西田/篤弘

地磁気静穏時活動におけるプラズマシート温度および密度の流速依存性

講演予稿:
　地磁気活動静穏時のDistant plasma sheet では、反地球向きの流速が大きいほど温度が高く密度が低いという傾向が明らかに見られる。このことは、lobe側から速度、密度を減じるとともに温度をやや増しながら到着したイオンが、プラズマシートに入って加速を受け速度が高められる際に、温度は速度と正の相関を持ち、密度は速度と負の相関を持つことを意味している。この観測事実をIMF北向きの時の尾部リコネクションモデルを基にして論ずる。

abstract:
　Systematic dependence of ion density and temperature on the tailward flow speed in the distant plasma sheet in geomagnetically quiet times A. Nishida, T. Mukai, M. Hoshino, Institute of Space and Astronautical Science and S. Kokubun STELAB, Nagoya University We found earlier that the magnetic reconnection occurs in the magnetotail not only when the IMF is southward but also when it is northward. This conclusion was reached from statistical analyses of plasma and magnetic field observations by Geotail ranging from the very near earth region (at 10 Re) to the distant tail (220 Re) [Nishida et. al., J. Geophys. Res., 103, 4409, 1998; Maezawa and Hori, in New Perspectives of the Earth's Magnetotail, AGU Monograph, 1998]. To comple- ment these statistical studies we have been examining individual cases of the plasma sheet observations in the distant tail, and we addressed the structure of the plasma sheet beyond the distant neutral line at the spring meeting of SGEPSS [Geophys. Res. Lett., in press]. In this sequel we report an interesting feature that is seen in the distant plasma sheet in geomagnetically quiet times. The feature is that density is lower but temperature is higher when tail- ward flow speed is higher, and this is exactly opposite to the trend seen in the lobe and the LLBL where density is higher but temperature is lower when tailward flow speed is higher. In the density versus Vx plot as well as in the temperature versus Vx plot, the regression lines for the plasma sheet and for the lobe intersect roughly at Vx = - 50 km/s, T = 200 eV and N = 0.5 /cc. These two trends can be interpreted to represent the gradual changes in the properties of the ions (1) as they reach the plasma sheet boundary (PSBL) from the lobe side and (2) as they are accelerated in the plasma sheet including PSBL. It is noteworthy that the plasma sheet data points fit a single, well defined trend over a range of Vx which spans -50 to -700 km/s. Bz in the plasma sheet is northward on average but tends to be southward when the tailward flow speed is fast. These observations will be discussed in the framework of the tail reconnection model developed in the papers cited above.

キーワード:
磁気圏尾部、磁力線再結合、リコネクション、プラズマシート

Keywords:
magnetotail, reconnection, plasma sheet