The Mars Thermosphere Terminator Structure and Variability Over a Solar Cycle.

Introduction:  The Extreme Ultraviolet Monitor (EUVM) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter measures thermospheric density from about 110 to 200 km via solar occultations using its 17-22 nm channel in addition to EUVM’s primary solar irradiance measurement [1]. The EUVM solar occultation (SO) data record [2] ranges from late 2014 through the present (2024), spanning nearly an entire solar cycle and 5 Mars years. Features inherent to solar occultations, namely their insensitivity to absolute calibration and inherently fixed local time, allow for the characterization of long-term variability of the upper atmosphere. 

Solar EUV Variability:  Solar EUV irradiance is the primary energy input to the Mars upper atmosphere. The EUVM SO data are analyzed for the dependence of exospheric temperature on solar EUV forcing. In general, these results are consistent with prior measurements of exospheric temperature sensitivity to EUV irradiance. However, the extensive nature of the EUVM SO data, spanning 5 Mars Years and much of a solar cycle, show that EUV temperature sensitivity varies between dawn and dusk and across seasons: The EUV temperature sensitivity tends to be higher at dawn and weaker at aphelion.

Latitudinal and Local Time Structure:  The EUV-induced variability is detrended from the EUVM SO data in order to characterize latitudinal and local time dependences. Thermospheric Polar Warming is evident in the winter pole at perihelion, as expected from measurements prior to MAVEN’s arrival at Mars. The EUVM SO data also show the first conclusive Thermospheric Polar Warming at the winter pole at aphelion [3]. This warming is observed to increase with the increase of dust in the summer hemisphere.

Seasonal Variability: The EUV-detrended data are also characterized for seasonal variability. Globally, there is no discernable seasonal variability beyond the modulation of solar EUV irradiance due to the Mars orbit eccentricity.

Sensitivity to Gravity Waves: The Mars Global Ionosphere Thermosphere Model (MGITM) [4] with a whole atmosphere nonlinear gravity wave parametrization [5] is compared to the EUVM SO observations to determine when small-scale gravity waves significantly influence exospheric temperature by adding and removing gravity waves to the model. In many cases, but not all, the inclusion of gravity waves in MGTIM yields better model-measurement agreement.

References:

[1] Eparvier, F. G., et al. Space Science Reviews 195 (2015): 293-301. [2] Thiemann, E. M. B., et al. Journal of Geophysical Research: Planets 123.9 (2018): 2248-2269. [3] Thiemann, Edward MB, et al. Geophysical Research Letters 51.5 (2024): e2023GL107140. [4] Bougher, S. W., et al. Journal of Geophysical Research: Planets 120.2 (2015): 311-342. [5] Yiğit, Erdal, et al. Journal of Geophysical Research: Atmospheres 114.D7 (2009).