| dc.description.abstract |
The Earth’s upper atmosphere, comprised of the thermosphere and ionosphere, is where neutral and charged particles interact causing complicated physical processes. The ionospheric electron density is highly variable with the altitude, latitude, longitude, local time, season, solar cycle. This dissertation shows the investigation of the nighttime features from the coupling between the ionosphere and thermospheric neutral wind. Two interesting phenomena associated with the electrodynamic processes are examined, which include (1) The Weddell Sea Anomaly (WSA) in southern mid to high-latitude and Siberia-Yakutsk Anomaly (SYA) in northern mid-latitude. The increasing anomalies of electron density are most prominent over the Weddell Sea region in the southern hemisphere and Siberia and Yakutsk areas in the northern hemisphere during local summer nighttime; and (2) The Plasma Depletion Bays (PDBs) at equatorial/low-latitude. These features of the electron density and TIMED/GUVI 135.6nm airglow emission are observed at the evening/night hours near magnetic equator in three longitude regions, North Atlantic, India Ocean, and Southeast Asia during May. Six microsatellites of the joint Taiwan-US satellite constellation mission, termed FORMOSAT-3/COSMIC (F3/C), were successfully launched in to a circle low Earth orbit at 01:40 UTC on 15 April 2006. Each satellite houses a GPS occultation experiment payload globally deriving the vertical electron density profile in the ionosphere. This constellation daily provides instantly more than 2000 profiles from 90 to 800 km altitude. Dense global electron density probing brings a new era of studying the space weather in the ionosphere.
In this dissertation work, the three-dimensional (3-D) plasma density structure constructed by electron density profiles from F3/C satellites are employed to study the diurnal, seasonal, latitudinal, and altitudinal variations of these anomalies and bay features. The results show that the WSA and SYA features occur prominently at about 300 km altitude, as well as yield the eastward shift of a single-peak plasma density along the WSA latitudes and a double-peak along the SYA latitudes during the period of 2007-2016. The thermospheric meridional and zonal winds simulated by Horizontal Wind Model 1993 (HWM93) is applied to interpret the plasma motions along the magnetic field lines associated with the WSA and SYA anomaly features. Results indicate that the meridional and vertical components of magnetic meridional wind can be responsible for the eastward shift of WSA single-peak and SYA double-peak plasma density. In fact, the WSA and SYA features constantly appear in whole day and all year round.
The PDB structures in the F3/C electron density prominently appear at 275 km altitude in the equatorial/low ionosphere. Three PDBs curving in the northern hemisphere around the magnetic equator situate in regions 30°–60°W (North Atlantic), 30°–110°E (India Ocean), and 120°–160°E (Southeast Asia) from April-September, while one PDB curving in the southern hemisphere appears in 80°–150°W (Southwest America) from October-March. A detailed study on the F3/C 3-D electron density structure shows that the four PDBs are intense mainly below the ionospheric peak density layer (~350 km altitude) in whole day and all seasons. A simulation of HWM93 suggests that the trans-equatorial plasma transports induced by the zonal wind result in the PDB features in the nighttime equatorial/low-latitude ionosphere. Blowing of the thermospheric neutral winds play an important role in the formation of the two anomalies and bay features. | en_US |