Nighttime Enhancement Of Thermospheric Temperature And Its Association With Terdiurnal Tides

The midnight temperature maximum (MTM) in the upper thermosphere represents a phenomenon in which the thermospheric temperature increases near midnight when solar heating is absent. A similar phenomenon also occurs in the daily variation of thermospheric mass density, which is called midnight density maximum (MDM). The MDM provides an indirect signature of the MTM under the assumption of diffusive equilibrium as well as the assumption that altitudinal variations in composition are insignificant. MTM/MDM have been reported by satellite and ground-based measurements since 1970s. Previous results demonstrated that the MTM occurs at the equator around 0-4 local time (LT) with an amplitude ranging from 40 K to 150 K. Only recently was it suggested that MTM could also occur in higher latitudes. In this dissertation, the variations of thermospheric nighttime temperature and density enhancements and their mechanisms, particularly the impacts of migrating terdiurnal tides from the lower thermosphere on MTM/MDM, are investigated. The results are summarized as follows:1. The similarities and differences between nighttime ion and neutral temperatures in the F region over Millstone Hill (42.5°N,71.5°W) are examined on the basis of incoherent scatter radar (ISR) and Fabry-Perot interferometer (FPI) observations during August 1-11,2011 and January 12-23,2012, which represent summer and winter seasons, respectively. Our results show that in winter both neutral and ion temperatures exhibit post-midnight enhancements, which occur during 3-4 LT and have an amplitude of about 30-90 K. However, in summer the situation is different during August 1-11,2011, when the post-midnight enhancement is only observed in neutral temperature so that the variation of the nighttime ion and neutral temperatures is almost out-of-phase. The causes of the post-midnight enhancements of neutral temperature in either winter or summer remain debatable. Our preliminary analysis indicates that the extension of MTM to the latitude of Millstone Hill is the preferred mechanism for the observed post-midnight enhancement of neutral temperature in winter and summer.2. We investigate the latitudinal variations of nighttime thermospheric density using CHAMP observations during 2002-2009. The sampling of the CHAMP satellite allows for a statistical analysis of MDM. From this analysis, it was found that the MDM of the thermosphere, a manifestation of the MTM, occurs in all seasons under both low and high solar activity, but its features vary significantly with season and solar activity. The peak of the MDM over the equatorial region occurs at-l:00 LT under low solar activity condition, whereas it happens about 1-2 hours earlier in high solar activity. We see an extension of the MDM from the equatorial region to southern mid-latitudes, especially during low solar activity condition.3. The formation and variation of the MTM/MDM are investigated by applying the migrating terdiurnal tide from the extended Canadian Middle Atmosphere Model (eCMAM) at the lower boundary of the NCAR Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIEGCM), along with the migrating diurnal and semidiurnal tides from the Global Scale Wave Model (GSWM). Several numerical experiments with different combinations of tidal forcing at the TIEGCM’s lower boundary were carried out to determine the contribution of each tide to MTM/MDM. We found that the interplay between diurnal, semidiurnal and terdiurnal tides determines the formation of MTM/MDM and their structure in the upper thermosphere. The decrease of thermospheric mass density after MDM reaches its maximum at-02:00 local time is mainly controlled by the terdiurnal tide. Furthermore, we examined the generation mechanisms of the migrating terdiurnal tide in the upper thermosphere and found that they come from three sources:upward propagation from the lower thermosphere, in-situ generation via nonlinear interaction between diurnal and semidiurnal tides and thermal excitation.4. The migrating terdiurnal tide at the mesosphere and lower thermosphere (MLT) is suggested to contribute to the formation of the well-known MTM/MDM in the upper thermosphere. Previous studies also showed the MTM/MDM occurrence strongly depends on seasons. The coupled TIEGCM and eCMAM simulations are employed to investigate the seasonal variations of the upward propagation of the terdiurnal tide in the MLT region. We found that background zonal and meridional winds can significantly affect the upward propagation of the terdiurnal tide. In addition, the terdiurnal tide in the MLT impacts not only the latitudinal distribution and the magnitude of the terdiurnal component in the upper thermosphere but also the influence of the background winds on the tidal upward propagation. Moreover, the hemispheric asymmetry in zonal mean and vertical gradient of the background temperature contributes to the seasonal variation of the tidal upward propagation as well.