We analyze ion temperature data near 350 km altitude over the years 1966-2012 to seek explanations for three outstanding questions concerning the long-term cooling observed in the upper thermosphere: (1) Why is the cooling so much larger than expected, (2) why has the cooling lasted so long, and (3) why is the thermospheric density response to the cooling so small? We speculate that gravity waves may cause this cooling and provide answers to these questions. Recent simulations have shown that gravity waves are expected to cool the upper thermosphere by an amount comparable to that observed over our data timeline. A gravity wave proxy formed from the nontidal fluctuations in temperature shows a positive long-term trend throughout its timeline, consistent with the increasing cooling observed. The time scales of the long-term trend and its decadal fluctuations are characteristic of the ocean, not the atmosphere. We suggest that the following scenario may explain these behaviors: (a) the climate regime shift of 1976-1977 launched slow Rossby waves across the oceans which continue to propagate to this day, (b) winds over this increasingly corrugated ocean have launched increasing fluxes of gravity waves into the atmosphere, and (c) these increasing fluxes of gravity waves have propagated to the thermosphere to produce increasing amounts of cooling. The strong thermospheric cooling seen would be expected to produce thermospheric density declines much larger than those observed via satellite drag. These temperature and density results would be compatible if the turbopause were lowered 4 km over the time span of observations.
All Science Journal Classification (ASJC) codes
- Space and Planetary Science