前沢/洌

遠方マグネトシースにおける太陽風プラズマの過膨張

講演予稿:
　地球から遠方のマグネトシースの磁場強度とプラズマ速度および密度を、 上流の太陽風中の値と比較して調べた。その結果、１５０Re以上遠方のマグ ネトシースの太陽風密度は、上流の太陽風の密度を下回ることがわかった。 これは、地球のバウショックにおいていったん圧縮されたプラズマが、バウ ショックから遠ざかるにしたがい急速に膨張することを意味している。十分 遠方では、マグネトシースの圧力は、上流の圧力と同じ値に戻るはずである が、温度はバウショックで非断熱的に暖められたため、元にもどらない。その ため、圧力は同じでも、密度は元の太陽風より低い状態まで、プラズマが過 膨張すると解釈できる。また、遠方のマグネトシースの磁場強度も、太陽風の 磁場強度より低いことがわかった。

abstract:
　Study of the plasma and magnetic field properties in the distant magnetosheath is important because they determine the pressure balance at the distant magnetopause and eventually control the cross-sectional size (and possibly the length) of the earth's magnetotail. We investigate the magnetic field (both magnitude and components) and the speed and density of the sheath plasma at x<-150 Re. In order to minimize the statistical fluctuations, the sheath quantities observed by the GEOTAIL spacecraft are normalized by the corresponding upstream solar wind quantities observed by the IMP-8 spacecraft. It is found that the sheath plasma speed at x<-150 Re is almost the same as the upstream solar wind speed, while the distant sheath plasma density is appreciably lower than the upstream solar wind density. We interpret this result as indicating that the magnetosheath plasma, which was once compressed at the earth's bow shock, tends to expand rapidly in the distant magnetosheath. We expect that the sheath plasma pressure will eventually return to the initial solar wind value when the plasma flows towards infinity. However, the plasma temperature would not return to its initial value, since the sheath plasma has gone through the non-adiabatic heating processes at the bow shock. As a matter of fact, adiabatic cooling in the magnetosheath cannot compensate for the nonadiabatic heating at the bow shock and the sheath plasma has higher entropy as compared to the initial solar wind. Thus at infinity, the sheath plasma is expected to approach a state which has the same pressure, but has a higher temperature and a lower density as compared to the solar wind. Our observation indicates that this kind of "over-rarefaction" really occurs in the distant magnetosheath at x< -150 Re. Our analysis also indicates that the magnetic field magnitude in the distant magnetosheath is lower than that of the upstream IMF. This result is consistent with the above finding concerning the expansion of the distant sheath plasma. The result is also consistent with the re-orientation of the magnetosheath magnetic field caused by the inhomogeneous sheath flow around the magnetosphere.

キーワード:
マグネトシース、太陽風、磁気圏尾部

Keywords:
magnetosheath, solar wind, magnetotail