Characterization And Modeling Of Thermospheric Density Variations

Thermospheric density is large enough to measurably impede the motion of lower orbiting objects,including the planning of satellite missions and lifetimes,orbit and reentry prediction,manoeuvre planning for ground-track maintenance and precise orbit determination.Moreover,due to the limited and discontinuous thermospheric density measurements,their characterizations are not fully understood yet.In this dissertation,we develop some empirical models of the thermospheric density based on different methods and observations,and then investigate the seasonal variations,annual asymmetry,and density cell structures of the thermospheric density.The main results are given as follows.1.Empirical model of thermospheric density and its correctionWe first review the progress of empirical modeling and model correction of the thermospheric density.Then,we develop empirical models to predict thermospheric density.Thermospheric mass densities,derived from GOCE satellite for sun-synchronous orbits during 2009-2013,have been used to develop the GOCE model at dawn/dusk local solar time sectors,based on the Empirical Orthogonal Function method.Moreover,the effective exospheric temperature,'derived from thermospheric densities measured by CHAMP satellite during 2002-2010,has been utilized to develop the ETM model with the aid of MSISE00 model.In addition,the artificial neural network method is applied to construct a global averaged density model,called ANNM.Finally,we correct the MSISE00 empirical model by adjusting its solar F10.7 index.Generally,the densities from our GOCE model,ETM and ANNM are in better agreement with the independent density measurements than those from the MSISE00 model.Overall,our models and calibration can greatly reduce the density errors and the deviation,especially the errors in different seasons and during the low solar activity periods.Furthermore,the ANNM model can well isolate the influences of various factors on the thermospheric density,and our results suggest that it is more suitable for studying the seasonal variations and long-term trend of thermospheric density.2.Seasonal and annual variations of thermospheric densityThe results from our GOCE model show that the dawn densities peak in the polar regions,but the dusk densities maximize in the equatorial regions.The seasonal variations show an obvious hemispheric asymmetry,with much larger amplitudes in the southern hemisphere.However,the largest seasonal variations in the relative sense at both dawn and dusk appear in the southern polar regions around June and July months.Moreover,the relative seasonal variations to the annual mean have similar patterns across all latitudes regardless of solar activity conditions.Both amplitude and phase of the annual variations have strong latitudinal and solar activity dependences,but the semiannual variations seem to be only controlled by the solar activity.Additionally,the annual variation is more significant at higher latitudes,while the semiannual variation is much more predominant at lower latitudes.We also find that the annual asymmetry and the effect of the Sun-Earth distance vary with latitude and solar activity.Thermospheric densities from the ETM also undergo significant seasonal variation,which depends on latitudes.Moreover,the ETM gives higher densities in March than in September under high solar activity conditions,but vice versa for the low solar activity levels,called equinox asymmetry,which are different from the MSISE00 results.Additionally,the amplitude of the seasonal variation in the southern hemisphere is also greater than that in the northern hemisphere,and the hemispheric asymmetry for seasonal variation at 400 km is much stronger than those from the GOCE and MSISE00 models.Using the ANNM,we also find the equinox asymmetry feature.The contributions of annual asymmetry of solar activity on the corresponding asymmetry of thermospheric density depend on the height,but become less above 525 km.It could be related to the transition height of atomic oxygen to Helium at higher altitudes.Regarding geomagnetic activity,its contribution to the annual asymmetry of thermospheric density dereases with the increasing height.3.Density cells in the low thermosphere at high latitudesWe have provided a climatological view of the low,high and wavy density cells in the low thermosphere at high latitudes,using thermospheric mass density at 270 km derived from GOCE satellite.The three different density cell structures show a significant hemispheric asymmetry,and they tend to occur in the northern hemisphere Moreover,they also have an obvious seasonal dependence,with a larger occurrence ratio in local summer.The low density cell mainly occurs around 75 0N?80°N in the dawn side,and the high and wavy density cells usually locate at?80°N around morning time.The main ranges of horizontal scales of low,high and wavy density cells are about 2000?4000 km,3000?3500 km,and 4000?5000 km.Additionally,the main relative variations of low,high and wavy density cells are about 30%,40%and 20%-30%to the background values.These characterizations may be associated with the distributions of interplanetary magnetic field,solar radiation,electric potential structure,particle precipitation or the background.In conclusion,our work can effectively improve the accuracy of thermospheric density prediction,and can also be used to improve our understanding of the seasonal variation,annual asymmetry and long-term trend of thermospheric density at different altitudes,and so on.The revealed charateristics of the density cell structure at high latitudes contribute to improve our understanding of the irregularity structures of the low thermosphere.