Investigation On The Variations Of The Topside Ionosphere Using Low Earth Orbit Satellite-based TEC

The Earth's ionosphere is an important region that connects outer space and the middle atmosphere.This region is critical to radio propagation and communication,navigation systems,and human's space activities.Therefore,it is important to study the ionosphere.As measurements of the topside ionosphere are relatively sparse,the characteristics of the topside ionosphere and the physical processes involved have not been fully understood.Recently,low Earth orbit(LEO)satellites equipped with Global Navigation Satellite System(GNSS)receivers have become new tools for total electron content(TEC)measurements on the topside ionosphere.They provide a great opportunity to investigate the variations of the topside ionosphere.In this dissertation,which uses LEO-based TEC,we focus on the investigations of vertical TEC mapping function,determination and variations of Differential Code Bias(DCB),longitudinal variations of the topside ionosphere,and responses of the topside ionosphere to geomagnetic storms.The main results of this dissertation are outlined as follows:1.Development of new methods for LEO-based TEC data processing(1)High-accuracy conversion of slant to vertical TEC using a mapping functionThe mapping function is commonly used to convert slant TEC to vertical TEC based on the assumption that the ionospheric electrons are concentrated in a layer.The height of the layer is called the ionospheric effective height(IEH)or shell height.Given that the orbital altitudes of LEO satellites are usually at or above the F2 region of the ionosphere,whether the commonly used mapping function for ground-based GNSS observations is still valid and how to choose the optimized IEH for the LEO-based TEC conversion have not been systematically studied yet.This study is intended to examine the applicability of three mapping functions for LEO-based GNSS observations.Two IEH calculating methods,namely the centroid method,based on the definition of the centroid,and the integral method,based on one half of the total integral,are discussed.It is found that the IEHs increase linearly with the orbital altitudes ranging from 400 to 1400 km.There is a negative correlation between the derived IEHs and solar activity.The derived IEHs from CHAMP observations are similar to the IEHs calculated from the model,although they showed stronger solar activity dependence.Model simulations are used to compare the vertical TEC converted by these mapping functions and the vertical TEC directly calculated by the model.Our results illustrate that the F&K(Foelsche and Kirchengast)geometric mapping function together with the IEH from the centroid method is more suitable for the LEO-based TEC conversion,though the thin layer model along with the IEH of the integral method is more appropriate for the ground-based vertical TEC retrieval.(2)Determination of DCB with high accuracy and reliabilityThe uncertainty of DCB is one of the main error sources in the retrieval of the topside TEC above the LEO satellites(LEO-based TEC).As the LEO-based TEC is usually much smaller than the ground-based TEC and LEO satellites move quickly,the DCB should be estimated more accurately in order to process high-precision TEC.In this study,we propose an improved DCB estimation method(ZERO method)by assuming that the LEO-based TEC can reach zero.The minimum relative TEC during each orbital revolution is obtained,then the lower quartile(25%)minimum among these minimum relative TECs is calculated.The combination of the lower quartile minimum relative TEC with the daily minimum relative TEC is used to further correct the lower quartile minimum.This improved ZERO method gives a stable and reliable DCB estimation.We also optimize the parameter configuration in the commonly used least square method(LSQ method).Theoretically,data from high elevations and lower cutoff vertical TECs are more in tune with regional spherical symmetry assumption.However,the analysis suggests that the 3-TECU cutoff vertical TEC with 10° cutoff elevation is considered to offer a reasonable DCB estimation,as the number of processed data is limited.Furthermore,the LSQ method is recommended to estimate the LEO DCB if the quality of the LEO satellite data is not high,albeit the improved ZERO method generally gives reliable results.