In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere

K G Hanley; J P McFadden; D L Mitchell; C M Fowler; S W Stone; R V Yelle; M Mayyasi; R E Ergun; L Andersson; M Benna; M K Elrod; B M Jakosky

doi:10.1029/2021JA029531

In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere

J Geophys Res Space Phys. 2021 Dec;126(12):e2021JA029531. doi: 10.1029/2021JA029531. Epub 2021 Dec 12.

Authors

K G Hanley¹, J P McFadden¹, D L Mitchell¹, C M Fowler^{1

2}, S W Stone³, R V Yelle³, M Mayyasi⁴, R E Ergun⁵, L Andersson⁵, M Benna^{6

7}, M K Elrod^{6

8}, B M Jakosky⁵

Affiliations

¹ Space Sciences Laboratory University of California Berkeley Berkeley CA USA.
² Department of Physics and Astronomy West Virginia University Morgantown WV USA.
³ Lunar and Planetary Laboratory University of Arizona Tucson AZ USA.
⁴ Astronomy Department Boston University Boston MA USA.
⁵ Laboratory for Atmospheric and Space Physics University of Colorado Boulder Boulder CO USA.
⁶ Center for Research and Exploration in Space Science and Technology NASA Goddard Space Flight Center Greenbelt MD USA.
⁷ University of Maryland Baltimore County MD USA.
⁸ University of Maryland College Park MD USA.

Abstract

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present $O_{2}^{+}$ temperatures in Mars' ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition instrument onboard the Mars Atmosphere and Volatile EvolutioN spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis altitude from the nominal 150 to 120 km for 1 week in order to sample the ionospheric main peak and approach the homopause. We use two independent techniques to calculate ion temperatures from the measured energy and angular widths of the supersonic ram ion beam. After correcting for background and instrument response, we are able to measure ion temperatures as low as 100 K with associated uncertainties as low as 10%. It is theoretically expected that ion temperatures will converge to the neutral temperature at altitudes below the exobase region (∼180-200 km) due to strong collisional coupling; however, no evidence of the expected thermalization is observed. 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. The source of the energy maintaining the high ion temperatures remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere.

Keywords: MAVEN; Mars ionosphere; electrostatic analyzer; ion chemistry; ion temperature; ionosphere.