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 ( τ ) of a TEC slab of uniform density ( N max ). 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 N max . 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 T n values of 250°K and 125°K. Using Mars Global Surveyor data, τ patterns show a mild dependence upon the solar EUV flux proxy F 10.7 , with Δ T n (°K) ~ 0.3° per unit change in F 10.7 at Mars.