Altitude and solar activity dependence of 1967–2005 thermospheric density trends derived from orbital drag

Abstract We examine 1967–2005 thermospheric mass density trends (as well as 1967–2013 trends) derived from satellite orbit data, as a function of altitude, solar flux, and geomagnetic activity. At 400 km altitude, the estimated 1967–2005 trend is −2.0 ± 0.5% per decade. The estimated trends become increasingly negative with increasing height between 250 and 575 km, suggesting an exospheric temperature trend of −1 to −2 K per decade, which is much smaller than temperature trends that have been inferred from ground‐based incoherent scatter radar measurements. The orbit‐derived trend height profiles are in good agreement with model simulations of the enhanced cooling that results from increasing concentration of CO 2 in the mesosphere and lower thermosphere. In contrast to earlier results, the solar flux dependence of the estimated trends is weak, relative to the trend uncertainty. There is some indication that the trends may be stronger during very low geomagnetic activity conditions. Estimation of the solar flux and geomagnetic activity dependence of the trends is complicated by monotonic decreases in these drivers over the past four solar minima together with the CO 2 increase, all of which drive interminima decreases in density.