Long-Term Trends in Fabry-Perot Interferometer Observations of Polar Thermospheric Nightside F-Region Neutral Temperature

Earth’s nighttime thermospheric neutral temperature trend over the period from 1995 to 2021 at F-region altitude (near 240 km) was analyzed using all-sky imaging Fabry-Perot spectrometer measurements of 630 nm emission spectra from atomic oxygen. Observation sites were located within the auroral zone in both hemispheres, and the data period spans more than two full solar cycles. Observed temperatures were compared with NRLMSISE-00 model temperatures. NRLMSISE-00 is an empirical atmospheric model with location, time, solar activity, and geomagnetic activity as model inputs. This model, however, does not incorporate the climatological trend over time. The difference between the trimonthly median of the measured temperatures and the model temperatures was estimated. Linear fit of these differences showed a trend of -14.9± 7.0 K/decade in thermospheric neutral temperature. This means there is less than a 5% chance that there is no cooling trend at all. Previous studies reported that the main cause of this trend is enhanced cooling due to the continued increase in carbon dioxide (CO2) concentration in the thermosphere. CO2 is very efficient in absorbing and re-emitting heat in the form of long-wave radiation. Unlike global warming in the troposphere, the heat radiated by thermospheric CO2 molecules escapes to space thereby decreasing the overall temperature of the region.