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

The total electron content (TEC) of a planetary ionosphere is dominated by plasma near and above the height of maximum electron density (N-max). The ratio TEC/N-max represents the thickness (tau) of a TEC slab of uniform density (N-max). For a photochemical ionosphere, relates to the scale height (H=kT/mg) of the ionized neutral gas as 4xH. Derived temperatures refer to 160km in thermosphere heightbelow the asymptotic temperature of the exosphere. The MARSIS instrument on Mars Express has produced data sets of TEC and N-max. We used them to form patterns versus solar zenith angle and solar cycle phase. For daytime (SZA<90 degrees) conditions, <tau >(day)similar to 50km, decreasing rapidly for solar zenith angle (SZA)>90 degrees to <tau >(night)similar to 25km. These correspond to T-n values of 250 degrees K and 125 degrees K. Using Mars Global Surveyor data, patterns show a mild dependence upon the solar EUV flux proxy F-10.7, with T-n(degrees K)0.3 degrees per unit change in F-10.7 at Mars.