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

Bifurcations have been observed in many current sheets throughout the solar system, including in terrestrial, Jovian and solar current sheets. Explanations for these observations include magnetic reconnection, tail lobe compression, and quasi-adiabatic ion motion.

In this study, we report the first observations of bifurcated current sheets in Mercury's magnetotail using MESSENGER magnetic field measurements. Although observations of Harris-like current sheets (HCT) have been made, we have observed significantly more bifurcated (BCS) (~52 observations) than Harris-type structures (~19 observations) (Figure 1). Furthermore, we investigated the causes of bifurcations in Mercury's magnetotail using their observed properties, spatial locations and information about, or proxies for magnetospheric environmental conditions at the times of current sheet crossings, and comparing these with predictions from existing explanations in the literature.

The observed symmetry about the magnetic equator in BCS's, as well as their spatial distribution, make it unlikely that magnetic reconnection is the sole explanation for the observed bifurcation. The tail lobe magnetic pressure for BCS observations is the same, within error, as that for HCSs, and so it is similarly unlikely that increased pressure in the tail lobes causes the bifurcations. Finally, although the orbit of individual ions cannot be measured, the interactions of ions with quasi-trapped orbits and those with transient orbits, appears to best explain the current sheet bifurcations in Mercury's tail based on the following observations: (1) The current density is reduced in the center of the current sheet rather than amplified on the flanks, (2) Both partial and total bifurcations of the current sheet have been observed, and (3) The total thickness of the current sheet is increased when it is found to be bifurcated. Based on our results, we suggest a timeline for the evolution of Mercury's tail current density.