List of Figures

View Image Figure 1:
Left: Adaptation of a sketch of the quiet solar corona made by Piddington (1972), based on prior drawings (Waldmeier, 1955) and photographs (Gold, 1955) of the 30 June 1954 eclipse. Right: Contrast-adjusted eclipse image taken with the POISE instrument on 26 February 1998, in Westpunt, Curaçao. The original image was made available courtesy of the High Altitude Observatory (HAO), University Corporation for Atmospheric Research (UCAR), Boulder, Colorado. UCAR is sponsored by the National Science Foundation.
View Image Figure 2:
X-ray corona (0.25 – 4.0 keV) observed by the Soft X-ray Telescope (SXT) on Yohkoh, on 6 December 2000. Yohkoh is a mission of the Institute of Space and Astronautical Sciences in Japan, with participation from the U.S. and U.K.
View Image Figure 3:
Polar view of the development of the north polar coronal hole from January to August 2001 (e.g., Carrington rotations 1972 to 1979), using reconstructed coronal hole boundaries from Kitt Peak He i 10830 Å maps. The maximum of solar activity occurred between late 2000 and early 2001. Data from the National Solar Observatory/Kitt Peak were produced cooperatively by NSF/NOAO, NASA/GSFC, and NOAA/SEL.
View Image Figure 4:
Summary of the largely unipolar magnetic field structure of polar coronal holes, with the fields of view successively widening from flux tubes in intergranular lanes (a), to a “funnel” rooted in a supergranular network lane (b), and finally to the extended corona (c). Adapted from Figure 1 of Cranmer and van Ballegooijen (2005).
View Image Figure 5:
Comparison of empirically determined densities in the upper solar atmosphere. Avrett and Loeser (2008) values of electron number density (solid black curve) and total hydrogen number density (dot-dashed black curve) are compared with various visible-light pB electron number densities for coronal holes (blue curves) and streamers (red curves); see text for details.
View Image Figure 6:
Radial dependence of empirical and model temperatures in polar coronal holes and fast wind streams. Mean plasma temperatures from a semi-empirical model (dashed black curve; Avrett and Loeser, 2008) and from a turbulence-driven coronal heating model (solid black curve; Cranmer et al., 2007). Te from off-limb SUMER measurements made by Wilhelm (2006) (dark blue bars) and Landi (2008) (light blue bars), Tp from UVCS measurements assembled by Cranmer (2004b) (see text), and perpendicular O+5 ion temperatures from Landi and Cranmer (2009) (open green circles) and Cranmer et al. (2008) (filled green circles). In situ proton and electron temperatures in the fast wind (r > 60R ⊙) are from Cranmer et al. (2009).
View Image Figure 7:
Combined image of the solar corona from 17 August 1996, showing the solar disk in Fe XII 195 Å intensity from EIT (yellow inner image) and the extended corona in O vi 1032 Å intensity from UVCS (red outer image). Axisymmetric field lines are from the solar-minimum model of Banaszkiewicz et al. (1998), and O vi emission line profiles (bottom) are from SUMER (Warren et al., 1997, left) and UVCS (Kohl et al., 1997, right).
View Image Figure 8:
Radial dependence of solar wind outflow speeds. UVCS Doppler dimming determinations for protons (red; Kohl et al., 2006) and O+5 ions (green; Cranmer et al., 2008) are shown for polar coronal holes, and are compared with theoretical models of the polar and equatorial solar wind at solar minimum (black curves; Cranmer et al., 2007) and the speeds of “blobs” measured by LASCO above equatorial streamers (open circles; Sheeley Jr et al., 1997).
View Image Figure 9:
Summary of Cranmer et al. (2007) models: (a) The adopted solar-minimum field geometry of Banaszkiewicz et al. (1998), with radii of wave-modified critical points marked by symbols. (b) Latitudinal dependence of wind speed at ∼ 2 AU for models with n = 1 (multi-color curve) and n = 2 (brown curve), compared with data from the first Ulysses polar pass in 1994 – 1995 (black curve; Goldstein et al., 1996). (c) T (r) for polar coronal hole (red solid curve), streamer edge (blue dashed curve), and strong-field active region (black dotted curve) models.
View Image Figure 10:
Solar wind specific entropy plotted as a function of solar wind speed, computed for both the ZEPHYR models at 1 AU (black symbols, curve) and from ACE/SWEPAM data (blue points).
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.