| Summary: | 博士 === 國立中央大學 === 太空科學研究所 === 87 === The global positioning system (GPS) has global long-term observation data and high spatial and temperal resolutions, which is an ideal tool to investigate large-scale ionospheric variations. In this dissertation, a study of the ionospheric equatorial anomaly in total electron content (TEC) has been developed by using the GPS to investigate the seasonal variations, solar eclipse effects, and magnetic storm effects.
Combination of dual-frequency observables of psudoranges and carrier phases are employed to develop a GPS-TEC observtaion technique and then construct a 3-dimensional (3D) TEC distribution in time and latitude. Therefore, the ionospheric variability in TEC magitude and spatial distribution over the geomagnetic equatorial, equatorial anomaly, and even midlatitude regions are investigated.
Observations for seasonal variations in the ionospheric northern and southern equatorial anomaly region in 1997 show that the TECs in equinotical seansons are greater than those in solstice seasons and a clear winter anomaly occurs in the northern hemisphere but not in the southern hemisphere. The asymmetry in the winter anomaly could be caused by different locations of the sub-solar points in summer and winter resulting in energy input imbalance between southern and northern hemispheres in this region. The appearing time of the equatorial anomaly crests are the earliest in winter, later in equinoxes, and the latest in summer. The two crests move significantly equatorward in winter but moderately poleward in summer and autumn. The appearing time and location of the two crests could be explained by combination of two theories: 1. A transequatorial neutral wind blowing from summer hemisphere to winter hemisphere; 2. The thermosphere-ionosphere-plasmasphere coupling which drives the equatorward thermospheric neutral wind in winter high latitude prevailing the poleward wind in low latitude.
In the ionospheric transient variability, two solar eclipse events on October 24, 1995 and March 9, 1997 occuring in the Pacific-Asian region have been examined. Results show that the solar eclipse effects occur throughout the geomagnetic equatorial, equatorial anomaly, and midliatitude regions, and lasted many hours after the maximum occultation. Four features of the effects named pre-ascension (PA), major depression (MD), sunset ascension (SA), and secondary depression (SD) are observed during the eclipse days. A detailed study shows that in geomagnetic low latitudes, PA is possibly related to the locations of the equatorial anomaly crest. The latitudinal location, amplitude, and occurrence time of MD suggest that the fountain effect is essential. SA and SD occuring in geomagnetic equatorial and low latitudes and appearing respectively before/around and after local indicate that in addition to the day-to-day variations, the prereversal enhancement plays an important role.
The effects of geomagnetic storm on ionospheric TEC have also been investigated by using the GPS data during two observations on January 10 and may 15, 1997, respectively. It is found that after the onset of sudden storm commencement (SSC), the equatorial anomaly crests move poleward, and the daytime TEC is significantly reduced one day after SSC. The penetration effect and distrubed dynamo mechanism in ionospheric electric fields are proposed to explain the above phenomena.
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