Ionospheric plasma depletions at Mars as observed by MAVEN

The Martian ionosphere, modulated by the solar wind from the topside and by remnant crustal magnetic fields close to the surface, possess unique structures different from Earth and Venus. Integrated observations by the plasma and magnetic field instruments onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft show clear evidence of ionospheric plasma depletions, independent of seasonal variations at Mars. During such depletions, the plasma density of all ionospheric ion species is reduced by up to an order of magnitude and, at the same time, the electron temperature increases abruptly. An automated algorithm for the identification of such plasma depletions is developed. Altogether, as many as 580 events are identified in 8619 orbits available from October 2014 to May 2021. A statistical investigation of these events reveals that they are more prominent on the night side and occur at altitudes between 150 and 500 km. Considering the spacecraft velocity and the observed event durations, we suggest that the depletions are bubble-like structures, more elongated horizontally than vertically. A possible mechanism of their formation is discussed.