Coronal Holes (0909.2847v1)

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 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.

• The paper characterizes coronal holes using UVCS and spectroscopic data to reveal their role in high-speed solar wind acceleration.
• The analysis links open magnetic field structures within coronal holes to continuous plasma outflows and enhanced ion heating.
• The findings highlight the importance of kinetic processes like ion cyclotron resonance for understanding solar wind dynamics and space weather impacts.

An Overview of "Coronal Holes"

The paper "Coronal Holes" by Steven R. Cranmer provides a comprehensive analysis of coronal holes, detailing their characteristics and significance in the solar atmosphere. Coronal holes, identified as the darkest and least active regions when the Sun is observed in ultraviolet and X-ray wavelengths, play a crucial role in solar wind dynamics due to their association with open magnetic fields and high-speed solar wind streams.

Key Highlights

1. Characterization and Observation: Coronal holes are regions on the Sun with low density and relatively unipolar magnetic fields which extend into the heliosphere, allowing charged particles to escape as part of the solar wind. These regions are best visible during low solar activity periods and are observed using a combination of spectroscopic and coronagraphic techniques.
2. Magnetic Structure and Solar Wind: The paper explains the linkage between coronal holes and the acceleration of the high-speed solar wind. The configuration of open magnetic field lines within coronal holes facilitates the outward flow of solar wind plasma. However, the extent to which this process is continuous or intermittent, influenced by open flux tubes or closed loops, remains under investigation.
3. Plasma Properties: Measurements within coronal holes reveal distinct plasma characteristics, notably the high degree of heating and acceleration of heavy ions. Utilizing data from the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), the paper describes surprising findings where heavy ions reach temperatures significantly higher than those of protons and electrons.
4. Theoretical Implications: The observations point towards kinetic plasma processes, specifically ion cyclotron resonance and turbulent dissipation, as potential mechanisms for the observed ion heating. However, complete theoretical models are yet to fully validate these processes.
5. Comparison with Other Solar Regions: Unlike active regions and magnetic loops where the magnetic fields are complex and closed, coronal holes represent simpler conditions that provide a natural starting point for modeling solar wind processes.
6. Geomagnetic Impact: The high-speed solar wind streams emanating from coronal holes contribute to geomagnetic storms on Earth, further highlighting the practical significance of understanding these solar phenomena.

Implications and Future Directions

The research on coronal holes offers valuable insights into broader heliophysics and astrophysical contexts, bridging solar observations with plasma physics. While strides have been made in understanding the fundamental mechanisms behind coronal heating and solar wind acceleration, the paper acknowledges that a number of critical questions remain unanswered. Continued observational advancements and theoretical modeling are necessary to unravel these complex processes.

Future work may focus on refining the models for kinetic plasma interactions, investigating the role of transient events like jets and waves in coronal holes, and utilizing forthcoming data from missions like Solar Probe and Solar Orbiter to improve our understanding.

This research significantly contributes to the field by providing a detailed framework for future investigations into the solar wind, its acceleration mechanisms, and its interactions with planetary environments, thereby enhancing predictions of space weather phenomena that impact technological systems on Earth.