太阳顶外弹性碰撞对星际中性氦的减速和加热
Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions Beyond the Heliopause
摘要：
对太阳附近的星际中性原子直接取样，可以研究日球层周围的非常局域的星际介质。到目前为止，ISN氦原子的主要种群一直被认为反映了太阳层顶原始的VLISM条件。因此，由星际边界探测器（IBEX）在1天文单位观测到的原子被用来确定VLISM相对于太阳的温度和速度，而不考虑与太阳层顶外其他物种的弹性碰撞。在这里，我们评估这些碰撞对主要ISN氦人口的影响。我们跟踪氦原子的轨迹，并跟踪它们与减速等离子体和星际氢原子的碰撞。原子通常会在日鞘外层碰撞几次，只有约1.5%的原子完全没有散射。我们使用计算的微分截面随机选择这些碰撞中的散射角。我们估计，由此产生的初级ISN氦原子在日球顶的减速~0.45公里/秒，加热~1100 K相比，原始VLISM。由此产生的速度分布是不对称的，并显示出一个延长的尾巴在反向阳方向。考虑到IBEX观测参数的变化，原始VLISM中太阳的相对速度为25.85 km/s，温度为6400 K。最后，本文作为氦原子弹性碰撞微分截面的一个来源。
英文摘要：
Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables studies of the very local interstellar medium (VLISM) around the heliosphere. The primary population of ISN helium atoms has, until now, been assumed to reflect the pristine VLISM conditions at the heliopause. Consequently, the atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used to determine the VLISM temperature and velocity relative to the Sun, without accounting for elastic collisions with other species outside the heliopause. Here, we evaluate the effect of these collisions on the primary ISN helium population. We follow trajectories of helium atoms and track their collisions with slowed plasma and interstellar hydrogen atoms ahead of the heliopause. Atoms typically collide a few times in the outer heliosheath, and only ~1.5% of the atoms are not scattered at all. We use calculated differential cross sections to randomly choose scattering angles in these collisions. We estimate that the resulting primary ISN helium atoms at the heliopause are slowed down by ~0.45 km/s and heated by ~1100 K compared to the pristine VLISM. The resulting velocity distribution is asymmetric and shows an extended tail in the antisunward direction. Accounting for this change in the parameters derived from IBEX observations gives the Sun's relative speed of 25.85 km/s and temperature of 6400 K in the pristine VLISM. Finally, this paper serves as a source of the differential cross sections for elastic collisions with helium atoms.
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