The equivalent slab thickness of Mars' ionosphere: Implications for thermospheric temperature

Abstract

The total electron content (TEC) of a planetary ionosphere is dominated by plasma near and above the height of maximum electron density (Nmax). The ratio TEC/Nmax represents the thickness (τ) of a TEC slab of uniform density (Nmax). For a photochemical ionosphere, τ relates to the scale height (H = kT/mg) of the ionized neutral gas as τ ~ 4 × H. Derived temperatures refer to ~160 km in thermosphere height—below the asymptotic temperature of the exosphere. The MARSIS instrument on Mars Express has produced data sets of TEC and Nmax. We used them to form τ patterns versus solar zenith angle and solar cycle phase. For daytime (SZA < 90°) conditions, <τ > day ~ 50 km, decreasing rapidly for solar zenith angle (SZA) > 90° to < τ > night ~ 25 km. These correspond to Tn values of 250°K and 125°K. Using Mars Global Surveyor data, τ patterns show a mild dependence upon the solar EUV flux proxy F10.7, with ΔTn(°K) ~ 0.3° per unit change in F10.7 at Mars.

The Total Electron Content of the Martian Ionosphere From MRO/SHARAD Observations

“…Given that the TEC integral is dominated by N e ( h ) contributions from the photochemical equilibrium (PCE) domain of h ≤ 200 km, Mendillo et al () applied PCE formalism to TEC values to create a candidate TEC module for the Mars Initial Reference Ionosphere (MIRI): …”

“…To use the extensive SHARAD database to create an improved TEC module for MIRI, we adopted a new approach that uses all available data for a parameterization—rather than calibrating a PCE equation using the best defined subset of the observations. The procedure is as follows: As done previously, assume that TEC is dominated by PCE conditions (Cartacci et al, ; Lillis et al, ; Mendillo et al, ) for heights ranging from the bottomside ionosphere to the MRO orbit at 270 km. Each SHARAD TEC value at distance ( d ) was thus transformed to its equivalent TEC value at 1.524 AU using the PCE conditions TEC ≈ 1/ d . For the date of a SHARAD TEC value, the difference in orbital longitudes between Earth and Mars was used to determine the “rotated Sun date” described above and thus arrive at the solar flux proxy at 1.524 AU given by equation . All TEC values, solar fluxes, and SZA angles at 1.524 AU were then used to derive their best fit linear relationship predicted by equation .…”

“…1. As done previously, assume that TEC is dominated by PCE conditions (Cartacci et al, 2013;Lillis et al, 2010;Mendillo et al, 2015) for heights ranging from the bottomside ionosphere to the MRO orbit at 270 km. Each SHARAD TEC value at distance (d) was thus transformed to its equivalent TEC value at 1.524 AU using the PCE conditions TEC ≈ 1/d.…”

“…The integration of these MGS profiles to achieve the first estimates of TEC and its variability was presented in Mendillo et al (). Subsequent studies used MGS‐derived TEC and N max to estimate atmospheric scales heights and temperatures (Mendillo et al, ). Today, the radio occultation experiment from the Mars Express (MEX) mission (Pätzold et al, ) and new Radio Occultation Science Experiments (ROSE) (P. Withers, MAVEN's radio occultation science experiment (ROSE), private communication, 2017) from the Mars Atmosphere and Volatile Evolution (MAVEN) mission (Jakosky, ) continue to provide N e ( h ) data sets and thus TEC patterns at high solar zenith angles from that technique.…”

The structure and variability of Mars dayside thermosphere from MAVEN NGIMS and IUVS measurements: Seasonal and solar activity trends in scale heights and temperatures

“…These limited dayside temperature measurements included in situ sampling from (a) Viking 1 lander and Viking 2 lander entry accelerometers (based on mass density scale heights) [ Seiff and Kirk , ], (b) Viking 1 lander and Viking 2 lander Upper Atmosphere Mass Spectrometers (UAMS) (based on neutral density scale heights) [ Nier and McElroy , ], (c) the Mars Global Surveyor (MGS) Accelerometer Experiment [e.g., Keating et al , , , ; Bougher et al , ], and (d) the MGS application of the precise orbit determination technique (which was used to derive densities and scale heights from 1999 to 2005) [ Forbes et al , ]. Recently, observations from Mars Express MARSIS (Mars Advanced RADAR for Subsurface and Ionospheric Studies) were used to find the equivalent slab thickness of the ionosphere from which thermospheric temperatures were derived [ Mendillo et al , ]. In addition, remote measurements of key dayglow emissions (e.g., CO Cameron bands and CO ultraviolet doublet (UVD)) were obtained by Mariners 4, 6, 7, and 9 and Mars Express and have been used to extract dayside thermospheric temperatures [e.g., Stewart , ; Stewart et al , ; Leblanc et al , ; Huestis et al , ; Stiepen et al , ].…”