Magnetic reconnection inside the Martian nightside ionosphere during the disappearing solar wind event

We report the first in-situ observation of magnetic reconnection within the nightside ionosphere of Mars during the disappearing solar wind (DSW) event on 26 December, 2022, a region where such processes were previously not expected. Data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission reveal that this event occurs between two open magnetic field lines. This event exhibits a complete set of reconnection signatures, which include changes in the magnetic field topology, clear Hall magnetic field reversal, ion acceleration, and sub-Alfvén outflow. Furthermore, we observed a notable similarity in the pitch angle distributions of photoelectrons and suprathermal electrons. We attribute this phenomenon to the radial expansion of the ionosphere, which caused the upward migration of ionospheric ions and neutrals. This upward movement allowed nightside ionospheric regions above the reconnection site to be illuminated by extreme ultraviolet radiation, leading to the generation of photoelectrons. These photoelectrons subsequently propagated towards the planetary surface along open magnetic field lines together with suprathermal electrons from the solar wind. This process underscores the profound reshaping of magnetosphere-ionosphere coupling dynamics that can occur under extreme solar wind conditions on Mars.