Monitoring and understanding upper atmospheric long-term cooling

Atmospheric climate change is characterized by global warming above the Earth’s surface associated with the increase of the greenhouse gas population since the start of the industrial era. In the upper atmosphere where both neutral and plasma gases are subject to substantial variability due to space and terrestrial weather, including low atmospheric forcing. Direct and long-term ionospheric observations with the incoherent scatter radar (ISR) technique provide an efficient way to quantify and understand the variation trend of the thermal status in the upper atmosphere. Since 2008, the ISRs have been providing some of the key evidence for the cooling in the ionosphere and thermosphere, particularly, its altitude dependence above 100 km. This cooling was increasing as a function of height and was initially interpreted as a greenhouse gas effect in the upper atmosphere which was also observed in satellite drag observations, however, almost all the ISR results suggested that the cooling appeared substantially large and therefore additional cooling processes beyond the greenhouse gases as a thermospheric cooling agent are needed. These include potential wave activity changes arising from climate change as well as secular changes in Earth’s main magnetic field. In this presentation, we provide an updated analysis of ISR-measured trends and discuss some progresses in understanding these results.