Long-term Trends in the Upper Atmosphere Using the Incoherent Scatter Radar Observations over Arecibo

Upper atmospheric long-term trends have been characterized through the analysis of the ionospheric ion temperature (T-i). Previous studies used T-i observations from various incoherent scatter radar (ISR) facilities located at different latitudes. In this paper, we analyze Arecibo Observatory's (AO) ISR (18 degrees 20'N, 66 degrees 45'W) data sets from 1985 to 2019 to detect T-i long-term trends as a function of altitude from similar to 140 to similar to 677 km. We empirically modeled the responses of T-i to the known forcings of solar activity, geomagnetic activity, and the annual and semi-annual oscillations. The Ti trend is determined through least squares fitting to the residuals of the T-i, which were estimated by removing the empirically modeled T-i from the observed T-i. Since the ions and neutrals are closely coupled, our results indicate that the upper atmosphere/ionosphere over Arecibo has been cooling over the 35 years studied. Above 350 km, a latitudinal dependency is seen by comparison of all ISR estimated Ti trends, which agrees with the earlier reported results. These observed cooling trends exceed the magnitude expected by the modeling studies from increased greenhouse gas (GHG) concentrations. These excess coolings are as high as -1.2 K/year below 320 km altitude, where an increase in GHG dominates. Nighttime cooling trends in the altitude of similar to 320-400 km might be caused by the increasing GHG concentrations and magnetic field variations since the trends of AO-ISR match with the Whole Atmosphere Community Climate Model eXtension simulations.