Global characteristics in the diurnal variations of the thermospheric temperature and composition

Abstract

Global characteristics in the diurnal components of Ogo 6 neutral mass spectrometer measurements near 450 km are discussed qualitatively as well as quantitatively on the basis of a theoretical model. Observations and conclusions are summarized: (1) during equinox the temperature maximum occurs after 1600 LT at the equator and shifts toward 1500 LT at the poles, whereas the oxygen concentration at 450 km peaks about 1 h earlier (this observed phase difference is attributed primarily to wind‐induced diffusion, which is most effective for the mass density near 250 km and thus can contribute considerably to the temperature‐density phase anomaly in the thermosphere); (2) there is general agreement between the magnitudes and the phases of the diurnal, semidiurnal, and terdiurnal temperature components at 450 km from theory as well as from Ogo 6 and radar backscatter measurements; (3) the maximum in the diurnal variation of He is observed near 1030 LT consistent with theoretical results that further emphasize the importance of dynamics and diffusion; (4) during solstice conditions the diurnal temperature maximum shifts toward later local times in substantial agreement with radar temperature measurements; and (5) the temperature‐oxygen density phase difference at 450 km is observed to decrease with latitude from the winter toward the summer hemisphere, where oxygen may even peak after the temperature at high latitudes. This effect is attributed to the diurnal variation in the O2 dissociation, which becomes most significant in the summer hemisphere, where the dissociation rate is enhanced and where the atomic oxygen abundance in the lower thermosphere is significantly reduced, owing to the annual component in the large‐scale circulation.

Simulations of the equatorial thermosphere anomaly: Physical mechanisms for crest formation

“…On the other hand, Fuller‐Rowell et al [1997] suggested that this zonal wind transport mechanism seems to work in the wrong sense, given that the increase of zonal winds at the magnetic equator due to small zonal ion‐drag would carry hot dayside neutral parcels into the evening sector, creating a temperature bulge at the equator. This is not consistent with the OGO 6 measurements shown in Mayr et al [1974] in which neutral temperature/density after sunset did not show a peak at the magnetic equator. Another scenario, proposed by Fuller‐Rowell et al [1997], involves chemical heating.…”

Tides and the midnight temperature anomaly in the thermosphere

“…Radar backscatter measurements at Arecibo (Harper, 1973;Cogger et al, 1974) have revealed occasional well deftned temperature maxima after midnight (~0200LT), where normally a fiat minimum is observed. Such a feature also appeared in the empirical OGO-6 model Mayr et al, 1974). More recently, in situ satellite measurements with the Neutral Atmospheric Temperature Experiment (Spencer et al, 1973) indicated pronounced temperature enhancements centered around midnight .…”

Model study on the formation of the equatorial mass density anomaly in the thermosphere

“…During solstices, the density crest in the summer hemisphere tends to occur 1–2 h earlier than that in the winter hemisphere [ Liu et al , 2007]. The model of Mayr et al [1974] predicted that the density crest in the winter hemisphere occurred 1–2 h earlier than that in the summer hemisphere. The reason for this discrepancy remains unclear.…”