Upstream Proton Cyclotron Waves at Mars During the Passage of ICMEs

The presence of proton cyclotron waves (PCWs) upstream from Mars indicates picked up protons originating from the Martian extended hydrogen (H) exosphere by the solar wind and loss of H to interplanetary space. The occurrences and properties of PCWs associated with normal solar wind conditions have been extensively investigated since their initial detection about 32 years ago, but little attention has been paid to PCW activity under extreme space weather conditions. Here we characterize PCWs during the passage of interplanetary coronal mass ejections (ICMEs) through the Martian environment using the measurements by the Mars Atmosphere and Volatile EvolutioN spacecraft from December 2014 to February 2019. It is found that PCWs occur more frequently during perihelion periods (Ls = 180°–360°) than during aphelion periods (Ls = 0°–180°) for disturbed times of ICME passage. In the perihelion season, the PCW occurrence rate is increased on average by a factor of about 2 during the ICME phase relative to the pre‐ICME and post‐ICME phases. Moreover, PCWs reveal more pronounced wave characteristics with larger amplitudes, higher ellipticity, and smaller propagation angle with respect to the ambient magnetic field direction during the ICME phase. Plain Language Summary Proton cyclotron waves (PCWs) upstream from Mars are generated by pickup of planetary protons originating from the extended hydrogen exosphere, and thus have important implications for understanding the loss of H to interplanetary space and therefore the water escape history. They have attracted considerable attention since their initial detection in 1989, focusing primarily on their properties associated with the nominal solar wind. Little attention has been paid to PCW activity during extreme space weather events, such as interplanetary coronal mass ejections (ICMEs). This work aims to explore the statistical properties of upstream PCWs during the passage of ICMEs. The results indicate that, in the perihelion season, the PCW occurrence rate is increased by a factor of about 2 during the ICME phase relative to the quiet phases and moreover PCWs show more pronounced wave characteristics during the ICME phase. Key Points Proton cyclotron waves (PCWs) occur more frequently during perihelion periods than during aphelion periods for disturbed times of interplanetary coronal mass ejection (ICME) passage The PCW occurrence rate is increased by a factor of ∼2 during the ICME phase relative to the pre‐ICME an