Evidence for superthermal secondary electrons produced by SEP ionization in the Martian atmosphere

The atmosphere of Mars, lacking a global magnetic field, is exposed to the precipitation of solar energetic particles (SEPs), resulting in impact ionization and the production of secondary electrons, some of which may escape the atmosphere. In this study, we examine upward traveling fluxes of superthermal electrons between similar to 100 and 650 eV, measured by the Mars Global Surveyor Magnetometer/Electron Reflectometer at 400 km altitude during nine of the largest and clearest SEP events of the last solar maximum from November 2000 until the Halloween storms of late 2003. We subtract the contribution from backscattered low-energy precipitating electrons and find that, for the highest and most rarely observed SEP fluxes, we detect a statistically significant flux of SEP-produced superthermal electrons escaping the Martian atmosphere. The measured fluxes are found to be in broad agreement with a calculation of expected upward electron fluxes resulting from ionization of neutrals by energetic proton impact. Peak SEP ionization rates on the nightside from the Halloween storms are found to be comparable to (although lower than) typical dayside photoionization rates and at least 3 orders of magnitude higher than average nightside electron impact ionization rates. Further advances in our knowledge of SEP effects on the Martian ionosphere await data from the Radiation Assessment Detector (RAD) instrument on the Mars Science Laboratory rover in 2012 and the MAVEN orbiter in 2014.