Disappearing Solar Wind at Mars: Changes in the Mars‐Solar Wind Interaction.

On 26 December 2022 the solar wind density dropped by over an order of magnitude and remained low for about a day. We have utilized in‐situ plasma measurements made by the Mars Atmosphere and Volatile EvolutioN mission to determine how this change affected the Mars‐solar wind interaction. During this time period, on inbound orbit segments, MAVEN sampled the terminator ionosphere, which switched from a magnetized to unmagnetized state immediately following the minimum in solar wind density. The magnetic field amplitude was typically 5–10 nT within the upper ionosphere prior to the event and consistently <1 nT after. During the event the magnetic pressure dominated immediately above the ionosphere while within the ionosphere the ionospheric plasma pressure dominated. The high altitude terminator ionosphere remained in this unmagnetized state throughout the event, suggesting that it was the new equilibrium state of the system. The terminator upper ionosphere returned to its original magnetized state once the solar wind density had recovered. The outbound orbit segments sampled the dayside subsolar region which remained magnetized throughout the event: the magnetization state of the ionosphere varied locally, dependent upon the solar zenith angle and corresponding incident solar wind dynamic pressure. Such conditions are different to the commonly reported unmagnetized ionospheric state at Venus during solar maximum conditions, where the interplanetary magnetic field is repelled from the entire dayside ionosphere. Drastic changes in the upstream solar wind are able to change the Mars‐solar wind interaction state on timescales less than one MAVEN orbit (∼3.5 hr). Plain Language Summary: Our Sun emits a continuous stream of charged particles radially outward into our solar system, known as the solar wind. Much like flowing water in a river is deflected around rocks and other objects in the river, the solar wind is also deflected around most obstacles in our solar system, including the planets. The physical forces that control this deflection differ depending upon the characteristics of the planet and the solar wind as it encounters the obstacle. We have studied a rare event at the planet Mars when the solar wind density dropped to very low values over the course of a day. Using orbiting spacecraft observations we have shown that the physical processes that control how the solar wind flows around the planet change directly in response to this event. These processes reverted back to their prior states once the solar wind density recovered a few days later, demonstrating that the planet‐wide system responds to large changes in the solar wind density on timescales of a few hours or less. Key Points: The Mars terminator ionosphere transitioned to an unmagnetized state in response to the 'disappearing solar wind event' in December 2022The Mars‐solar wind interaction system responds to changes in the solar wind density on timescales <3.5 hrThe event shares similarities with when the Venus ionosphere is unmagnetized, but the localized nature is an important difference [ABSTRACT FROM AUTHOR]