(3)Understanding of the variations of DCBThe DCBs of Global Positioning System(GPS)satellite show a similar long-term variation with solar activity proxy F10.7 index during 2002-2013.Some studies considered that the solar-cycle-like dependence of GPS DCB is related to ionospheric variations.This study has examined whether this variation of the GPS DCBs is associated with ionospheric variability.The GPS observations from the LEO satellites of CHAMP,GRACE and Jason-1 are used to address this issue.The GPS DCBs estimated from the LEO-based observations at different orbital altitudes show a similar tendency with the DCBs obtained from ground-based observations.However,this solar-cycle-like dependency disappeared when the DCBs of 13 continuously operating GPS satellites are constrained to zero-mean.Our results thus revealed that ionospheric variation is not responsible for the long-term variation of the GPS DCBs.Instead,it is attributed to the update with new GPS satellites of different types and the zero-mean DCB condition applied on all satellites.Moreover,our results also revealed that the LEO DCBs underwent obvious long-term variation and periodic oscillations of months.The CHAMP data illustrated that the long-term variation of LEO DCBs is partly associated with the GPS satellite replacement,and the periodic variation can be attributed to the variation of the hardware thermal status,represented by the receiver CPU temperature.2.New aspects of the variations of the topside ionosphere(1)Longitudinal structure of the topside ionosphereThe upward-looking ionospheric TEC from the MetOp-A and TSX satellites during 2008-2015 has been used to systematically study the longitudinal variations of the topside ionosphere and plasmasphere.The results of this study are summarized as follows:There are significant longitudinal variations in the topside ionosphere and plasmasphere at low latitudes.The TEC maximum during the June solstice over the Western and Central Pacific Ocean corresponds to a TEC minimum at the same location during the December solstice;but the opposite seasonal variation occurs over South America and the Atlantic Ocean.During the solstices,the relative longitudinal variations in the geomagnetic equatorial region do not have a strong dependence on local time and solar activity.The TEC in the winter hemisphere decreases with increasing solar activity,especially at higher altitudes and at night.This TEC depletion with solar activity depends on longitude.The longitudinal variations of upward-looking TEC are different from variations of electron densities both around the F2 peak and at orbital altitudes.This indicates that the topside ionosphere structure is strongly influenced by the physical processes in the topside region,rather than being a pure reflection of the ionospheric F2 peak structure.(2)Responses of the topside ionosphere to stormsTopside ionospheric TEC observations from multiple LEO satellites have been used to investigate the local time,altitudinal,and longitudinal dependence of the topside ionospheric storm effect during the March 2015 geomagnetic storm.Our results show,for the first time,that a persistent topside TEC depletion was found.This depletion lasted for more than 3 days after the storm main phase at most longitudes,except in the Pacific Ocean region,where the topside TECs during the storm recovery phase were comparable to the quiet time ones.The long-duration TEC depletion in the topside ionosphere might be produced largely by the field-aligned plasma diffusion effect and the long-lasting low O/N2 ratio in the bottomside ionosphere at higher latitudes,since the magnetic field lines in the topside ionosphere are connected to the bottomside ionosphere at higher magnetic latitudes.Moreover,the topside TEC patterns observed by MetOp-A(832 km)were different from those seen by other LEO satellites with lower orbital altitudes during the storm main phase and at the beginning of the recovery phase,especially in the evening sector.The differences suggest that field-aligned plasma diffusion might be more important than the uplift of the ionosphere and composition changes at these altitudes.In conclusion,the results of this dissertation have enabled the enhancement of the accuracy and reliability of LEO-based topside TEC in space weather applications,and have advanced our knowledge of the variations and of the topside ionosphere and their response to geomagnetic storms.Further,study of the characteristics of the topside ionosphere and plasmasphere and the physical processes involved using high-precision LEO-based topside TEC,is very important in many ways,such as revealing the relationship of mass and energy exchange in different height regions,understanding the coupling process between the ionosphere/thermosphere and topside ionosphere-plasmasphere,and explaining unknown physical mechanisms in the ionospheric F2 region.This has provided the opportunity to improve ionospheric models,and give better forecasts of space weather.