Characterizing Plasma Depletion Events on Mars: Spatial and Temporal Dynamics from MAVEN and Mars Express

The Martian ionosphere is primarily influenced by solar radiation on the dayside, while on the nightside, it is controlled by impact ionization from precipitating particles and the transport of ions from the dayside. Occasionally, the ionosphere exhibits abrupt plasma density reductions—characterized by an order-of-magnitude decrease relative to the background—referred to as Plasma Depletion Events (PDEs). These events, often accompanied by elevated electron temperatures and electrostatic fluctuations, are poorly understood yet potentially critical to understanding ion escape and ionospheric variability. Characterizing their dimensions, recurrence, and temporal behavior provides valuable insight into the plasma environment of Mars. This study investigates over 1,000 PDEs detected by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft between October 2014 and May 2021. By analyzing recurring MAVEN orbits, we identify 80 PDEs reappearing at the same locations within 18 to 30 hours, suggesting that these events may recur periodically. Additionally, conjugate observations by MAVEN and Mars Express reveal that PDEs can span up to 750 km and persist for several hours. These findings suggest PDEs to be large-scale, recurring phenomena with implications for plasma instabilities, ion escape, and Martian ionospheric dynamics.