A ground-to-exosphere Martian general circulation model: 1. Seasonal, diurnal, and solar cycle variation of thermospheric temperatures

We present the extension to the thermosphere of a Martian general circulation model, the first able to self-consistently study the whole Martian atmosphere from the surface to the exosphere. We describe the parameterizations developed to include physical processes important for thermospheric altitudes. The results of a simulation covering 1 full Martian year are presented, focusing on the seasonal, diurnal, and day-to-day variability of the temperatures in the exobase region. The seasonal variation of the zonal mean temperatures in the upper atmosphere is of about 100 K, mostly due to the variation of the solar forcing. The temperature of the mesopause ranges between 115 and 130 K, with little seasonal and day-night variations. Its pressure level undergoes significant seasonal and day-night variations. Comparisons with SPICAM observations show that the modeled mesopause is too low and too warm. A similar study for the homopause shows that it is located higher in the atmosphere during solstices, owing to reinforced mixing by a stronger circulation. Important day-night temperature differences are found in the thermosphere, ranging from about 60 K at aphelion to 110 K at perihelion. This diurnal cycle is slightly perturbed by the day-to-day variations of temperature, dominated by waves with periods of 2 to 6 sols and amplitude of 30 K. The model reproduces the observed solar cycle variation in temperatures when using a UV heating efficiency of 16%, slightly lower than the theoretical value. The seasonal variation of temperatures is overestimated by the model, in comparison with the available measurements.