Alternative Title磁気嵐に伴う極冠電離圏でのプラズマ密度構造に関する研究
Note (General)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 scenario that the series of plasma density depletions is a result of the poleward drift of the undulating boundary of the tongue of ionization; this undulating boundary is created in the cusp roughly 20 min before the ESR observation by the azimuthal intrusion, in response to the rapid prenoon shift of the footprint of the reconnection line, of the low-density plasmas originating in the morning sector. In Chapter 3, Storm-time enhancements of 630.0-nm airglow associated with polar cap patches, we study the vertical structures of both neutral and ionized gases in the polar cap. We examined the brightness of 630.0-nm airglow, I630, associated with polar cap patches observed during a magnetic storm that occurred on 22 January 2012. Brightness was measured using an all sky imager (ASI) located at Longyearbyen, Svalbard. The observed I630 was compared with the F-region electron density observed by the EISCAT Svalbard Radar (ESR). The I630 was positively correlated with the F2-layer peak electron density, NmF2, and inversely correlated with the altitude of the F2-layer peak electron density, hmF2, as expected from the known relationship between these parameters. To estimate the altitude of the peak emission of the airglow, we performed model alculations of the volume emission rate, V630, under quiet and disturbed conditions, using MSIS-modelled neutral gas profiles and the electron density profile obtained from the ESR data. In order to validate the V630 calculation, I630 was calculated by integrating the V630 along altitude, and then compared with the ASI-observed I630. During the observation periods the measured brightness frequently exceeded the calculated I630; we infer that, in most cases, low energy particle precipitation is responsible for the extra brightness. However, when there was less particle precipitation, the observed values were in good agreement with the calculated values. Under the magnetically disturbed conditions during our observations, the model calculation showed that the altitude of V630 peak increases, the thickness of the emission layer increases, and patch brightness increases. The results clearly show the previously unknown vertical structure of polar-patch airglow under magnetic storms.
2013
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2018-01-02T17:18:43+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション