Multi-Fluid MHD Study of the Disappearing Solar Wind Event Observed by MAVEN: Effects of Solar Wind Density

Abstract

Using the latest Mars Multi-fluid Magneto-hydrodynamic (MHD) model (Y. J. Ma et al., 2019, https://doi.org/10.1029/2019ja027091), we examined an extremely low solar wind density event observed by Mars Atmosphere and Volatile Evolution (MAVEN) on 26 December 2022. Simulation results show that the solar wind density plays a crucial role in plasma interactions. The interaction region expands significantly from less than 2 R_M under typical solar wind density conditions to more than 4 R_M along the subsolar line for extremely low solar wind density (<0.1 cm⁻³), consistent with MAVEN observations during the event. Under low solar wind density conditions, the ion pickup plume becomes smaller and is associated with a lower flow speed. Model results also show a nonlinear correlation between the solar wind density and the ion escape rates. Specifically, when the solar wind density is relatively high (>0.2 cm⁻³), the total ion loss rate positively correlates with the solar wind density. However, when the solar wind density is extremely low, ion escape rates are nearly constant despite further decreases in solar wind density. For this event, the solar wind density dropped by over an order of magnitude, while the MHD model predicts the total ion escape rate reduced by approximately a factor of ∼3. Additionally, we investigated possible reasons for the sudden drop in magnetic field strength during the inbound segment of two consecutive orbits and found that a sudden change of the interplanetary magnetic field direction to radial configuration could also attribute to the observed field decrease.