The plasmasphere plays a central role in many of the complex processes in Earth's inner magnetosphere. Four decades of studies by a variety of in-situ techniques revealed the dynamical aspect of the plasmasphere changing depending on geomagnetic conditions. However, the in-situ measurements are local observations with which it is difficult to separate the temporal development and the spatial structure of the plasmasphere. Recent advances in satellite-based imaging techniques have overcome this difficulty and made it possible to routinely obtain full global images of the plasmasphere. The Extreme Ultraviolet Imager (EUV) on the IMAGE satellite produced consecutive images of the plasmasphere from the high-latitude view with a time resolution of 10 min and a spatial resolution of 0.1 Re.

The EUV images have revealed the details of a highly structured and dynamic entity of the plasmasphere, and demonstrated that the remote sensing with EUV techniques is a promising means to explore the plasmasphere. On the other hand, their remains some unsolved problems which are difficult to clarify by the imaging from a limited perspective, i.e. only from the high-latitude view. For example, a radial structure of enhanced brightness called a "finger" was found by the EUV instrument but the cause of this structure is still not understood. Furthermore, the gap between the results of the EUV measurements and the conventional theories for the plasmapause formation has been pointed out. In order to solve these problems, the imaging of the plasmasphere from a different perspective has been emphasized.

We have developed the Telescope of Extreme Ultraviolet (TEX) onboard the lunar orbiter KAGUYA launched in September 2007. Data available from the TEX instrument allow us for the first time to study the plasmasphere from a meridian perspective. The TEX instrument images the He+ distribution in the plasmasphere by detecting the resonantly scattered solar 30.4-nm radiation and produces images encompassing the entire plasmasphere. The author has participated intensively in the laboratory and in-orbit calibrations and data evaluation for the onboard instrument.

The performance of the TEX instrument after launch was verified using the in-orbit data obtained from February 2008 to May 2009. The result of this in-orbit calibration is presented in Chapter 2. From March to June 2008, the TEX instrument had produced the first sequential images of the plasmasphere viewed from the meridian perspective. The analysis and evaluation of these first images are demonstrated in Chapter 3. The corotation and the erosion of the plasmasphere were identified in the sequential images during the geomagnetically quiet (Kp < 3) and disturbed period (Kp=5), respectively. The angular rotation rate for the corotation and the inward velocity of the nightside plasmapause for the erosion are estimated by analyzing these images, and the results are consistent with those derived from IMAGE/EUV observations. This confirms that the TEX instrument successfully detected the spatial distribution and temporal development of the plasmasphere.

Furthermore, a new striking feature of enhanced brightness in the plasmasphere, called as a plasmaspheric "filament", was found in the TEX image during a prolonged quiet period (Kp < 2). The shape of the filament was closely aligned to the dipole magnetic field line, and this suggests that the filaments are caused by isolated magnetic flux tubes filled with denser plasmas than their neighbors. The nature of the filament agrees with that of the finger structure observed by IMAGE/EUV. Consequently, thanks to the novel view of the TEX instrument from the meridian perspective, this study reveals that the finger structure seen in the EUV images are the equatorial projection of isolated flux tubes filled with denser plasmas than its neighbors.

The formation mechanism of the plasmapause was studied in Chapter 4. The sequential TEX images during the geomagnetic disturbance (Kp=5) on 1-2 May 2008 were analyzed. The plasmapause positions at the post-midnight observed from the meridian perspective clearly agreed with those predicted by the dynamic simulations based on the interchange mechanism. Furthermore, after the convection enhancement, the He+ column density in the nightside plasmasphere decreased by ~30% only at the low latitudes (< 20 deg). This suggests that the formation of the new plasmapause occurs first near the equatorial region during a geomagnetically disturbed period. These results agree with the formation mechanism of the plasmapause based on the quasi-interchange instability.

For the next plasmaspheric imaging from the International Space Station (ISS), new multilayer coatings having high reflectivity at 30.4nm and low reflectivity at 58.4nm were developed. Chapter 5 deals with this topic. For the He II imaging from ISS, the contamination from geocoronal He I (58.4nm) emission should be eliminated more effectively than the TEX instrument because ISS orbits at a low altitude of ~350km, inside the geocorona. The newly developed Y2O3/Al multilayer coating designed for normal incidence reflection had higher reflectivity (24.9%) at 30.4nm and significantly lower reflectivity (1.3%) at 58.4nm than the conventional coatings such as Mo/Si. The temporal stability of the Y2O3/Al multilayer coating was also evaluated and found to be highly stable under vacuum, dry N2 purge, and normal atmosphere. Then, based on these results, the Y2O3/Al multilayer coating was selected to apply for the flight mirror of the Extreme Ultraviole Imager (EUVI) onboard ISS. The calibration of the flight mirror was performed, and it was confirmed that EUVI can produce the plasmaspheric images from ISS with a high SNR of 16.

