Intense Cross-Tail Field-Aligned Currents in the Plasma Sheet at Lunar Distances

Field-aligned currents in the Earth's magnetotail, typically observed at the plasma sheet boundary layer, are believed to be driven by transient plasma flows and strong plasma pressure gradients at equatorial footpoints. Magnetotail currents (transverse to the magnetic field), usually observed within the plasma sheet, flow duskward (cross-tail) and are believed to be diamagnetic. These two current systems, field aligned and transverse, can be easily distinguished because of their differing propagation direction. By statistically analyzing magnetotail current sheet crossings by the two lunar-orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun spacecraft (probes), we demonstrate that the duskward magnetotail current in the lunar distance magnetotail (similar to 55-65R(E)) can, at times, be predominantly field aligned. In about half the current sheet crossings examined, there is a significant equatorial, unidirectional cross-tail magnetic field component near the peak cross-tail current density. Because of the alignment of the magnetic field with the cross-tail current, a significant part of the total cross-tail current (similar to 1 to 10 nA/m(2)) is field aligned. This magnetic field component contributes significantly to the vertical pressure balance and is therefore important for plasma sheet dynamic stability. The generation mechanism and closure of the resultant field-aligned current system and its role in the dynamic stability of the current sheet are important to investigate further.