Martian Electron Temperatures in the Subsolar Region: MAVEN Observations Compared to a One‐Dimensional Model

Abstract

Prior to the Mars Atmospheric Volatile EvolutioN (MAVEN) mission, altitude profiles of the electron temperature in the Martian thermosphere were measured only twice. Because the rates of several geophysically important processes depend strongly on the electron temperature, models of the Martian thermosphere and atmospheric escape rates have not been well constrained. In this paper, we use densities and temperatures measured by MAVEN instruments and the one‐dimensional model of Matta et al. (2014, https://doi.org/10.1016/j.icarus.2013.09.006) to test our understanding of the processes that determine the electron temperature. Our analysis is limited to inbound orbits where the magnetic field is within 30° of horizontal and the satellite is within 30° of the subsolar point at altitudes from 120 to 250 km. We introduce empirically adjusted electron temperatures below 180 km, where the MAVEN electron temperature measurements are known to be biased high. We introduce the concept of a local electron heating efficiency, which we define at a given altitude as the ratio of electron heating from photoionization to the total extreme ultraviolet energy deposited. Our analysis shows that MAVEN observations are consistent with the one‐dimensional model below ~210 km if the electron heating efficiency varies with altitude, and the electron temperature is within the empirical bounds below 180 km we introduced. It indicates that above ~210 km electron heat conduction dominates extreme ultraviolet heating in determining electron temperature. Our analysis also suggests that in the subsolar region electrons and neutrals are in thermal equilibrium below 120 km.

Low Electron Temperatures Observed at Mars by MAVEN on Dayside Crustal Magnetic Field Lines

“…The measured electron temperature is higher when EUV irradiances are higher in both crustal and draped‐field regions except for the very highest EUV case (which we do not understand), and it is consistent with results of the EUV deposition, which takes place in the ionosphere below 210‐km altitude (Peterson et al, ). It suggests that the high solar irradiance raises the electron temperature.…”

Subsolar Electron Temperatures in the Lower Martian Ionosphere

“…The black line with cross symbols in Figure shows energy deposition associated with photoionization calculated from the neutral densities shown in Figure and a solar EUV spectrum constrained by MAVEN observations (Thiemann et al, ) and available as a Level 3 data product. The other lines in Figure show the sum of thermal electron cooling rates, convection, advection, and expansion rates calculated using MAVEN data shown in Figures and and the five low‐altitude temperature extrapolations shown in Figure as described in Peterson et al (). Note that these calculations and those by Peterson et al () use the Campbell et al () inelastic e‐CO 2 cross section instead of the Delgarno () cross sections given as equation A12 by Matta et al ().…”

“…The other lines in Figure show the sum of thermal electron cooling rates, convection, advection, and expansion rates calculated using MAVEN data shown in Figures and and the five low‐altitude temperature extrapolations shown in Figure as described in Peterson et al (). Note that these calculations and those by Peterson et al () use the Campbell et al () inelastic e‐CO 2 cross section instead of the Delgarno () cross sections given as equation A12 by Matta et al (). This approach includes electron—ion and electron—neutral collisions and approximates the ion temperature (T I ) as the average of T E and T N .…”

“…Below ~700 K, MAVEN electron temperatures reported in the Level 2 LPW data product are known to be biased high (Ergun et al, ; Fowler, ; Peterson et al, ). The upward bias in the value reported in the Level 2 data likely arises from nonideal behavior with regards to the operation of the Langmuir probe in the highly variable plasma environment at Mars, including variable sheath shape and size, ion wakes behind the probes, and nonuniform probe surfaces due to atomic oxygen contamination of the probe surfaces (Fowler, ).…”

“…Thus, the photoelectron flux observations above 3 eV shown in Figures and do not directly determine electron heating rates; the secondary electrons with energies below 3 eV they produce both locally and deeper in the atmosphere determine electron heating rates. Peterson et al () developed an alternative method to calculate the local electron heating rates, which is used in this paper and discussed below.…”