Coronal Holes

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.

Statistical Study of Network Jets Observed in the Solar Transition Region: a Comparison Between Coronal Holes and Quiet-Sun Regions

“…This was suggested as a formation mechanism for large, Type I solar spicules (see also de Pontieu, 1999;Kudoh and Shibata, 1999;Matsumoto and Shibata, 2010). Cranmer and Woolsey (2015) recently extended this idea to smaller-scale turbulent Alfvénic motions in CH network regions, and found that nonlinear mode conversion into chromospheric shocks may provide intermittent upflows of the right order of magnitude. Such a model produces vertical excursions in the height of the transition region of order 1 to 6 Mm over timescales of 20 to 60 s, which gives rise to apparent jet-like velocities of 50 to 200 km s −1 .…”

“…Numerous investigations are being carried out to understand where the solar wind originates and how it is accelerated (for recent reviews see Cranmer (2009) and Hansteen and Velli (2012)). Dark regions in coronal images indicate coronal holes (CHs), which are the commonly accepted large-scale source regions of high-speed solar wind.…”

The magnetic field in the solar atmosphere

“…The magnetic field of CHs is mainly open, i.e., organized in magnetic For techniques used to analyze observations of CHs, it makes a difference whether the holes are observed on the disk or above the solar limb. Discussing the corresponding challenges and issues, however, lies outside the scope of this work and we refer to the review by Cranmer (2009). Here, we focus on the magnetic field structure in CHs.…”

Spectroscopic observations of propagating disturbances in a polar coronal hole: evidence of slow magneto-acoustic waves

“…Magnetohydrodynamic (MHD) waves are important candidates for the heating of the solar corona and the acceleration of the fast solar wind (Cranmer 2009;Ofman 2010). Recently, McIntosh et al (2011a) reported the ubiquitous presence of outwardpropagating transverse oscillations with amplitudes of nearly 20 km s −1 and periods between 100 s and 500 s throughout the quiescent atmosphere, which according to these authors are sufficiently energetic to accelerate the fast solar wind and heat the quiet corona.…”