The Martian Ionospheric Response to the Co-rotating Interaction Region that caused the Disappearing Solar Wind Event in December 2022.

Abstract

In December 2022, the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observed a highly unusual drop in solar wind proton densities below 1 particles/cc over the course of 2 days; normally, these densities are on the order of ~10 particles/cc. For ~8 hours on December 26, 2022, the solar wind “disappeared” to densities below 0.2 particles/cc. This study presents evidence that the disappearing solar wind event was caused by a Corotating Interaction Region (CIR) (see Figure). CIRs are caused by fast moving solar wind overtaking preceding slow moving solar wind, leaving a wake of low-density solar wind plasma trailing behind the CIR interface. This event presents an excellent opportunity to analyze the importance of different solar wind conditions in driving dynamics in the Martian ionosphere and the induced magnetosphere. The statistical location where heavy cold ionospheric plasma transitions to shocked solar wind plasma, which at Mars is called the Ion Composition Boundary (ICB), has been analyzed over the entire MAVEN mission. Changes in dominant plasma pressure from different parts of the CIR structure are compared to determine the main forcing mechanisms of Martian ionospheric dynamics. For example, as the foreshock of the CIR interacts with Mars, high solar wind dynamic pressure compresses the ionosphere over a ~3.5 hour period. During the rarefaction period following the CIR, which coincides with the disappearing solar wind event on December 26ᵗʰ, the Martian ionosphere extended beyond any location previously observed throughout the mission (see bottom Figure). This study indicates how energy from the solar wind drives ionospheric dynamics if new or different regions of current develop based on changes in the solar wind and interplanetary magnetic field. The highly unusual disappearing solar wind event observed by MAVEN allows us to isolate the influence of solar wind conditions on the Martian thermosphere-ionosphere-magnetosphere system. Analysis of Mars' ionospheric response to the disappearing solar wind event gives key insight to the prominent forcing mechanisms, which are unrestrained by external solar wind forces.