Long-term variations of exospheric temperature inferred from f o F 1 observations: A comparison to ISR Ti trend estimates

Abstract

June noontime monthly median f(o)F(1) ionosonde observations at Sodankyla (auroral zone), Juliusruh, and Rome (middle latitudes) were used to retrieve exospheric temperature, Tex long-term variations over the (1958-2015) period. After removing solar activity effects the residual linear trends were found to be small (0.05-0.6)% per decade and statistically insignificant at middle latitudes. Therefore, the revealed Tex long-term variations are mainly due to long-term variations of solar activity, i.e., they have a natural (not anthropogenic) origin. Large trends in ion temperature, Ti inferred from incoherent scatter radar (ISR) observations which the researchers identify with trends in neutral temperature, Tn may be related to the incoherent scatter method routinely based on a fixed model of ion composition (O+/Ne ratio and mean ion mass, correspondingly) under varying geophysical conditions. Mean ion mass number manifests a negative trend at 175 km which should correspond to a negative trend in Ti contrary the results obtained below 200 km with ISRs. Therefore, routine ISR observations based on a fixed model of ion composition may be not appropriate for long-term trend analyses.