Subsolar Electron Temperatures in the Lower Martian Ionosphere

Abstract

Martian subsolar electron temperatures obtained below 250 km are examined using data obtained by instruments on the Mars Atmosphere Evolution Mission (MAVEN) during the three subsolar deep dip campaigns and a one‐dimensional fluid model. This analysis was done because of the uncertainty in MAVEN low electron temperature observations at low altitudes and the fact that the Level 2 temperatures reported from the MAVEN Langmuir Probe and Waves instrument are more than 400 K above the neutral temperatures at the lowest altitudes sampled (~120 km). These electron temperatures are well above those expected before MAVEN was launched. We find that an empirical normalization parameter, neutral pressure divided by local electron heating rate, organized the electron temperature data and identified a similar altitude (~160 km) and time scale (~2,000 s) for all three deep dips. We show that MAVEN data are not consistent with a plasma characterized by electrons in thermal equilibrium with the neutral population above 100 km. Because of the lack data below 120 km and the uncertainties of the data and the cross sections used in the one‐dimensional fluid model above 120 km, we cannot use MAVEN observations to prove that the electron temperature converges to the neutral temperature below 100 km. However, the uncertainty of the electron temperature altitude profile below 120 km does not impact our understanding of the role of electron temperature in determining ion escape rates because ion escape is determined by electron temperatures above 180 km.

Response of Mars's Topside Ionosphere to Changing Solar Activity and Comparisons to Venus

“…We parameterize the electron temperature by adapting the electron temperatures measured during MAVEN Deep Dip 9 and reported in Peterson et al. (2020). Peterson et al.…”

Perspective on Energetic and Thermal Atmospheric Photoelectrons

“…There have been too few simultaneous observations of thermal neutral, ion, and electron temperatures in the high-density region of a planetary thermosphere to confirm or refute the assumptions associated with photoelectron thermalization processes included in current models. However, recent results from the MAVEN spacecraft (Hanley et al, 2020;Peterson et al, 2020) have shown that the observed ion, neutral, and electron temperatures on Mars are not consistent with current models. In particular, Hanley et al have measured ion temperatures using the MAVEN SupraThermal and Thermal Ion Composition (STATIC) instrument (McFadden et al, 2015).…”

“…MAVEN observations of ion temperatures reported by Hanley et al (2020) show that thermalization occurs on Mars at higher density (>10 11 cm −3 ) and lower fractional ionization (<2 × 10 −6 ) than expected. The analysis of observed electron and neutral temperatures presented in Peterson et al (2020) suggests thermalization on Mars occurs below 100 km where the Bougher et al (2015) model gives a density ∼10 12 cm −3 and a fractional ionization of ∼10 −8 . Energetic photoelectron production by solar EUV and X-rays occurs well below Earth's mesopause (∼80 km, density > ∼10 14 cm −3 ) where it is strongly modulated by solar activity (e.g., Chamberlin et al, 2007).…”

“…They include thermal electrons with temperatures of <1 eV or 1.16 • 10 4 • K. The electron data below 3 eV were obtained at Mars from the Langmuir Probe and Waves (LPW) instrument on MAVEN (Andersson et al, 2015). They are presented as a Maxwellian distribution representing the average electron density (1.2 • 10 5 cm −3 ) and average electron temperature (760 • K) observed between 120 and 125 km from April 24 to 30, 2018 during the deep dip #9 interval (Peterson et al, 2020). Above 3 eV the data are from the MAVEN Solar Wind Electron Analyzer (SWEA) instrument (Mitchell et al, 2016) averaged over the same intervals.…”

Impact of the 2018 Mars Global Dust Storm on the Ionospheric Peak: A Study Using a Photochemical Model

“…(2015) cautioned that the electron temperature measured by the LPW instrument may be overestimated for temperatures less than 750 K (<180 km). A comparison between the T e profiles of Wang and Nielsen (2003) with the T e profiles measured by LPW during the deep dip campaigns 2, 8, and 9 (Peterson et al., 2020) showed the latter being larger than the theoretical profiles by a factor of ∼3 at an altitude of 180 km. The work of Mukundan et al.…”