Intense cross-tail field-aligned currents in the plasma sheet at lunar distances

Abstract

Field-aligned currents in the Earth's magnetotail are traditionally associated with transient plasma flows and strong plasma pressure gradients in the near-Earth side. In this paper we demonstrate a new field-aligned current system present at the lunar orbit tail. Using magnetotail current sheet observations by two ARTEMIS probes at $\sim60 R_E$, we analyze statistically the current sheet structure and current density distribution closest to the neutral sheet. For about half of our 130 current sheet crossings, the equatorial magnetic field component across-the tail (along the main, cross-tail current) contributes significantly to the vertical pressure balance. This magnetic field component peaks at the equator, near the cross-tail current maximum. For those cases, a significant part of the tail current, having an intensity in the range 1-10nA/m$^2$, flows along the magnetic field lines (it is both field-aligned and cross-tail). We suggest that this current system develops in order to compensate the thermal pressure by particles that on its own is insufficient to fend off the lobe magnetic pressure.