Coronal Holes

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.Comment: 61 pages, 12 figures. Accepted by the online journal "Living Reviews in Solar Physics." The abstract has been abbreviated slightly, and some figures are degraded in quality from the official version, which will be available at http://solarphysics.livingreviews.org

Turbulence in the Solar Corona

The solar corona has been revealed in the past decade to be a highly dynamic nonequilibrium plasma environment. Both the loop-filled coronal base and the extended acceleration region of the solar wind appear to be strongly turbulent, but direct observational evidence for a cascade of fluctuation energy from large to small scales is lacking. In this paper I will review the observations of wavelike motions in the corona over a wide range of scales, as well as the macroscopic effects of wave-particle interactions such as preferential ion heating. I will also present a summary of recent theoretical modeling efforts that seem to explain the time-steady properties of the corona (and the fast and slow solar wind) in terms of an anisotropic MHD cascade driven by the partial reflection of low-frequency Alfven waves propagating along the superradially expanding solar magnetic field. Complete theoretical models are difficult to construct, though, because many of the proposed physical processes act on a multiplicity of spatial scales (from centimeters to solar radii) with feedback effects not yet well understood. This paper is thus a progress report on various attempts to couple these disparate scales.Comment: 6 pages, 1 figure (AIP 6x9 style), to appear in AIP Conference Proceedings: "Turbulence and Nonlinear Processes in Astrophysical Plasmas" (6th Annual IGPP International Astrophysics Conference), Waikiki, March 16-22, 200