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Living Rev Sol Phys
2009[]; 6
(?): 3
PMID27194961
show ga
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.
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