Seki Kanako, Hirahara Masafumi, Hoshino Masahiro, Terasawa Toshio, Elphic Richard C, Saito Yoshifumi, Mukai Toshifumi, Hayakawa Hajime, Kojima Hirotsugu, Matsumoto Hiroshi
Solar-Terrestrial Environment Laboratory, Nagoya University, Honohara 3-13, Toyokawa, Aichi 442-8507, Japan.
Nature. 2003 Apr 10;422(6932):589-92. doi: 10.1038/nature01502.
Most visible matter in the Universe exists as plasma. How this plasma is heated, and especially how the initial non-equilibrium plasma distributions relax to thermal equilibrium (as predicted by Maxwell-Boltzman statistics), is a fundamental question in studies of astrophysical and laboratory plasmas. Astrophysical plasmas are often so tenuous that binary collisions can be ignored, and it is not clear how thermal equilibrium develops for these 'collisionless' plasmas. One example of a collisionless plasma is the Earth's plasma sheet, where thermalized hot plasma with ion temperatures of about 5 x 10(7) K has been observed. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earth's plasma sheet in coexistence with thermalized hot ions. This cold component cannot be detected by plasma sensors on satellites that are positively charged in sunlight, but our observations in the Earth's shadow show that the density of the cold ions is comparable to that of hot ions. This high density is difficult to explain within existing theories, as it requires a mechanism that permits half of the source plasma to remain cold upon entry into the hot turbulent plasma sheet.
宇宙中大部分可见物质以等离子体形式存在。这种等离子体如何被加热，尤其是初始非平衡等离子体分布如何弛豫到热平衡状态（正如麦克斯韦 - 玻尔兹曼统计所预测的那样），是天体物理和实验室等离子体研究中的一个基本问题。天体物理等离子体通常非常稀薄，以至于二元碰撞可以忽略不计，而对于这些“无碰撞”等离子体如何达到热平衡尚不清楚。无碰撞等离子体的一个例子是地球的等离子体片，在那里已经观测到离子温度约为5×10⁷K的热化热等离子体。在此，我们报告了日食期间等离子体分布函数的直接观测结果，揭示了地球等离子体片中冷离子与热化热离子共存的现象。这种冷成分无法被在阳光下带正电的卫星上的等离子体传感器检测到，但我们在地球阴影中的观测表明，冷离子的密度与热离子相当。这种高密度在现有理论中难以解释，因为这需要一种机制，使得一半的源等离子体在进入热湍流等离子体片时保持低温。
