Characterising Mars’ extended hydrogen exosphere from waves at the local ion cyclotron frequency

Ultra-low frequency plasma waves at a local ion gyrofrequency have been detected upstream of the bow shock at every planet with an extended atmosphere. These waves are observed as left-hand elliptically polarised, propagating mostly parallel to the ambient interplanetary magnetic field. They originate from solar wind pickup of ionised exospheric neutrals, especially H⁺, for which they are called Proton Cyclotron Waves (PCWs), and depend on the cone angle between the solar wind flow and the magnetic field. Excluding the foreshock, the wave analysis provides constraints for the exospheric species density at the origin of the waves. Using 10 years of magnetometer measurements from MAVEN, we show at Mars how the wave occurrence rate and inferred neutral densities evolve with solar longitude and solar wind cone angle. This method is used to extend to other masses than hydrogen, such as mass 2 (D, H₂), and we discuss the consequences of our results on Mars’ planetary atmospheric evolution.