Presentation #215.07D in the session Planetary Space Physics Talks (Oral Presentation)
In situ measurements of thermospheric and ionospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to cold gas and plasma. For electrostatic analyzers measuring cold ions, the instrument response function can be comparable to the ion temperature, so the data must be carefully corrected for background and instrument response for accurate ion temperatures to be calculated. We have used data from the SupraThermal And Thermal Ion Composition (STATIC) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) to derive temperatures of O2+in Mars’ ionosphere, correcting for instrument response and background on over 10,000 MAVEN orbits spanning a wide range of local time, latitude, and altitude.
We show that ion temperatures are typically enhanced over the neutral atmospheric temperature by dozens of Kelvins, even deep in the atmosphere where high collision rates are expected to force ions and neutrals to the same temperature. We have eliminated many possible energy sources for ions, including photoionization, chemical heating, Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. Our results suggest that a fundamental piece of physics is missing from our understanding of energy balance in the Martian ionosphere.
We have also fit drifting Maxwell-Boltzmanns to each STATIC distribution in order to determine whether suprathermal ions are present. Suprathermal ions are observed at altitudes just above the exobase at all local times. The effects of remanent crustal magnetism on ion distributions are also investigated. Our results suggest that Martian crustal fields shield planetary plasma from energization via the solar wind interaction on the dayside, while nightside crustal fields provide conditions under which ion energization is enhanced.
Our results provide important context for understanding how ions are accelerated from low energies at low altitudes to escape energy at high altitudes. Understanding ion energization processes is critical to forming a complete picture of cold ion outflow at Mars and other unmagnetized bodies, and therefore understanding how planetary atmospheres can escape to space.