Figure 5 illustrates a selection of visible-light measurements of the electron density in coronal holes and compares them to similar measurements of streamers and to a semi-empirical model of the chromosphere, transition region, and low corona (Avrett and Loeser, 2008). The blue coronal hole curves were adapted from the results of Fisher and Guhathakurta (1995) (dotted), Cranmer et al. (1999b) (solid), Doyle et al. (1999) (dashed), and Guhathakurta et al. (1999a) (dot-dashed). The red curves for equatorial helmet streamers were adapted from the results of Sittler Jr and Guhathakurta (1999) (solid) and Gibson et al. (1999) (dashed).
Note that streamers are denser than coronal holes by about a factor of 10, but the hole measurements themselves can often exhibit variations in the electron density by factors of the order of 2 – 3. Much of this spread is likely to be the result of different lines of sight passing through regions that contain varying numbers of polar plumes (see, e.g., Cranmer et al., 1999b, 2008). Some of the cited pB observations were optimized to avoid bright concentrations of plumes, and others have been purposefully averaged over the full range of coronal hole substructure. It is also possible that absolute calibration uncertainties may still exist between the different instruments used to determine pB and , and this could compound the reported range of variation in coronal hole electron densities.
For a steady-state solar wind outflow, the conservation of mass demands that the product of density, flow speed, and cross-sectional area of the flux tube remain constant. Thus, if the magnetic geometry and the electron density are known, mass conservation allows the solar wind outflow speed to be computed. Kohl et al. (2006) used the representative values of shown in Figure 5 together with a range of estimates for the superradial flux-tube expansion of coronal holes to determine outflow speeds in coronal holes. Figure 41a of Kohl et al. (2006) illustrates the result of this process, which shows a large range of values reflecting the uncertainties in both and the flux-tube area factor. Despite these uncertainties, though, the electron densities that became available in the 1990s demonstrated that the fast solar wind accelerates rapidly in coronal holes – probably reaching half of its asymptotic terminal speed () by heights no larger than .
The increased sensitivity of the LASCO instrument over earlier coronagraphs revealed a nearly continual release of low-contrast density inhomogeneities, or “blobs,” from the cusps of helmet streamers (Sheeley Jr et al., 1997; Tappin et al., 1999; Wang et al., 2000; Chen et al., 2009). These features were seen to accelerate up to speeds of order 300 – 400 km s–1 by the time they reached the outer edge of the LASCO field of view (); see also Figure 8 below. The blobs are typically only about 10% to 15% brighter or dimmer than the surrounding streamer material. Because of this low contrast, these features do not appear to comprise a large fraction of the mass flux of the slow solar wind. However, it is still unclear whether blobs are passive “tracers” that flow with the solar wind speed, or whether they are wavelike fluctuations that propagate relative to the background solar wind reference frame. This diagnostic tool has been much more difficult to apply in coronal holes than it has in the bright streamers, so no firm measurements of the fast wind acceleration yet exist from this technique.
Visible light measurements have also revealed evidence for compressive magnetohydrodynamic (MHD) waves that propagate along open field lines in coronal holes. Intensity oscillations measured by the UVCS and EIT instruments on SOHO were found to have periodicities between about 10 and 30 minutes and are consistent with being upwardly propagating slow-mode magnetosonic waves (DeForest and Gurman, 1998; Ofman et al., 1999, 2000). The relative amplitude of the density fluctuations () for these waves was found to range between about 0.03 and 0.15 (see Cranmer, 2004a). This is consistent with measurements of the density fluctuation amplitudes made at larger distances via radio scintillations (Spangler, 2002) and in situ instruments (Tu and Marsch, 1994). There have also been claims that low-frequency oscillations have been measured in H i Ly emission (Morgan et al., 2004; Bemporad et al., 2008; Telloni et al., 2009). In these cases, however, it is extremely important to take into account all of the relevant instrumental effects. These measurements still appear to be provisional.
As seen in Section 2 above, coronal holes have long been observed as the sites of thin, ray-like polar plumes. The earliest measurements of polar plume properties were made in broad-band visible light, and these dense strands are often seen to stand out distinctly from the ambient interplume corona. It is not clear, though, to what extent off-limb observations (which integrate over long optically thin lines of sight) ever capture only the “pure” plume or interplume plasmas. Space-based observations from, e.g., Spartan 201 and SOHO improved our ability to measure the physical properties in and between plumes (e.g., Fisher and Guhathakurta, 1995; DeForest et al., 1997; Cranmer et al., 1999b; DeForest et al., 2001). Although the brightest plumes are still discernible at the uppermost heights observed by LASCO (i.e., ), the plume/interplume density contrast becomes too low to measure clearly in interplanetary space (). However, indirect and possibly plume-related signatures in the in situ data have been reported by Thieme et al. (1990), Reisenfeld et al. (1999), and Yamauchi et al. (2002). The disappearance of plumes probably is the result of some combination of transverse pressure balance (i.e., dense plumes expanding to fill more of the available volume; see Del Zanna et al., 1998) and MHD instabilities that can mix the two components (Parhi et al., 1999; Andries and Goossens, 2001).
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