Role of oxygen ions in the formation of a bifurcated current sheet in the magnetotail

Abstract

Cluster observations in the near-Earth magnetotail have shown that sometimes the current sheet is bifurcated, i.e., it is divided in two layers. The influence of magnetic turbulence on ion motion in this region is investigated by numerical simulation, taking into account the presence of both protons and oxygen ions. The magnetotail current sheet is modeled as a magnetic field reversal with a normal magnetic field component B_n, plus a three-dimensional spectrum of magnetic fluctuations δB, which represents the observed magnetic turbulence. The dawn-dusk electric field E_y is also included. A test particle simulation is performed using different values of δB, E_y and injecting two different species of particles, O⁺ ions and protons. O⁺ ions can support the formation of a double current layer both in the absence and for large values of magnetic fluctuations (δB/B₀ = 0.0 and δB/B₀ ≥ 0.4, where B₀ is the constant magnetic field in the magnetospheric lobes).