Coronal Holes

TL;DR

Coronal holes are dark, active regions on the Sun linked to high-speed solar wind, with recent spectroscopic and coronagraphic measurements revealing complex plasma heating and acceleration processes involving wave physics and turbulence.

Contribution

This review synthesizes plasma measurements in coronal holes, highlighting evidence for open magnetic flux tubes and wave-driven heating, and discusses the role of turbulence and kinetic physics in solar wind acceleration.

Findings

Heavy ions are heated to hundreds of times proton and electron temperatures.

Spectroscopic data support ion cyclotron resonance as a heating mechanism.

Progress has been made in understanding plasma processes despite incomplete models.

Abstract

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 UVCS that heavy ions are heated to hundreds of times the temperatures of protons and electrons. These observations point to specific kinds of collisionless Alfven wave damping (i.e., ion cyclotron resonance), but complete 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 responsible for producing the observed properties of coronal holes.