Discovery of a wind-driven diamagnetic current system in the Martian magnetosphere

Magnetic fields around Mars shape the pathways of energy input into, and ion escape from, the Martian atmosphere, thereby potentially affecting the atmospheric evolution of Mars. The Martian magnetic field environment is primarily determined by the global electric current system resulting from the solar wind interaction with the upper atmosphere and crustal magnetic fields of Mars. Recently, it has been recognized that neutral wind-driven ionospheric dynamo currents generate magnetic field perturbations observable in orbits and on the surface of Mars. However, the entire system of near-Mars currents is not fully understood. Here we present a newly identified current system in the Martian magnetosphere directly coupled to the wind-driven ionospheric currents, based on the spherical harmonic analysis of magnetic field data obtained by the Mars Atmosphere and Volatile Evolution spacecraft. This magnetospheric current system can be explained by diamagnetic currents associated with magnetic fields generated by the wind-driven ionospheric dynamo currents. Our results demonstrate a close coupling between the neutral atmospheric dynamics and the high-altitude magnetospheric current system at Mars mediated by wind-driven ionospheric dynamo currents. This reveals a previously unrecognized role of the neutral atmosphere in controlling magnetic field environments of unmagnetized planets.