Equinoctial transitions in the ionosphere and thermosphere

Equinoctial summer/winter transitions in the parameters of the F2-region ar e analyzed using ground-based ionosonde and incoherent scatter observations . Average transition from one type of diurnal NmF2 variation to another tak es 20-25 days, but cases of very fast (6-10 days) transitions are observed as well. Strong day-time NmF2 deviations of both signs from the monthly med ian, not related to geomagnetic activity, are revealed for the transition p eriods. Both longitudinal and latitudinal variations take place for the amp litude of such quiet time NmF2 deviations. The summer-type diurnal NmF2 var iation during the transition period is characterized by decreased atomic ox ygen concentration [O] and a small equatorward thermospheric wind compared to winter-type days with strong poleward wind and increased [O]. Molecular N-2 and O-2 concentrations remain practically unchanged in such day-to-day transitions. The main cause of the F2-layer variations during the transitio n periods is the change of atomic oxygen abundance in the thermosphere rela ted to changes of global thermospheric circulation. A possible relationship with an equinoctial transition of atomic oxygen at the E-region heights is discussed.