Millstone Hill ISR observations of upper atmospheric long-term changes: Height dependency

Abstract

Ionospheric ion temperature is an excellent approximation to neutral temperature in the upper atmosphere, especially, for altitudes below 300 km. This analysis of long-term ionospheric ion temperature changes between 100 and 550 km at noon is based on a database of incoherent scatter radar observations spanning more than three solar cycles during 1968-2006 at Millstone Hill and provides direct evidence of long-term changes and their height dependency in the upper atmospheric temperature. A cooling trend at altitudes above 200 km and an apparent warming trend below 200 km are found. The cooling increases with height and shows variability with solar activity. The apparent warming varies with season and solar activity. It may result from the thermal subsidence caused by atmospheric contraction and pressure level change and from the ion temperature overestimation in the F1 region due to ion composition long-term changes. These long-term changes in ion temperature are accompanied by changes in electron density, being lower above the F2 peak and higher below the F2 peak. Electron temperature is accordingly enhanced. All these changes appear to be suggestive of a long-term greenhouse gas effect. Copyright © 2011 by the American Geophysical Union.