The Impact of Crustal Magnetic Fields on the Thermal Structure of the Martian Upper Atmosphere

Abstract

Using the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer data, we investigate the possible impact of crustal magnetic fields on the thermal structure of the Martian upper atmosphere. Our analysis reveals a clear enhancement in temperature over regions with strong crustal magnetic fields during two deep dip campaigns covering the periods of April 17-22 and September 2-8, both in 2015. Several controlling factors, such as solar EUV irradiance, relative atomic O abundance, and non-migrating tides, do not help to explain the observed temperature enhancement, and a magnetically driven scenario is favored. We evaluate the roles of several heating mechanisms that are likely modulated by the presence of crustal magnetic fields, including Joule heating, ion chemical heating, as well as electron impact heating via either precipitating solar wind electrons or locally produced photoelectrons. The respective heating rates of these mechanisms are substantially lower than the solar EUV heating rate, implying that none of them is able to interpret the observations.

Ionization Efficiency in the Dayside Martian Upper Atmosphere

“…Since photoelectrons also play a crucial role in the local energy balance of both neutrals and thermal electrons, we expect enhanced neutral and electron temperatures encountered near magnetic anomalies as well. The observation of enhanced neutral temperature was reported by Cui et al (2018) with the aid of the NGIMS data acquired during several DD campaigns (see also Leblanc et al 2006), but these authors argued that photoelectron trapping was unlikely to be a viable mechanism since the difference in photoelectron impact heating between regions with and without strong crustal magnetic fields was far insufficient to account for the difference in neutral temperature. Meanwhile, Flynn et al (2017) showed that regions over magnetic anomalies featured low electron temperatures, in contrast to our ideal expectation.…”

“…These measurements were recently used to examine the ionization balance (Mendillo et al 2017) and thermal structure (Cui et al 2018) in the Martian upper atmosphere. The line-ofsight optical depth for photoabsorption at EUV/X-ray is typically smaller than unity for a nominal MAVEN orbit, with extinction not exceeding 5% near the periapsis as long as the portion of the orbit below 250km remains sunward of 60°.…”

The Morphology of the Topside Martian Ionosphere: Implications on Bulk Ion Flow

“…Here the neutral temperature is obtained from the isothermal fitting to the respective CO 2 densities measured by the NGIMS in the Closed Source Neutral mode (Mahaffy, Benna, Elrod, et al, ). Only the inbound NGIMS data are used to avoid possible contamination by wall adsorption and chemistry (Cui et al, ). The above parameterization of the ion temperature is likely subject to considerable uncertainties, but these uncertainties should not be important for the purpose of the present study due to the dominance of the magnetic pressure gradient force (see below).…”

Structural Variability of the Nightside Martian Ionosphere Near the Terminator: Implications on Plasma Sources

“…The spherical nature of the atmosphere is taken into full account (Smith & Smith, ). Only the inbound portion of each orbit is included to avoid contamination by possible reactions on the NGIMS antechamber walls (e.g., Cui et al, ). The photoionization rate profiles obtained for all orbits are then combined to construct the median distribution of the photoionization rate with respect to altitude and SZA for each of the four geophysical conditions.…”