The combined effect of solar and geomagnetic activity on high latitude thermospheric neutral winds. Part I. Observations

Abstract

Long-term averages of Fabry-Perot Interferometer (FPI) observations of the night-time OI ( 1 D) emission at 630 nm from the F-region thermosphere at a high latitude site are presented here. The data base contains measurements of thermospheric neutral winds for every winter period from November 1981 to April 1989 inclusive. This covers nearly one complete solar cycle in terms of the radio and UV/EUV flux variation, from the last solar maximum to the present solar maximum. The instrument is located in Kiruna, Sweden, which is situated at the equatorward edge of the auroral oval at quiet to moderate levels of geomagnetic activity and beneath the oval at higher levels of activity. From this location, the FPI has sampled the response of the upper thermosphere winds at night to a wide range of geomagnetic and solar activity conditions. The data presented here show a significant correlation between the solar radio or EUV fluxes and the response of thermospheric neutral winds to geomagnetic conditions in the auroral oval. For K p < 2 the auroral oval is polewards of Kiruna and neutral winds are similar at solar minimum and solar maximum. At higher levels of geomagnetic activity, for a given level of activity, the neutral winds are a factor of two greater at high levels of solar flux, than at low levels of solar flux. Alternately, the same winds are seen at K p = 3 at high solar flux levels, as at K p = 5 for low flux levels. Comparing the situation for high and low solar activity, for a given level of geomagnetic activity, in the dusk auroral oval, the sunward or westward winds are a factor of two larger at solar maximum. In the midnight period, the equatorward wind is a factor of two larger at high solar activity, compared with low solar activity. In this paper, the observations will be discussed, with the implication that K p is a rather poor indicator of momentum and energy coupling from the solar wind to the upper thermosphere. In a second paper, the solar-terrestrial processes which cause this phenomenon will be discussed and modelled.