In situ measurements of thermospheric and ionospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present O2+ temperatures in Mars' ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition (STATIC) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis from the nominal 150 km to 120 km for one week in order to sample the ionospheric main peak and approach the well-mixed lower atmosphere. Two independent techniques are used to calculate temperatures from the measured energy and angular widths of the ion beam. After correcting for background and instrument response, temperatures as low as 100° K can be measured with associated uncertainties as low as 10%. It is theoretically expected that strong collisional coupling will force ion and electron temperatures to converge to the neutral temperature at altitudes below the exobase (~180-200 km). The neutral temperature is obtained from data collected by MAVEN’s Neutral Gas and Ion Mass Spectrometer (NGIMS), and the electron temperature is measured by MAVEN’s Langmuir probe; the expected thermalization is not observed. Instead, ion temperatures are enhanced over the neutral gas temperature by more than 100° K on the dayside, and electron temperatures are even higher (see figure). We have eliminated several possible explanations for the observed temperature difference between ions and neutrals, including Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. Our current study leaves one plausible heating mechanism, the release of internal energy from O2+ that becomes vibrationally excited as a result of atmospheric chemistry, but future work is needed to assess its validity. For more information and references, readers are referred to the preprint of this submitted manuscript (doi: 10.1002/essoar.10506971.1).
Figure: Temperature profiles as a function of CO2 density for O2+ (black), Ar (blue) and electrons (red) measured during the inbound segment of each orbit of the indicated Deep Dip (~40 orbits). Median values are shown by thick lines; shaded regions illustrate upper and lower quartiles. The spacecraft’s solar zenith angle (SZA) at the top and bottom of each profile are indicated on the plots.