Coronal Holes and the Solar Wind

Coronal holes are the darkest regions of the ultraviolet and X-ray Sun, both on the disk and above the 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 of coronal holes and how these measurements have been used to put constraints on theoretical models of coronal heating and solar wind acceleration. Heat deposition at the dense and collisional coronal base is of comparable importance (in determining, e.g., temperature gradients and asymptotic outflow speeds) as extended heating in the collisionless regions above 2 solar radii. Thus, a complete understanding of the physics requires both observations of the solar disk and inner corona (Yohkoh, EIT, CDS, SUMER) and coronagraphic observations of the wind’s acceleration region (UVCS, LASCO). Although strong evidence has been found to suggest that the high-speed wind is driven mainly by proton pressure, the differences between proton, electron, and heavy ion velocity distributions are extremely valuable as probes of the dominant physical processes. INTRODUCTION To be published in the proceedings of the Yohkoh 10th Anniversary Meeting, “Multi-wavelength Observations of Coronal Structure and Dynamics,” 21–24 January 2002, Kailua-Kona, Hawaii. The existence of coronal holes was first recognized by Waldmeier (1957, 1975), who noticed long-lived regions of negligible intensity in coronagraphic images of the 5303 Å green line. Waldmeier called the features that appeared more-or-less circular when projected onto the solar disk Löcher (holes), and the more elongated features were called Rinne (grooves) or Kanal (channels). The fact that coronal holes coincide with regions of open magnetic field that extend into interplanetary space was realized during the first decade of in situ solar wind observations (e.g., Wilcox, 1968). Coronal holes were effectively “re-discovered” in the early 1970s as discrete dark patches on the X-ray and ultraviolet solar disk, and their connection with the high-speed component of the solar wind soon became evident (Krieger et al., 1973; Zirker, 1977). The term “coronal hole” thus has come to denote both the on-disk features and their open-field extensions off the solar limb. This paper provides a brief review of the physics of coronal holes and the acceleration of the high-speed solar wind. Coronal holes become distinguishable from neighboring quiet and active regions several Mm above the photosphere, where the temperature exceeds 105 K. At these low coronal heights, holes exhibit lower densities and temperatures than other regions (see, e.g., Esser & Habbal, 1997). At larger heights, as the plasma becomes less collisiondominated, coronal hole densities remain relatively low but the temperatures of different plasma components begin to depart strongly from thermal equilibrium, with Te < Tp < Tion. Despite the large-scale identification of coronal holes with open magnetic field lines, they contain a wide variety of magnetic structures, from X-ray bright points and spicules on the smallest scales to plumes and jets on larger scales (see Figure 1). At the minimum of the Sun’s 11-year