The velocity distribution of interstellar pickup ions (PUIs) has typically been described as evolving through fast pitch angle scattering followed by adiabatic cooling while being transported radially outward with the solar wind. In combination, the ionization rate, which controls the radial profile of the interstellar neutrals, and the cooling process determine the slope of the observed PUI distributions. Thus far, a cooling index of 3/2 for the PUI velocity distributions has been used in almost all studies. This value is based on the implicit assumptions of immediate PUI isotropization due to pitch angle scattering and solar wind expansion with the square of the distance from the Sun. Here we determine the observed cooling index in a comparison of He + PUI distributions taken for 1 month in the upwind direction with ACE SWICS from 1999 through 2010 over the past solar cycle with such an isotropic PUI model, treating the cooling index as a free parameter. The ionization rate is obtained simultaneously from independent observations. To separate effects of slow pitch angle scattering of PUIs, the comparison is repeated for times restricted to perpendicular interplanetary magnetic field (IMF). When averaged over the entire data set, the cooling index is very close to 3/2. However, it varies substantially from 1.1 to 1.9 between samples, shows a distinct variation with solar activity, and has a significant correlation with sunspot number when data are restricted to nearly perpendicular IMF ( θ B v SW > 60 ° ) excluding the slow pitch angle scattering in the radial IMF direction. The potential influence of slow pitch angle scattering, solar wind structures, and electron ionization on the cooling index and its variations is discussed. Key Points We studied PUI cooling behavior by comparison with observations. We found PUI cooling behavior is correlated with solar activity. Potential influences on the cooling behavior and its variations is discussed.
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机译：星际拾取离子（PUI）的速度分布通常被描述为通过快速俯仰角散射，随后进行绝热冷却，同时随太阳风径向向外传输而发展。结合起来，控制星际中性粒子径向分布的电离速率和冷却过程决定了观察到的PUI分布的斜率。到目前为止，几乎所有研究都使用了PUI速度分布的3/2冷却指数。此值基于俯仰角散射和太阳风膨胀与距太阳距离的平方的即时PUI各向同性的隐含假设。在这里，我们使用这样的各向同性PUI模型，通过比较过去的太阳周期中1999年至2010年ACE SWICS在上风向1个月的He + PUI分布与ACE SWICS得出的观测冷却指数，将冷却指数作为自由参数。电离速率是通过独立观察同时获得的。为了分离PUI的慢俯仰角散射的影响，对限于垂直行星际磁场（IMF）的时间重复进行比较。当对整个数据集进行平均时，冷却指数非常接近3/2。然而，当数据被限制在几乎垂直的IMF（θB v SW> 60°）（不包括慢俯仰角）时，样品之间的差异在1.1到1.9之间变化很大，并显示出太阳活动的明显变化，并且与黑子数具有显着的相关性。沿径向IMF方向散射。讨论了慢俯仰角散射，太阳风结构和电子电离对冷却指数及其变化的潜在影响。重点我们通过与观察值的比较研究了PUI的冷却行为。我们发现PUI的冷却行为与太阳活动有关。讨论了对冷却行为及其变化的潜在影响。
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