Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions beyond the Heliopause
Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions beyond the Heliopause
复制标题
日球层顶以外的弹性碰撞对星际中性氦的减速和加热
DOI：
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发表时间：
2021
影响因子：
7.9
通讯作者：
J. Heerikhuisen
中科院分区：
文献类型：
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作者：
P. Swaczyna;F. Rahmanifard;E. Zirnstein;D. McComas;J. Heerikhuisen
对靠近太阳的星际中性（ISN）原子进行直接采样可以研究日光层周围的局部星际介质（VLISM）。到目前为止，ISN 氦原子的主要种群一直被认为反映了日球层顶的原始 VLISM 条件。因此，星际边界探测器 (IBEX) 在 1 个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，而不考虑与日球层顶以外其他物种的弹性碰撞。在这里，我们评估了这些碰撞对初级 ISN 氦族的影响。我们追踪氦原子的轨迹，并追踪它们在日球层顶之前与减速的等离子体和星际氢原子的碰撞。原子通常在外日鞘中碰撞几次，只有 ∼1.5% 的原子根本没有散射。我们使用计算出的微分横截面来随机选择这些碰撞中的散射角。我们估计，与原始 VLISM 相比，在日球层顶产生的初级 ISN 氦原子减慢了~0.45 km s−1，并加热了~1100 K。由此产生的速度分布是不对称的，并且在反太阳方向上显示出延长的尾部。考虑到 IBEX 观测得出的参数的这种变化，得出原始 VLISM 中太阳的相对速度为 25.85 km s−1，温度为 6400 K。最后，这封信作为氦原子弹性碰撞的微分截面的来源。
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−1 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−1 and temperature of 6400 K in the pristine VLISM. Finally, this Letter serves as a source of the differential cross sections for elastic collisions with helium atoms.