are caused by the nonconservation of the quasi-adiabatic invariant, are investigated. The jump of the invariant ΔI_z is shown to depend on the small parameter according to the power-law ΔI_z ∼ κ^h, where the exponent h varies between unity and 3/4, depending on the level of current sheet bifurcation. The obtained dependence of ΔI_z on κ coincides with analytic expressions in the limiting cases of nonbifurcated and completely bifurcated current sheets.