Bifurcated Current Sheets in Mercury's Magnetotail: Observations and Implications

Cross-tail current sheets play an important role in magnetospheric dynamics and plasma transport. We present a comprehensive study of the structure and behavior of Mercury's cross-tail current sheet, as well as constraints on the plasma environment imposed by this structure, using magnetic field measurements from the MESSENGER spacecraft. We find that the current sheet dominantly exhibits a bifurcated, or double-peaked, current density structure throughout the tail region, but that a single-peaked sheet is sometimes observed. The time-averaged current distribution in the nightside magnetosphere is as follows: close to the planet on the midnight plane at an altitude of 0.15 R-M (where R-M is Mercury radius = 2,440 km), current density peaks are 0.2 R-M north and south of the magnetic equator. The peak-to-peak separation decreases with down-tail distance, until the current density profile resembles that of a single-peaked sheet. We find no correlation between the sheet geometry and long-period variations in solar wind and magnetospheric conditions. We attribute the observed bifurcated structure in the current sheet to non-adiabatic behavior (Speiser orbits) of the major plasma constituents in Mercury's magnetosphere (H+, Na+ and, to a lesser extent e(-)). Finally, we infer the spatial distribution of the current carriers by comparing a simple model of their resultant current sheets to observations. We find that, close to the planet, a combination of planetary sodium ions and solar wind protons with energies of a few keV are the main current carriers, but in the far-tail region, the current is dominantly carried by solar wind protons.