The Martian Ionospheric Response to the Co‐Rotating Interaction Region That Caused the Disappearing Solar Wind Event at Mars.

An unusually low density solar wind event was observed in December 2022 moving past both Earth and Mars. The source was traced back to a coronal hole and active region on the Sun's surface. The resulting solar wind lead to the development of a co‐rotating interaction region (CIR) and trailing rarefaction region that lasted for multiple solar rotations. Within this structure, the solar wind conditions, including density, velocity, and magnetic field magnitude and orientation drastically changed. In this study we analyze the response of the Martian ionosphere using MAVEN data to these changing solar wind conditions. The low density solar wind region associated with the December event resulted in the expansion of the Martian ionospheric boundaries. We show that the ion composition boundary (ICB) is located at extreme altitudes that are beyond previously observed locations from the MAVEN mission between 2015 and 2018. Furthermore, the boundary between shocked solar wind and the Martian ionosphere identified using electron and ion data moved together on the dayside of the planet with the changing solar wind conditions. However, at the flank region these boundaries do not move together, and we show here that the decoupling of the two boundaries may be the result of a change in the interplanetary magnetic field azimuthal angle. Plain Language Summary: The Sun constantly emits fast moving charged particles into interplanetary space in what is known as the solar wind. In December 2022, a region of fast moving solar wind overtook a region of slower moving solar wind. The fast solar wind acted as a broom sweeping up and compressing the slower wind preceding it. This interaction of two solar wind speeds is observed to rotate with the Sun. Trailing this interaction region is a region of low density solar wind. In December 2022, this type of interplanetary solar wind structure interacted with Mars' electrically charged atmosphere, or ionosphere. Throughout this period, the ionosphere boundary characterized by a transition from solar wind ions to planetary ions moved up and down relative to the planet near dusk. As the rarefaction region passed through the system, the ionosphere expanded to unusually high heights as it was not compressed by the solar wind. However, the flapping of the ionospheric boundary seems to depend on the orientation that the solar wind interacts with Mars. Key Points: The disappearing solar wind event observed at Mars in December 2022 was caused by a co‐rotating interaction regionMars' ionosphere expanded further than previously observed locations between 2015 through 2018The flank ion composition boundary experiences large fluctuations in altitude after a change in the interplanetary magnetic field direction [ABSTRACT FROM AUTHOR]