田口/聡

講演予稿:
　地球から 45 地球半径以内の磁気圏尾部にてGeotail 衛星によって得られた磁場 およびプラズマデータを解析して、尾部から地球方向に動くプラズモイドの特性を 明らかにした。地球から離れる方向に動く通常のプラズモイドの発生と比較して、 地球向きに動くプラズモイドは３分の１の程度の頻度で起こっており、極めて通常 の現象であることがわかった。地球向きに動くプラズモイドの尾部側では、南北の ローブ領域から磁気中性面に向かって100 - 250 km/s 速度でプラズマの流れが起 こっていた。これは、プラズモイドがその尾部側で起こっているローブのリコネク ションによって地球方向に駆動されていることを示している。

abstract:
Many analyses of Geotail, ISEE 3 and IMP 8 from the mid- or distant tail have indicated that plasmoids are ejected tailward in association with auroral substorms. Plasmoids ejected tailward are recognized from a bipolar excursion of the magnetic field direction, first northward and then southward, often together a high By core coincident with high-speed tailward flow in the central plasma sheet. The tailward moving plasmoids are also identified as a traveling compression region (TCR) which is a northward and then a southward (bipolar) field variation coincident with the field compression in the lobe or the plasma sheet boundary layer (PSBL). Measurements in the central plasma sheet or the lobe have also suggested that plasmoid can move earthward. South- north bipolar field tilting identified in these regions is consistent with the picture of the earthward propagating plasmoid. However, because the evidence in the central plasma sheet observation is very limited and also because a lobe S-N compression can be produced by an external increase in solar wind pressure, whether the earthward moving plasmoid is a common entity in the magnetosphere or a rare event is not understood. In this study we survey Geotail mid-tail lobe and PSBL data for the magnetic field signatures of the earthward moving plasmoids. We first identified the tail lobe and PSBL intervals for 20 min or longer from the data obtained earthward of X= -45 Re for November 1994 through January 1998 with appropriate criteria based on the magnetic field and plasma data. From the identified intervals, S-N field tilting and only north field tilting with a compression were searched. About 100 events were identified in the region of X= -20 to -45 Re. When this number is compared to the number (about 300) of the usual TCR events associated with tailward moving plasmoids, it indicates that the earthward moving plasmoids are common phenomena in the mid-tail. The plasma data for the most of the S-N (or only N) events show that the lobe plasma velocity towards the neutral sheet is enhanced and peaked around the most northward tilting of the field. The coincidence of the plasma motion toward the neutral sheet with the north field tilting denies the possibility that the S-N events are produced by an external increase in solar wind pressure. If the S-N events are so produced, the velocity toward the neutral sheet would occur with the south tilting of the field. The peak magnitude of the enhanced velocity lies from 100 to 250 km/s which are roughly 10% or less of the lobe Alfven speed. We interpret the velocity enhancement having these characteristics as plasma inflow by rarefaction due to the earthward/tailward transport of lobe fluxes from the reconnection region tailward of the satellite. Hence, the earthward moving plasmoid is driven by reconnection that reaches the lobes tailward of the satellite before reconnection at a near-Earth region proceeds to the lobes.

キーワード:
ジオテール衛星、磁気圏尾部、プラズモイド、リコネクション

Keywords:
Geotail satellite, magnetotail, plasmoids, reconnection