Taking up space: the expansion of Mars’ ionosphere in response to the disappearing solar wind density event in December 2022

Mars has a thin atmosphere and ionosphere, but unlike Earth, it does not have an intrinsic magnetic field. However, there are crustal magnetic fields, remnant from when the red planet did have an intrinsic magnetic field, that reside mainly in the southern hemisphere. This produces a hybrid magnetosphere where the Martian ionosphere and crustal fields can directly interact with the solar wind and interplanetary magnetic field. 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 time period presents an excellent opportunity to analyze the importance of solar wind densities in driving 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. The Martian ionosphere and ICB during the disappearing solar wind event on December 26th was found to extend beyond any location previously observed throughout the mission. This study demonstrates that electron temperatures and densities are similar to those during nominal solar wind conditions below the ICB, as is expected. Changes in dominant plasma pressure from times with normal density conditions to those with low density are compared. This study indicates how energy from the solar wind drives ionospheric dynamics if new or different regions of current developed based on changes in the magnetic field. The highly unusual disappearing solar wind event observed with MAVEN allows us to isolate the influence of solar illumination on the Martian thermosphere-ionosphere-magnetosphere system. This gives key insight to ionospheric dynamics that are unrestrained by external solar wind forces.