Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA.
Southwest Research Institute, San Antonio, Texas 78238, USA.
Phys Rev Lett. 2018 Aug 17;121(7):075102. doi: 10.1103/PhysRevLett.121.075102.
Nonthermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed that PUIs should be preferentially heated at quasiperpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (H^{+}) PUIs at an interplanetary shock by the New Horizons' Solar Wind Around Pluto (SWAP) instrument at ∼34 au from the Sun. At this shock, H^{+} PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of H^{+} SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. H^{+} SWIs lose ∼85% of their energy flux across the shock and H^{+} PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all H^{+} PUIs is approximately six times that of H^{+} SWIs. We find that H^{+} PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.
非热 pickup 离子（PUIs）是在太阳风中通过太阳风离子（SWIs）与慢星际中性原子之间的电荷交换而产生的。长期以来，人们一直理论上认为，与热 SWIs 相比，PUIs 在准垂直激波处应该优先加热，但尚未直接观测到。我们通过新视野号环绕冥王星的太阳风（SWAP）仪器在距太阳约 34 个天文单位处的星际氢（H^{+}）PUIs 在行星际激波处提供了原位观测。在这个激波处，H^{+}PUIs 仅占总质子密度的百分之几，但包含了大部分内部粒子压力。在激波之前，观察到 SW 流速逐渐降低和 H^{+}SWIs 同时加热，这表明上游存在高能粒子压力梯度。H^{+}SWIs 在激波处损失了约 85%的能量通量，而 H^{+}PUIs 则被优先加热。此外，在激波下游还观察到了 PUI 尾，使得所有 H^{+}PUIs 的能量通量大约是 H^{+}SWIs 的六倍。我们发现，H^{+}PUIs，包括其超热尾部，在激波框架下几乎包含了总下游能量通量的一半。
