Lower thermosphere mean temperatures, densities, and winds measured by EISCAT: Seasonal and solar cycle effects

Abstract

The European Incoherent Scatter (EISCAT) UHF radar in northern Scandinavia has now been making regular measurements of lower thermosphère temperatures, densities, and meridonal winds for 10 years, covering almost a complete solar cycle. Daily averages from a total of 110 days of operations have been used to study the seasonal, solar flux and magnetic activity dependence of these parameters. In the case of densities, lower values occur in summer than in winter, but no significant solar flux or magnetic activity dependence can be resolved. The daily mean meridional winds are only a few meters per second and show large day‐to‐day variability. Between 100‐ and 120‐km altitudes, there is a suggestion of a reversal from equatorward to poleward prevailing wind from winter to summer, particularly at solar minimum. As regards temperature, the seasonal variations are as expected, with lower values in summer than in winter below 98‐km altitude but with a summer maximum at higher altitudes. A clear dependence on magnetic activity is seen above 110‐km altitude in all seasons and amounts to about 1 K/Ap unit between 110‐ and 120‐km altitudes. In winter, magnetic activity effects seem to penetrate to lower altitudes. No clear dependence on solar flux is seen below 100‐km altitude. At higher altitudes, a positive dependence on solar flux is seen during summer, increasing with altitude and reaching 0.25 K/10.7 cm flux unit. In winter, however, at altitudes 100–115 km, temperatures are significantly lower (by about 10 K) at solar maximum than at solar minimum.