Variability of Upstream Proton Cyclotron Wave Properties and Occurrence at Mars Observed by MAVEN

The presence of plasma waves upstream from the Martian bow shock, with frequencies near local proton cyclotron frequency in spacecraft frame, constitutes, principle, an indirect signature for existence planetary protons ionization exospheric hydrogen. In this study, we determine "proton wave” (PCW) occurrence rate between October 2014 and February 2020, based on Magnetometer Solar Wind Ion Analyzer measurements Mars Atmosphere Volatile EvolutioN mission. We characterize its dependence several wave solar wind (SW) properties, longitude ranges. confirm a previously reported long-term trend more PCWs perihelion, likely associated changes hydrogen density. Furthermore, report first time decrease median PCW amplitude each consecutive perihelion. Such variability cannot be attributed to differences distribution SW conditions. This could inputs, foreshock effects, asymmetries due convective electric field influencing newborn protons. addition, observe frequently low intermediate interplanetary magnetic (IMF) cone angles, slower speeds, higher densities. IMF angle preference results trade-off linear growth rates, saturation energies, pick-up Moreover, dependencies speed density indicate importance characteristic transit timescale charge exchange process coupling exosphere.