4.1 Magnetic connectivity with the solar wind

Although the magnetic field in the solar corona is generally too weak to be measured directly, the overall morphology of the field lines can be extrapolated from magnetograms taken at the level of the photosphere. One popular technique is the “potential field source surface” (PFSS) method, which assumes the corona is current-free out to a spherical surface (set typically at a radius between 2.5 and 3.5 solar radii, or R ⊙), above which the field is radial (e.g., Schatten et al., 1969Altschuler and Newkirk Jr, 1969Hoeksema and Scherrer, 1986). The PFSS method has been shown to create a relatively good mapping between the Sun and the heliosphere (Arge and Pizzo, 2000Jump To The Next Citation PointLuhmann et al., 2002Jump To The Next Citation PointWang and Sheeley Jr, 2006Jump To The Next Citation PointWang, 2009), although the results can be problematic for regions dominated by stream-stream interactions (Poduval and Zhao, 2004).

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, 1968Krieger 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., 1999Habbal et al., 2001Woo, 2005Woo 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., 1999bCranmer et al., 1999bJump To The Next Citation PointJones, 2005Wang and Sheeley Jr, 2006Jump To The Next Citation PointWang 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., 2000Jump To The Next Citation PointStrachan et al., 2002Susino 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., 1976Jump To The Next Citation PointNeugebauer et al., 1998Jump To The Next Citation PointZhang et al., 2003) and active regions (Hick et al., 1995Liewer et al., 2004Sakao 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. (2007Jump To The Next Citation Point) and Wang et al. (2009Jump To The Next Citation Point); see also Section 5.

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