Steven R Cranmer Submitted: 2009-09-16 09:07
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
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.
Authors: Steven R. Cranmer
Publication Status: Living Reviews in Solar Physics, in press. (61-page review paper)
Last Modified: 2009-09-17 11:00