Vol. 6 (2009) > lrsp-2009-3

doi: 10.12942/lrsp-2009-3
Living Rev. Solar Phys. 6 (2009), 3

Coronal Holes

1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Mail Stop 50, Cambridge, MA 02138, U.S.A.

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Article 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 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.

Keywords: Solar wind, Solar wind plasma, Kinetic and MHD theory, Turbulence, Corona, Coronal heating, Waves and instabilities, Plasma heating, Coronal holes, Solar cycle, Wave-particle interactions, Solar wind turbulence, Magnetohydrodynamic waves, Space plasmas, Magnetohydrodynamics (MHD), Stellar winds, Cyclotron resonance, Alfv\'{e}n waves, UV radiation

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Article Citation

Since a Living Reviews in Solar Physics article may evolve over time, please cite the access <date>, which uniquely identifies the version of the article you are referring to:

Steven R. Cranmer,
"Coronal Holes",
Living Rev. Solar Phys. 6,  (2009),  3. URL (cited on <date>):

Article History

ORIGINAL http://www.livingreviews.org/lrsp-2009-3
Title Coronal Holes
Author Steven R. Cranmer
Date accepted 15 September 2009, published 29 September 2009
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