Study On The Mid-latitude Ionospheric Trough-statistical Analysis, Modeling And CT Reconstruction

The mid-latitude ionospehric trough (MIT) is a symbolic structure of the subauroral ionosphere; it is not only controlled by many local physical processes in the ionosphere, but also influenced by convection electric field from the magnetosphere and geomagnetic disturbances and so on. The investigation of the morphological and dynamic characteristics of MIT and its controlling factors can help us to understand the magnetosphere-ionosphere coupling mechanisms, and has significant value to modeling and forecasting the mid-and high-latitude ionosphere.Making use of nearly a decade (2000-2009) plasma density data observed by CHAMP satellite, this dissertation analyzes the effects of geographic longitude, season, solar activity, interplanetary magnetic field (IMF), and solar wind electric field on the MIT occurrence rate in both southern and northern hemisphere; then study the dynamic characteristics of the MIT through typical case analysis and statistical analysis, and then analyzes the influences of geographic longitude, geomagnetic activity, and solar wind electric field on the movement of MIT, and builds an empirical model about the variation of MIT position with the geographic longitude and SYM-H index. In addition, using GRACE beacon TEC measurements, this dissertation carries out some CT reconstructions in the topside ionosphere and lower plasmasphere, and then make some investigations of the altitude variation of the location, width, and depletion depth of MIT during the major magnetic storm on15May2005based on the reconstructed results.The main studies performed and results obtained in this dissertation can be concluded as follows:1. Using CHAMP plasma density observations, statistical analysis of the MIT occurrence rate has been made, some meaningful results are found:(1) The distributions of the MIT occurrence rate with magnetic local time (MLT) in the northern and southern hemisphere are in good agreement with each other, while the occurrence rate in the northern hemisphere is overall higher than that in the southern hemisphere. (2) During the period of21-05MLT, the features about the MIT occurrence rate are as follow:(a) Variations of MIT occurrence rate with invariant magnetic latitude in two conjugate hemispheres exhibit Gaussian distribution characteristics. And peak values of MIT occurrence rate in two conjugate hemispheres locate near the invariant magnetic latitude of59°.(b) The MIT occurrence rate in the years near high solar activity (2000-2005) is obviously greater than that in the years near low solar activity (2006-2009).(c) There exist significant distinctions of the seasonal effects of MIT occurrence rate in different geographic longitude regions, and the seasonal effects show hemispheric differences.(d) The MIT occurrence rates in both northern hemisphere and southern hemisphere during the period of21-05MLT significantly increase with the growth of Kp index, But the growth speeds in the two hemispheres are different.(e) Both the IMF-Bz and-By can put impact on the MIT occurrence rates through their magnitude, the MIT occurrence rates increase with the growth of the values of IMF-Bz and-By. The MIT occurrence rate with a southward Bz is greater than that with a northward Bz. But the influences from the polarity of By is unconspicuous.(f) The Ey component of the solar wind electric field can affect the MIT occurrence rate when it has a positive value, other than has a negative value.2. Through typical case studies and statistical analysis, the dynamic characteristics of the MIT, and the influences of geographic longitude, geomagnetic activity, and solar wind electric field on the movement of MIT are investigated. Results show that:(1) Under geomagnetic quiet conditions, geographic longitude effect of the MIT location is found in both northern hemisphere and southern hemisphere, and they indicate hemispheric asymmetry. (2) During geomagnetic storm time, the equatorward movement of the MIT is almost synchronized to the variation of the SYM-H index.(3) The position variation of the MIT correlates significantly with SYM-H index, and the correlation coefficients of the northern hemisphere and southern hemisphere are0.98and0.95, respectively.(4) The magnitude of dawn-dark component of solar wind electric field can also significantly influence the position of MIT, but the influence of its polarity is relatively weaker.(5) An empirical model about the variation of MIT position with geographic longitude and SYM-H index is established based on statistical analysis. And the established models are proved to be good in predicting the MIT locations.3. Using the total electron content (TEC) recorded by the GRACE-borne GPS receivers, CT reconstructions are carried out during the major magnetic storm on15May2005to obtain the electron density distribution of the MIT in the topside ionosphere and the lower plasmasphere, main results are as follows:(1) Developed a application program for ionosphere and plasmasphere CT, and successfully reconstructed the electron density distribution of the MIT.(2) The main trough obviously drifts equatorward to lower latitude (more than14°) during this storm, and extends in the meridional direction from-5°at quiet day to-21°during the storm. During the initial phase of the storm, the equator-side width of the MIT is larger than its polar-side width. However, the equator-side width of the MIT becomes smaller than its polar-side width.(3) The MIT minimum slowly moves to higher latitudes with the increment of altitude during the quiet times. However, during the storm time, the MIT minimum moves to lower latitudes with the increment of altitude.The MIT depletion depth increases with altitude during the initial phase and main phase of the storm, and it decreases with altitude during the recovery phase of the storm.(4) During the recovery phase of the May2005super-storm, a main trough with the double-minimum structure is found in the meridian of-160°W. At almost the same time and in the adjacent longitude sector, the DMSP F15borne equipment also detects the main trough with fine double-minimum structure. It indicates that space based CT can reconstruct reliable electron density profiles.The innovation of this dissertation can be concluded as:1. This paper reveals that the dawn-dask component of solar wind electric field Ey can significantly influence both the occurrence rate and the position of the MIT, it influences the MIT occurrence rate through its polarity, but influences the position of MIT mainly through its the magnitude.2. Preliminary builds an empirical model about the variation of MIT position with geographic longitude and SYM-H index.3. A software about space-based ionospheric and plasmaspheric CT is developed, and it is successfully used in reconstructing the electron density distribution of the MIT.