MAVEN/NGIMS Dayside Exospheric Temperatures over the Solar Cycle and Seasons: The Role of Dayside Thermal Balances in Regulating Temperatures

Modern Earth global warming of its lower atmosphere is complemented by enhanced CO2 15-micron cooling in its upper atmosphere. This CO2 cooling process also has a counterpart in the CO2 dominated upper atmospheres of Venus and Mars. From the MAVEN mission, significant and largely periodic variability of Mars dayside exospheric temperature is now revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) datasets collected throughout solar cycle 24. These NGIMS dayside mean exospheric temperatures (Texo) are shown to vary by ~80 K (~180-260 K) over solar cycle 24. This corresponds to a ΔTexo/ΔEUV sensitivity of ~50 K m² mW^-1, where Lyman-α is the solar index. Previous Mars Global Surveyor (MGS) derived Texo values (~170-300 K) yield a sensitivity of ~45 in the same units, for the much stronger solar cycle 23. This close correspondence suggests that the underlying dayside energy balances are similar over these two solar cycles. Corresponding Mars Global Ionosphere-Thermosphere Model (M-GITM) simulations show that molecular thermal conduction largely balances EUV heating for the Mars dayside thermosphere, while CO2 15-micron cooling is secondary in importance, along with global winds. It is valuable to compare this Mars sensitivity to that computed for the dayside thermosphere of Venus. Pioneer Venus dayside datasets imply a sensitivity of ~16-19 units for solar cycle 21, a factor of ~3 smaller than computed for Mars. This is so because Venus CO2 cooling strongly balances EUV heating near its peak, thereby serving as an efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars, leaving molecular thermal conduction to largely balance the EUV heating layer, thereby yielding larger solar cycle variations in Texo. It is noteworthy that numerical model predictions of ancient Mars exospheric temperatures have been shown to be strongly regulated by enhanced CO2 15-micron cooling rates, similar to present day Venus.