Coronal holes are the darkest and least active regions of the Sun, as observed both on the solar disk and above the solar limb. Coronal holes are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. This paper reviews measurements of the plasma properties in coronal holes and how these measurements are used to reveal details about the physical processes that heat the solar corona and accelerate the solar wind. It is still unknown to what extent the solar wind is fed by flux tubes that remain open (and are energized by footpoint-driven wave-like fluctuations), and to what extent much of the mass and energy is input intermittently from closed loops into the open-field regions. Evidence for both paradigms is summarized in this paper. Special emphasis is also given to spectroscopic and coronagraphic measurements that allow the highly dynamic non-equilibrium evolution of the plasma to be followed as the asymptotic conditions in interplanetary space are established in the extended corona. For example, the importance of kinetic plasma physics and turbulence in coronal holes has been affirmed by surprising measurements from the UVCS instrument on SOHO that heavy ions are heated to hundreds of times the temperatures of protons and electrons. These observations point to specific kinds of collisionless Alfvén wave damping (i.e., ion cyclotron resonance), but complete theoretical models do not yet exist. Despite our incomplete knowledge of the complex multi-scale plasma physics, however, much progress has been made toward the goal of understanding the mechanisms ultimately responsible for producing the observed properties of coronal holes.

中文翻译：

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冕洞。

正如在太阳盘和太阳边缘上方观察到的那样，冕洞是太阳最黑暗和最不活跃的区域。冕洞与快速扩张的开放磁场和高速太阳风的加速有关。本文回顾了日冕洞中等离子体特性的测量，以及如何使用这些测量来揭示加热日冕和加速太阳风的物理过程的细节。目前仍不清楚太阳风在多大程度上是由保持开放的通量管供给的（并由足点驱动的波状波动提供能量），以及在多大程度上有多少质量和能量从闭环间歇性地输入到太阳风中。开放区域。本文总结了这两种范式的证据。还特别强调光谱和日冕测量，当在扩展日冕中建立行星际空间的渐近条件时，可以跟踪等离子体的高度动态非平衡演化。例如，SOHO 上 UVCS 仪器的令人惊讶的测量结果证实了动态等离子体物理和日冕洞湍流的重要性，即重离子被加热到质子和电子温度的数百倍。这些观察结果指出了特定类型的无碰撞阿尔文波阻尼（即离子回旋共振），但完整的理论模型尚不存在。然而，尽管我们对复杂的多尺度等离子体物理学的了解还不完全，但在理解最终负责产生所观察到的冕洞特性的机制方面已经取得了很大进展。