Seasonal Transitions in the Thermosphere Inferred from Ionospheric Observations

Ionospheric observations along with CHAMP/STAR neutral gas density measurements were used to retrieve thermospheric parameters and to check whether the equinox transition season exists separately from the December solstice and June solstice seasons. Juliusruh and Boulder ionosonde stations located in "far-from-pole" and "near-pole" longitudinal sectors were analyzed during deep solar minimum in 2008-2009. The results were compared to GOLD column O/N-2 ratio observations. The retrieved thermospheric parameters have shown that equinoctial transition period exists separately from the winter one at Juliusruh, while column O/N-2 ratios, exospheric temperatures Tex, and vertical plasma drifts related to thermospheric winds retrieved at Boulder for the winter season do not significantly differ from vernal values. This means that the December solstice season just does not exist as it merges with the vernal season in the "near-pole" longitudinal sector. The obtained results indicate that two longitudinal sectors manifest different seasonal variations both in thermospheric circulation and neutral composition.