6 Summary and Conclusions

The last decade has seen significant progress toward identifying and characterizing the processes that produce coronal holes. As remote-sensing plasma measurements have become possible in the extended solar corona (i.e., the region of primary acceleration of the solar wind), the traditional gap between solar physics and in situ space physics has become narrower. However, there are still many unanswered questions: How and where in the solar atmosphere are the relevant waves and turbulent motions generated? Which kinds of fluctuation modes (i.e., linear or nonlinear; Alfvén, fast, or slow; high k∥ or high k⊥) are most important? What frequencies dominate the radially evolving power spectrum? What fraction of the interplanetary solar wind comes from filamentary structures such as polar plumes and polar jets? Are there relatively simple “scaling laws” that will allow us to use only the measured properties at the solar surface to predict the resulting amount of coronal heating and solar wind acceleration?

Answering the above questions involves moving forward in both the theoretical and observational directions. Section 5 described the large number of suggested physical processes for energizing the plasma in coronal holes. The validity of many of these processes still needs to be assessed, and their relative contributions to the heating and acceleration of the actual solar corona need to be quantified. If, at the end of this process, there are still a number of mutually inconsistent theories that are still viable, the only way forward is to determine what future measurements would best put the remaining models to the test. These activities are ongoing with the planning of inner heliospheric missions such as Solar Probe (McComas et al., 2007) and Solar Orbiter (Marsden and Fleck, 2007), as well as next-generation ultraviolet coronagraph spectroscopy missions that would follow up on the successes of UVCS/SOHO (see, e.g., Cranmer, 2002bGardner et al., 2003Kohl et al., 2006).

The development of more physically sound models of the solar wind feeds back in many ways to a wider understanding of stellar outflows and star/planet evolution. Figure 11View Image shows some of the the early stages of evolution for a representative solar-type star. At all ages, cool stars are inferred to exhibit some kind of wind or jet-like outflow (Lamers and Cassinelli, 1999Wood, 2004Güdel, 2007Cranmer, 2008a). Young stars first become visible as dust-obscured cloud cores and protostars (e.g., Lada, 1985Hartmann, 2000), and these objects are often associated with bipolar, collimated jets. These outflows indicate some kind of transfer of energy from the accretion disk’s orbital motion to torqued magnetic fields (rooted on the stellar surface) that relieve the buildup of angular momentum and eject plasma out the poles (e.g., Blandford and Payne, 1982van Ballegooijen, 1994). As the accretion rates decrease over time, protostars become visible as classical T Tauri stars (CTTS), and there remains ample evidence for polar outflows in the form of both “disk winds” and true stellar winds (Hartigan et al., 1995Ferreira et al., 2006Cranmer, 2008b). The primordial accretion disk is dissipated gradually as the star enters the weak-lined T Tauri star (WTTS) phase, with a likely transition to a protoplanetary dust/debris disk. Strong stellar magnetic activity remains evident during these stages from, e.g., X-rays (Feigelson and Montmerle, 1999). Many “post T Tauri” stars, once they reach the zero-age main sequence (ZAMS), remain rapidly rotating, and for young ZAMS stars such as AB Dor there is evidence for a range of X-ray emitting plasma from dark polar spots (which probably do not correspond to open magnetic field regions like coronal holes) to huge “slingshot prominences” extending over several stellar radii (e.g., Güdel et al., 20012003Jardine and van Ballegooijen, 2005).

View Image

Figure 11: Illustration of the evolving circumstellar environment of a solar-mass star (see text), showing various kinds of open-field structures that may be analogous to present-day coronal holes.

Learning about the fundamental physics responsible for solar coronal holes has relevance that reaches into other areas of study besides astrophysics, including plasma physics, space physics, and astronautical engineering. The practical benefits of improving long-term predictions for the conditions of the Earth’s local space environment are manifold (see, e.g., Feynman and Gabriel, 2000Eastwood, 2008). In addition, parallel research into the expansion of the polar wind from the Earth’s ionosphere has led to an improved understanding of kinetic processes in plasmas on the boundary between collisional and collisionless conditions (Lemaire and Pierrard, 2001Barakat and Schunk, 2006). Finally, a growing realization that strict topical compartmentalization is often a hindrance to making progress has given rise to greater interest in interdisciplinary studies of “universal processes in heliophysics” (Crooker, 2004Davila et al., 2009).

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