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First Observations of Aphelion Winter Polar Warming in the Mars Thermosphere

Presentation #203.05 in the session The Martian Upper Atmosphere.

Published onOct 20, 2022
First Observations of Aphelion Winter Polar Warming in the Mars Thermosphere

Polar warming in the Mars thermosphere is a phenomena caused by an interhemispheric circulation cell with its upward leg in the summer hemisphere and descending leg in the winter hemisphere. The resulting adiabatic heating causes a sharp increase in temperature at high latitudes in the winter hemisphere, and is predicted to be more pronounced on the nightside. Winter polar warming in the Mars thermosphere was first observed in the northern winter hemisphere using MGS aerobraking data taken near perihelion. Subsequent measurements by other instruments confirmed polar warming in the northern winter hemisphere, but measurements in the southern winter hemisphere remained elusive. While some general circulation models indicated southern winter polar warming should occur, the lack of observational evidence suggested that the requisite circulation cell was possibly too weak at aphelion, due to the lower insolation and corresponding energy input into the summer hemisphere. We present first observations of southern winter polar warming in the Mars thermosphere measured via solar occultations by two independent instruments, the EUVM instrument onboard MAVEN and the NOMAD instrument onboard TGO. The observed warming occurs in the dawn sector and temperatures increase by ~35 K between 30o S and 70o S, which is about half as large as the warming observed at Northern winter. No polar warming is observed in the dusk sector at either Northern or Southern winter. Instead, a cooling trend with increasing latitude is observed in both hemispheres at dusk near aphelion. The dusk cooling trend is more pronounced in the northern hemisphere, which may be due to adiabatic cooling in the summer hemisphere. These observations indicate that, while the magnitude of winter polar warming is smaller at aphelion, sufficient insolation exists for observable southern winter polar warming.

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