Thermospheric parameters long-term variations retrieved from ionospheric observations in Europe

Abstract

Recently developed method to retrieve thermospheric parameters (Tex, O, O2, and N2) and the total solar EUV flux with λ<1050 Å from routine foF1 ionosonde observations has been applied to monthly median foF1 data on Rome, Slough/Chilton, and Juliusruh stations to analyze long-term trends in the thermospheric parameters. For the first time exospheric temperature and neutral composition were obtained for June noontime conditions over the period of ~ 5 solar cycles. The retrieved parameter manifested solar cycle and long-term (some solar cycles) variations with a rising phase in 1965–1985 and falling phase in 1985–2008. The retrieved thermospheric parameters were shown to be close to the MSIS-86 model ones exhibiting very small (<1% per decade) and statistically insignificant linear trends estimated either over all 56 years or only over the years of solar minimum. No peculiarities in long-term variations in relation with the last deep solar minimum have been revealed. The source of the thermospheric parameter long-term trends is the Sun, i.e., they have a natural (not anthropogenic) origin and are mainly controlled by long-term variations of solar and geomagnetic activity.