Meridional distribution of the Earth's plasmasphere derived from extreme ultraviolet images (極端紫外光による撮像から明らかにする地球プラズマ圏の子午面分布)と題するこの論文は6章より成り、極端紫外領域での技術開発により自ら作りあげた測器を用い、これまで行われたことのない視点からの観測によって、地球プラズマ圏の構造・変動に関して得た新しい知見を議論している。

第1章ではこれまでのプラズマ圏に関する観測と理論がまとめられている。第2章では月周回衛星「かぐや」に搭載された極端紫外光望遠鏡(TEX)の軌道上較正作業が述べられている。第3章では先行するNASAのIMAGE衛星による極軌道からの赤道面撮像に対し、月周回軌道からの子午面撮像という新しい視点に立った観測を記述しており、第4章ではこの視点から得られた新しい知見が議論されている。第5章では近年中に国際宇宙ステーションに搭載され、プラズマ圏を電離圏側から観測予定の新測器のための性能向上努力が記述されており、第6章がまとめとなっている。

本研究の成果の中心は「これまでにない視点からの観測により、地球プラズマ圏の形成・状態・変動に関する新知見を得た」という点にある。プラズマ圏の振る舞いを把握することは太陽風が磁気圏内に引き起こす物理現象を理解する上で重要な鍵となる。近年までプラズマ圏の観測は、主に衛星に搭載されたプラズマ計測器による局所的測定が多かった。しかし、局所的測定では見出された変化が時間変化なのか場所変化なのかもわからず、しかも1衛星上だけでは全体像が見えてこない。そこで、地球プラズマ圏全体をHe+の30.4nm共鳴線を用いて撮像し、現象の全体像に迫ろうという試みが行われるようになった。He+ 30.4nm共鳴線による観測は2000年に打ち上げられたNASAのIMAGE衛星により先行されたが、本研究では2007年打ち上げの月周回衛星「かぐや」にTEXを搭載することにより、プラズマ圏子午面を赤道面付近から撮像した。その結果、IMAGE衛星による極軌道上からの赤道面撮像では解釈が確定できなかった現象について、確定的な結論を得ることに成功した。例えば、IMAGEにより発見されたFINGERと呼ばれる現象は(1)濃いプラズマの詰まった磁力管なのか、あるいは(2)放射状構造を持ったプラズマの粗密波なのかIMAGEのデータでは判断できなかったが、TEXによる赤道面上からの子午面撮像により、本質は(1)であることを示すことができた。また、夜側におけるプラズマ圏界面の位置や界面形成時の形状について、プラズマの共回転・対流に基づく古典論と、さらに重力・遠心力を取り入れた新たな力学モデルとの比較検討を行い、新モデルの観測とのより大きな整合性を見出し、その妥当性を立証した。

さらに、測器開発それ自体にも独創性がある。つまり、ここで用いている30.4nm,という波長域には透過材がなく、しかも鏡材の反射率も低いため結像光学系が簡単には作れない。論文提出者は様々な工夫の末に反射材を独自に選定して新たな極端紫外用反射鏡を開発し、宇宙ステーション搭載予定の測器のために30.4nmで高効率かつ58.4nmで低効率というやや背反する条件を満たすことに成功している。

このように、論文提出者の作業は、これまで適切な光学材料がなかったため未開拓に近い分野であった極端紫外分光という領域において、光学部品をはじめとする測器主要部を自ら開発するという努力によって困難を乗り越えることに始まり、衛星への搭載、データ取得・解析、新事実の発見と解釈にまで至っている。つまり、理学研究のすべてを含みかつ独創性に満ちたまれに見る豊かな研究といえる。

なお、本論文第5章などは本専攻の吉川一朗准教授ほか多くの人々との共同研究であるが、論文提出者が主体となって行ったものであり、その寄与は充分であると判断する。

以上より、論文提出者に博士(理学)の学位を授与できると認める。