North-South Plasma Asymmetry Across Mercury's Near-Tail Current Sheet

Among nearly 300 near-Mercury tail current sheet crossings performed by the MESSENGER spacecraft, we identified 37 traversals of an asymmetric current sheet, wherein the lobe densities on opposite sides differ by a factor of three or more. These asymmetric current sheet crossings primarily occur on the dawnside. A global magnetohydrodynamic (MHD) simulation was found to be in excellent agreement with the observations. The results suggest that the north-south density asymmetry is caused by solar wind entering via an upstream-connected window in one hemisphere. Furthermore, the Parker spiral interplanetary magnetic field (IMF) controls the near-tail density asymmetries, whereas Mercury's offset dipole magnetic field controls those in mid- or distant-tail regions. We propose that hemispheric asymmetries in Mercury's magnetospheric convection occur under strong IMF conditions. Mercury possesses a small magnetosphere owing to its weak planetary magnetic field and strong interactions with the solar wind in the inner heliosphere. The transport process of the solar wind mass and energy into its magnetosphere remains unclear. Previous MESSENGER observations suggest that although the Earth-like plasma mantle is detected inside the near-tail magnetopause in normal IMF magnitudes, it is not a permanent feature of Mercury's magnetosphere. Here we report, for the first time, that solar wind ions can enter deep into the near-tail region via an upstream-connected window in one hemisphere and form a density-asymmetric current sheet under strong IMF conditions. Through MHD simulations, we revealed tail dawn-dusk asymmetries during the transport of solar wind plasma. Advanced data expected from BepiColombo will further improve our understanding of the solar wind-magnetosphere coupling. North-south asymmetries exist in Mercury's magnetotailBoth the interplanetary magnetic field (IMF) BX polarity and intrinsic offset dipole magnetic field control the north-south density asymmetry in Mercury's tailThe IMF Parker spiral results in a dawnside preference for the plasma asymmetric current sheet