論文内容の要旨

論文題目

Signatures of the cold plasma sheet in the near-Earth magnetotail（地球磁気圏近尾部の冷たいプラズマシートの性質）

氏

西 野名真 木The cold plasma sheet formation under northward interplanetary magnetic field (IMF)
has been an important issue of the magnetospheric physics. In order to obtain clues tounderstanding the cold plasma sheet formation under northward IMF, we perform
following studies basing upon spacecraft observations.
In Chapter 2 and 3, we study the temperature anisotropies of electrons andtwo-component protons observed by the Geotail spacecraft. The two-component
protons result from mixing of the cold component from the solar wind and the hotcomponent of the magnetospheric origin, and may be the most eloquent evidence for thetransport process across the magnetopause. The cold component occasionally has astrong anisotropy in the dusk flank, and the sense of the anisotropy depends on theobserved locations: the parallel temperature is enhanced in the tail flank while theperpendicular temperature is enhanced on the dayside. The hot component is nearly
isotropic in the tail while the perpendicular temperature is enhanced on the dayside. The
parallel anisotropy of electrons is stronger than that of the cold proton component,
which is attributed to selective heating of electrons. We further find that strengths of theparallel anisotropies in the tail flank depend on the latitudinal angle of the IMF; strong
parallel anisotropies occur under strongly northward IMF. We discuss that theKelvin-Helmholtz vortices developed under strongly northward IMF and resultant
magnetic reconnection therein may lead to the strong parallel anisotropies observed inthe tail flank.
In Chapter 4, we perform a case study of a duskside Kelvin-Helmholtz (KH) vortices
event on 24 March 1995. We have found that the parallel anisotropy of electrons starts

to be enhanced on the magnetosheath side of a current layer in the KH vortical structure,and low-energy bidirectional electron beams or flat-topped shape of the electron
distribution functions in the direction along the local magnetic field are apparent on themagnetosphere side of the current layer. The protons consisted of two separate (coldand hot) components in the magnetosphere-like region inside the KH vortical structure,
where the cold proton component have a strong parallel anisotropy and it consisted ofbidirectional beams in the region near the boundary. We discuss that the bidirectionalityof electrons and the cold proton component implies magnetic reconnection inside theKH vortical structure. In addition, we suggest selective heating of electrons inside thevortical structure. Comparing temperatures in the magnetosphere-like region inside thevortical structure with those in the cold plasma sheet, we show that further heating istaking place in the cold plasma sheet or on the way from the vortices to the cold plasma
sheet.
In Chapter 5, we deal with the cold plasma sheet found in the midnight region. Throughthe effort to obtain clues toward understanding of transport of cold plasma in thenear-Earth magnetotail under northward IMF, we find that two-component protons areobserved in the midnight plasma sheet under northward IMF. Since the two-component
protons are frequently observed on the duskside during northward IMF intervals but
hardly on the dawnside, those found in the midnight plasma sheet are thought to comefrom the dusk flank. The cold proton component in the midnight region occasionallyhas parallel anisotropy, which resembles that in the tail flank on the duskside. The flows
in the plasma sheet with two-component protons were quite stagnant or slightly going
dawnward, which supports the idea that the observed two-component protons in themidnight region are of duskside origin. Because the two-component protons in themidnight plasma sheet emerge under strongly northward IMF with the latitudinal angle
larger than 45 degrees, and because the lag from the strongly northward IMF to theemergence can be as short as a few h, we suggest that prompt plasma transport fromduskside to midnight region occurs under strongly northward IMF. In addition, we alsosuggest that gradual cooling of hot protons under northward IMF is a global
phenomenon in the near-Earth magnetotail.