Anomalous behavior of the thermosphere during solar minimum observed by CHAMP and GRACE

Abstract

High‐resolution density observations inferred from accelerometer measurements on the Challenging Mini‐Satellite Payload (CHAMP) and Gravity Recovery and Climate Experiment (GRACE) satellites for the period 11–17 December 2008 during the solar minimum are analyzed and compared to reference model predictions. The density distribution as a function of altitude can be studied because the satellites were in the same 8.4/20.4 hour (0824/2024) local solar time plane, CHAMP at a mean altitude of 332 km and GRACE 144 km higher. The mean observed and model density profiles at 476 km reveal considerable differences, which, except for a 25% bias, is not the case at an altitude of 332 km. These differences result from the fact that during this solar minimum the transition from predominantly O to He occurs at a much lower altitude in connection with the winter helium bulge than is predicted by the model. When averaged over the 1 week period, striking wave‐3 structures are revealed with respect to longitude that are remarkably consistent between CHAMP and GRACE. We interpret these as the manifestations of nonmigrating tides propagating upward from the lower atmosphere. Comparisons are made with similar data from August 2008, which reveal a wave‐4 structure. However, whereas previous results and conventional wisdom would suggest that these features are produced by the eastward propagating diurnal tides with zonal wave numbers s = −2 (DE2) and s = −3 (DE3), respectively, the asymmetries and day‐night phase differences that are present suggest the predominance of semidiurnal tides SE1 and SE2, respectively, along with the aforementioned diurnal tides as well as tides SW5 and SW6.