Inferring IMF Orientation From Martian Current Sheet Crossings Using MAVEN Data

The flowing solar wind carries with it the Interplanetary Magnetic Field (IMF), and the directions of the two together determine the orientation of the solar wind convection electric field. Since the solar wind velocity varies little in its direction (toward Mars from the Sun), the orientation of the electric field is controlled by the `clock angle' of the IMF, or the direction of the IMF perpendicular to the Mars-Sun direction. The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has observed that particle escape is organized by the electric field direction. However, due to its precessing elliptical orbit, MAVEN spends months at a time without visiting the unperturbed solar wind. Thus, the IMF is not directly measured, and we cannot properly organize measurements of escaping plasma during these periods. Here we present a proxy estimate for the IMF clock angle during times when measurements in the solar wind are not possible. We use the orientation of the cross-tail current sheet formed in the draped two-lobed magnetotail behind Mars, which should be directly related to the upstream IMF clock angle. We infer the clock angle direction based on of the normal vector to the current sheet, as determined by Minimum Variance Analysis. To validate the proxy, we analyzed orbits when the spacecraft both crossed the current sheet and entered into the solar wind. For 24% of total MAVEN orbits, we can approximate the IMF orientation to within 82° at a 1σ level. We note that the IMF is likely to rotate during the time (≈2.5 hours) between when the spacecraft takes upstream IMF measurements and when it crosses the current sheet. Using unperturbed solar wind data at Earth, we find that the IMF clock angle remains consistent to within 72° at a 1σ level over 2.5 hours.