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- Title
Bifurcated Current Sheets in Mercury's Magnetotail: Observations and Implications.
- Authors
Al Asad, Manar M.; Johnson, Catherine L.; Philpott, Lydia C.
- Abstract
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 RM (where RM is Mercury radius = 2,440 km), current density peaks are 0.2 RM 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. Plain Language Summary: Hot, ionized gas, known as plasma, that is continuously emitted from the Sun interacts with magnetized bodies like Mercury. It compresses Mercury's magnetic field on the dayside, stretches it on the nightside, and concentrates plasma flows in specific regions, creating current sheets. We investigate the structure and properties of one of these regions, the tail current sheet, and observe an unusual average structure compared with that at other magnetized bodies. Mercury's tail current sheet is typically bifurcated into two sheets that occur to the north and south of the magnetic equator. Specifically, we find a ∼7:1 occurrence rate of a bifurcated versus a single current sheet. The north‐south distance between the sheets decreases away from the planet, and they converge toward a single sheet at the limit of our observations, but the total current carried by them remains the same. Using the geometry and occurrence rates of the bifurcated current sheets, we conclude that they are formed by the orbits of relatively high energy current carriers around magnetic field lines. We use the observed distance between the two sheets to constrain the type of current carriers to solar wind protons and sodium ions that are likely of planetary origin. Key Points: Global mapping of Mercury's cross‐tail current sheet reveals a dominantly bifurcated structureThe current sheet structure is stable against long‐period (>10 hr) variations in magnetospheric and external conditionsThe structure is consistent with planetary sodium ions as the primary current carriers in the near‐tail region and protons in the far‐tail
- Subjects
CURRENT sheets; MAGNETOTAILS; MERCURY (Planet); SOLAR wind; MAGNETOSPHERE
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 11, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2021JA029417