Solar-wind-dependent streamline model for Mercury's magnetosheath: A hydrodynamic magnetosheath model for Mercury

Context. Mercury's magnetosphere and magnetosheath are unique in the Solar System plasmas as they are highly time dependent, since the planet has only a small-scale magnetosphere originating from the weak intrinsic planetary magnetic field. Yet, it is believed that the plasma therein reaches a quasi-stationary state, that is, the ground state of magnetospheric dynamics, when the solar wind smoothly passes by the magnetosphere without energy or momentum exchange in quiet conditions. Aims. Here, we aim to construct a semi-analytical streamline model for Mercury's magnetosheath to extend the modeling effort from the magnetospheric plasma to the magnetosheath plasma. The magnetosheath model should have the capability of determining the plasma density and the bulk velocity as a function of the radial distance from the planet, the zenith angle to the Sun, and the solar wind condition. Methods. Our magnetosheath model was constructed with (1) the steady-state continuity equation around a magnetospheric obstacle where the bow shock and magnetopause location may depend on the solar wind condition, (2) the jump conditions at the bow shock, and (3) the adiabatic behavior in the magneotsheath. Results. Our magnetosheath model reasonably explains and reproduces the in-situ measurements around Mercury by the MErcury Surface, Space Environment, GEochemistry and Ranging (MESSENGER) spacecraft as well as numerical simulations. Conclusions. The presented streamline model of Mercury's magnetosheath serves as a useful tool for the on-going two spacecraft BepiColombo mission when analyzing the plasma data by tracing the plasma parcel along the streamline both forward from one spacecraft to another and backward, locating the shock crossing coordinate, or when estimating the elapsed time of plasma parcel after the shock crossing.