| The Earth's ionosphere is a region around 60-1000 km altitudes that consists of charged particles and neutral gas,which is the main geospace area for human's activities and aircraft operation.It can significantly affect the propagation of radio waves.In the ionosphere,there are different ionospheric structures with various scales.Because these ionospheric structures are different with that of background ionosphere.The transmission of radio waves when they propagate through the regions of ionosphere with ionospheric structures canbe severely impacted by the ionospheric structures,especially regarding the communication,navigation,orbit determination and positioning of the space crafts.Such as,the ionosphere scintillnation and even loss of satellite signals occur.Thus,to investigate the ionospheric structures with various scales have great scientific significance and application prospective.In recent years,the Beidou geostationary satellite(GEO)provided fidelity TEC observations to study the ionospheric variability,including the mesoscale ionospheric structures,as the GEO ionospheric observations from the Beidou navigation system are not affected by spatial movement of the satellites.In this dissertation,the mesoscale ionospheric structures are extensively studied on the basis of the Beidou GEO ionospheric observations,which are combined with the airglow and GPS observations.The characteristics of medium-scale traveling ionospheric disturbances(MSTIDs)at middle latitudes,and the mesoscale wave-like ionospheric structures at low latitudes are observed and analyzed.Furthermore,the characteristics of daytime ionospheric longitudinal variations within the zonal range of 1000 km are examined for the first time,and the influences of geomagnetic activities on these longitudinal variations are explored.The main results are summarized as follows:(1)Characteristics of MSTIDs at middle latitudes revealed from multiple ionospheric observations.The characteristics of nighttime MSTIDs at middle latitudes are firstly observed by the Beidou GEO total electron content(TEC)observations,which are compared with those from the airglow images and Global Positioning System(GPS)TEC observations.Three peaks in a year for the seasonal variations of the nighttime MSTID occurrence rate in 2016 are firstly found from these three types of observations.Meanwhile,the results show that the features of nighttime MSTIDs from the Beidou GEO TEC are in better agreement with those from the airglow images than those from the GPS TEC,given that the GPS TEC results are affected by Doppler shift and ionospheric background TEC due to movement of GPS satellites.In addition,the characteristics of MSTIDs during daytime are analyzed using the Beidou GEO TEC observations,which are compared with those of nighttime MSTIDs at same regions.Obvious differences in the characteristics of MSTIDs are present between daytime and nighttime,which could be attributed to different generation mechanisms of the MSTIDs during daytime and nighttime.The daytime MSTIDs could 'be induced by the atmospheric gravity waves from the low atmosphere,whereas the nighttime ones could be associated with the electrodynamic processes including the Perkin instability,E and F region coupling,and inter-hemispheric coupling.(2)New understanding of daytime ionospheric periodic wave-like structures at low latitudes.The daytime ionospheric periodic wave-like structures with the periods of about 18-28 minutes are for the first time statistically analyzed and explored in the low-latitude ionosphere over the Asian-Australian sector using TEC from Beidou GEO observation chain during 2016-2017.The daytime ionospheric periodic wave-like structures frequently occur during 11:00-17:00 local time(LT)in the winter at latitudes ranging between 170°N and 25°N(10°-180°N MLAT)in the Northern Hemisphere,and they have a maximum occurrence rate of 80%at?21°N(140°N MLAT).In the Southern Hemisphere,daytime ionospheric periodic wave-like structures are also observed during 11:00-15:00 LT in the winter within latitudes ranging between 6.0° S and 11.1° S(15.4°-21.6° S MLAT),although the peak occurrence rate is only approximately 40%.The daytime periodic wave-like structures are generally consistent with those of stratospheric GWs in terms of the seasonal and latitudinal variations.These suggest that daytime ionospheric periodic wave-like structures in the low-latitude ionosphere could be triggered by GWs from the lower atmosphere,and generated in the low-latitude ionosphere rather than propagating from the other latitudes.In addition,the characteristics of daytime ionospheric periodic wave-like structures at the low latitudes of Northern and Southern Hemispheres are compared.No clear correlation between the two Hmeispheres is found,which suggests that the inter-hemispheric coupling process is not the major contributor to generate the observed daytime periodic wave-like structures at low latitude ionosphere(3)New features of the ionospheric longitudinal variations within the zonal area of 1000 km at low and middle latitudes.For the first time we used TEC data from a regional Beidou GEO receiver network to observe and investigate ionospheric longitudinal variations within the zonal range of 1000 km over Central China during the period from July 2015 to December 2016.Pronounced daytime ionospheric longitudinal gradients within the zonal scale of 1000 km are present in Beidou GEO TEC.The maximum TEC longitudinal gradient can excess 40 total electron content unit(TECU).Sometimes the daytime ionospheric longitudinal gradients in TEC is larger in the west than in the east sides,while sometimes vice versa.The occurrence frequence of the former condition is slightly higher than that of the latter.The occurrence frequencies of these two daytime ionospheric longitudinal gradients have similar local time variations,which mostly occur from noon to afternoon.In addition,the occurrence rate of daytime ionospheric longitudinal gradients present a semiannual variation with two main peaks in spring and in autumn,and the minima appears in winter and summer.(4)The dependence of the daytime ionospheric longitudinal variations with the zonal range of several hundred kilometers on the geomagnetic activity are found.The daytime ionospheric longitudinal variations within the zonal range of 1000 km are investigated during the geomagnetic storm of September 2017 using the Beidou GEO ionospheric observation chain.The characteristics of daytime ionospheric longitudinal gradients during the geomagnetic storm are obviously different from those under geomagnetically quiet time.In addition,the association between daytime ionospheric longitudinal gradients and geomagnetic activities is statistically analyzed using the Beidou GEO observations over central China during December 2015 to December 2016 The occurrence frequencies of daytime ionospheric longitudinal gradients under different geomagnetic activities are generally similar,whereas strong daytime ionospheric longitudinal gradients tend to occur under the moderate and strong disturbance of the geomagnetic activities.In addition,for most events,the obvious daytime ionospheric longitudinal gradients are accompanied by the background TEC enhancements.These suggest that the electric field disturbances originating from the geomagnetic activities could have significant effects on producing the observed ionospheric longitudinal gradients.In conclusion,the results of this dissertation are beneficial to improve our understanding of the mesoscale ionospheric structures at middle and low latitudes,and to strengthen the application of the Beidou navigation system.It has the great significance in improving the accuracy of the communication,navigation,positioning and the space weather forecast model. |
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