Wang Linghua, Lin Robert P, Larson Davin E, Luhmann Janet G
Physics Department, University of California, Berkeley, California 94720-7300, USA.
Nature. 2008 Jul 3;454(7200):81-3. doi: 10.1038/nature07068.
The solar wind blows an immense magnetic bubble, the heliosphere, in the local interstellar medium (mostly neutral gas) flowing by the Sun. Recent measurements by Voyager 2 across the termination shock, where the solar wind is slowed to subsonic speeds before entering the heliosheath, found that the shocked solar wind plasma contains only approximately 20 per cent of the energy released by the termination shock, whereas energetic particles above approximately 28 keV contain only approximately 10 per cent; approximately 70 per cent of the energy is unaccounted for, leading to speculation that the unmeasured pickup ions or energetic particles below 28 keV contain the missing energy. Here we report the detection and mapping of heliosheath energetic ( approximately 4-20 keV) neutral atoms produced by charge exchange of suprathermal ions with interstellar neutral atoms. The energetic neutral atoms come from a source approximately 60 degrees wide in longitude straddling the direction of the local interstellar medium. Their energy spectra resemble those of solar wind pickup ions, but with a knee at approximately 11 keV instead of approximately 4 keV, indicating that their parent ions are pickup ions energized by the termination shock. These termination-shock-energized pickup ions contain the missing approximately 70 per cent of the energy dissipated in the termination shock, and they dominate the pressure in the heliosheath.
太阳风在流经太阳的局部星际介质（主要是中性气体）中吹出一个巨大的磁泡，即日球层。“旅行者2号”最近穿越终端激波的测量发现，在终端激波处太阳风减速至亚音速后进入日鞘，被激波作用后的太阳风等离子体仅包含终端激波释放能量的约20%，而能量高于约28 keV的高能粒子仅包含约10%；约70%的能量无法解释，这引发了一种推测，即未被测量的拾取离子或能量低于28 keV的高能粒子包含了缺失的能量。在此，我们报告了对日鞘高能（约4 - 20 keV）中性原子的探测和绘图，这些中性原子是由超热离子与星际中性原子进行电荷交换产生的。高能中性原子来自一个经度约60度宽的源，横跨局部星际介质的方向。它们的能谱与太阳风拾取离子的能谱相似，但在约11 keV处有一个拐点，而不是约4 keV，这表明它们的母离子是被终端激波加速的拾取离子。这些被终端激波加速的拾取离子包含了在终端激波中耗散的约70%的缺失能量，并且它们主导了日鞘中的压力。
