Abstract
Abstract. Equinoctial summer/winter transitions in the parameters of the F2-region are analyzed using ground-based ionosonde and incoherent scatter observations. Average transition from one type of diurnal NmF2 variation to another takes 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 median, not related to geomagnetic activity, are revealed for the transition periods. Both longitudinal and latitudinal variations take place for the amplitude of such quiet time NmF2 deviations. The summer-type diurnal NmF2 variation during the transition period is characterized by decreased atomic oxygen concentration [O] and a small equatorward thermospheric wind compared to winter-type days with strong poleward wind and increased [O]. Molecular N2 and O2 concentrations remain practically unchanged in such day-to-day transitions. The main cause of the F2-layer variations during the transition periods is the change of atomic oxygen abundance in the thermosphere related to changes of global thermospheric circulation. A possible relationship with an equinoctial transition of atomic oxygen at the E-region heights is discussed.Key words. Atmospheric composition and structure (thermosphere – composition and chemistry) – Ionosphere (ionosphere- atmosphere interactions; ionospheric disturbances)
Highlights
Two types of diurnal foF2 variation have been known for many years (e.g. Yonezawa, 1959)
The summertype diurnal NmF2 variation during the transition period is characterized by decreased atomic oxygen concentration [O] and a small equatorward thermospheric wind compared to winter-type days with strong poleward wind and increased [O]
Perturbations of the F2-layer, due to geomagnetic disturbances, mask the date of transitions but the analysis has shown that, usually, the transition period ends with a geomagnetic storm after which a new type of the diurnal foF2 variation is established
Summary
Two types of diurnal foF2 variation (winter and summer) have been known for many years (e.g. Yonezawa, 1959). In the late 60s, was probably the first to show the effect of equinoctial transitions in the F2-region parameters using Millstone Hill incoherent scatter observations; he proposed a relationship of this effect with changes in the global thermospheric circulation (Evans, 1970, 1973, 1974) According to his observations the main differences between the winter and summer F2-region are the following: 1) large diurnal NmF2 variations in winter (up to an order of magnitude), while in summer the NmF2 day/night ratio is only about a factor of 2; 2) the maximum in the diurnal NmF2 variations takes place around 13 LT in winter, while in summer it shifts towards 18–20 LT, a morning peak can frequently occur; 3) summer day-time hmF2 values are higher by about 20 km and in summer the layer is broader than in winter for the same geophysical conditions. There are theoretical and experimental indications of strong and sudden changes in the thermospheric circulation pattern around equinoxes and related changes in neutral composition
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