By far, the strongest causal link between a specific type of coronal structure (measured via remote sensing) and a particular type of quasi-steady solar wind flow (measured in situ) is the connection between large coronal holes and high-speed streams (Wilcox, 1968; Krieger et al., 1973). The general interpretation of this correlation – together with the results of magnetic extrapolation models such as PFSS – is that coronal holes represent a bundle of open flux tubes that flare out horizontally as distance increases. In other words, the coronal hole flux tubes expand superradially. Although there are some observations that appear to support other interpretations (Woo et al., 1999; Habbal et al., 2001; Woo, 2005; Woo and Druckmüllerová, 2008), the preponderance of evidence seems clearly to support the idea that fast solar wind streams emerge mainly from superradially expanding coronal holes (e.g., Guhathakurta et al., 1999b; Cranmer et al., 1999b; Jones, 2005; Wang and Sheeley Jr, 2006; Wang et al., 2007).
In contrast to the rather definitive correlation between large coronal holes and the fast solar wind, the coronal sources of the more chaotic slow-speed solar wind are not as well understood (see Schwenn, 2006). Two regions that are frequently cited as sources of slow wind are: (1) boundaries between coronal holes and streamers, and (2) narrow plasma sheets that extend out from the tops of streamer cusps (Wang et al., 2000; Strachan et al., 2002; Susino et al., 2008). These regions tend to dominate around solar minimum. Note that the former type of boundary region tends to contain flux tubes that may be classified as coronal holes when using the theoretical definition (i.e., footpoints of field lines that are open; see Section 1) but would not be defined as such when using the observational definitions (i.e., low emission or low density).
During more active phases of the solar cycle, there is evidence that slow solar wind streams also emanate from small coronal holes (e.g., Nolte et al., 1976; Neugebauer et al., 1998; Zhang et al., 2003) and active regions (Hick et al., 1995; Liewer et al., 2004; Sakao et al., 2007). During the rising phase of solar activity, there seems to be a relatively abrupt ( 6 month) change in the locations of slow wind footpoints: from the high-latitude hole/streamer boundaries to the low-latitude active region and small coronal hole regions (Luhmann et al., 2002). The ability of many of these kinds of regions to produce slow wind was modeled by Cranmer et al. (2007) and Wang et al. (2009); see also Section 5.
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