| Summary: | 電気通信大学 博士（理学） 2013 The polar-cap ionosphere is directly coupled with the magnetosphere, which makes the region unique compared with the low- and mid-latitude ionosphere. Because of this coupling with the magnetosphere, the polar cap ionosphere is disturbed during magnetic storms. In this thesis, we examine ionospheric plasma density structures in the polar cap during magnetic storms. In Chapter 1, after reviewing the solarterrestrialenvironment in general, we present some important features of the polar upper atmosphere and ionosphere with special attention to magnetic storms. Following this general introduction, Chapters 2 and 3 are dedicated to specific researches. In Chapter 2, Steep plasma depletion in dayside polar cap during a CME-driven magnetic storm, we investigate a horizontal structure of dayside polar cap ionosphere during a magnetic storm. A series of steep plasma depletions was observed in the dayside polar cap during an interval of highly enhanced electron density on 14 October 2000 through EISCAT Svalbard Radar (ESR) field-aligned measurements and northward-directed low-elevation measurements. Each depletion started with a steep dropoff to as low as 10^11 m^-3 from the enhanced level of ～3×10^12 m^-3 at F2 region altitudes, and it continued for 10-15 min before returning to the enhanced level. These depletions moved poleward at a speed consistent with the observed ion drift velocity. DMSP spacecraft observations over an extended period of time which includes the interval of these events indicate that a region of high ion densities extended into the polar cap from the equatorward side of the cusp, i.e., a tongue of ionization existed, and that the ion densities were very low on its prenoon side. Solar wind observations show that a sharp change from IMF BY > 0 to BY < 0 is associated with each appearance of the ESR electron density dropoff. These facts present the first observational evidence for some of the previously speculated theories on patch formation. In addition, we propose a ...
|
|---|