  The impact of a rarest solar wind phenomenon [disappearing solar wind (DSW)
event during 26-28 December 2022] on the Martian nightside ionosphere is
investigated, for the first time, using MAVEN datasets. During an extremely low
solar wind density, the Martian top nightside ionospheric species underwent
significant enhancements. At a given altitude, the density of electron
increased by ~2.5 times, while for ions (NO+, O2+, CO2+, C+, N+, O+, and OH+),
it increased by >10 times, respectively, compared to their typical average
quiet-time periods. The extremely low dynamic pressure during the DSW event,
contrasting with relatively higher nightside ionospheric thermal pressure
nearly by an order, suggests an upward ionospheric expansion as a direct
consequence. This causes an increased topside ionospheric density.
Additionally, the day-to-night plasma transport may also be a contributing
factor to increased plasma density. Thus, this study presents a new insight
about planetary atmosphere/ionosphere response to the rarest event.
