velocities (Hyder & Lites, 1970; Noci et al., 1987; Withbroe et al., 1982). These observations applied to the O VI doublet lines have shown that the fast solar wind becomes supersonic much closer to the Sun than the slow solar wind. The fast O5+ ions reach speeds in excess of 600 km/s within 4 solar radii from the solar surface (Antonucci et al., 2000). Identical techniques were also used to study flows in the vicinity of streamers, adjacent but not above the helmet, and found that the slow solar wind accelerates more slowly, with its outflow speed remaining below 200 km/s at least until 4 solar radii (Abbo et al., 2010; Strachan et al., 2000). Figure 1.2 presents a summary of these results.
Figure 1.2 Outflow velocity (km/s) of the solar wind for the considered four regions as a function of the heliocentric distance (in solar radii). The gray band from 4 to 10 R⊙ shows the range of outflow velocities for the slow wind obtained with LASCO (Sheeley et al., 1997). The solid curve up to 3 R⊙ represents the values of the fast wind obtained from the UVCS data (Antonucci et al., 2000), and the dashed curves show the results by Telloni et al. (2007) of the fast wind velocity. The error bars (small in many cases) have been estimated based on the propagation of the statistical uncertainties of the observed OVI 1032 and 1037 line intensities.
(Source: Reproduced from Abbo et al., 2010. © 2010, Elsevier.)
A similar analysis of the Doppler dimming technique has been applied to the Lyman‐ α (121.6 nm) emission of neutral hydrogen. The latter moves outward in response to rapid charge‐exchange coupling with the heated protons that eventually form the bulk of the solar wind. Neutral hydrogen therefore acts as a proxy for protons at heights up to 2.5 solar radii in coronal holes, and higher heights in more dense structures. In some coronal structures, He+ can act as a proxy for alpha particles, which are also an important component of the solar corona. The UVCS/SoHO instrument has provided H I Lyman‐ α spectral line data over 18 years. This makes it possible to study proton outflow speeds throughout the solar cycle, focusing on the coronal region sampled by the spectrometer field of view, such as coronal streamers (Susino et al., 2008; Zangrilli & Poletto, 2016) and coronal holes (Antonucci et al., 2000; Strachan et al., 1993; Teriaca et al., 2003).
In a recent analysis (Bemporad, 2017), UVCS daily Lyman‐ α synoptic data were combined to provide the first 2D images of coronal Lyman‐ α emission, representative of future data that will be acquired by the Metis coronagraph onboard Solar Orbiter (Antonucci et al., 2017). These have been directly combined with classical 2D coronagraphic images acquired in white light with LASCO to derive 2D maps of HI outflow speeds, with a technique originally described by Withbroe et al. (1982) that neglects line‐of‐sight integration effects. As pointed out by Bemporad (2017), because both the radiative component of Lyman‐ α emission and the white‐light polarized emission depend on the electron density distribution integrated along the line of sight, this latter quantity can be simplified by directly taking the ratio between the two UV and white‐light intensities.
Figure 1.3 shows an example of a 2D outflow velocity map, ranging between 1.5 and 4.0 R⊙, that was obtained this way (Bemporad, 2017). The map shows an increasing speed with altitude from about 150–200 km/s in the equatorial regions to 400 km/s in the polar regions. As we shall see, these values are in agreement with the expected latitudinal distribution of slow and fast solar wind components during solar minimum, corresponding to equatorial regions, and mid‐latitude and polar regions, respectively. Alternatively, as shown more recently by Dolei et al. (2018), line‐of‐sight integration effects can be fully taken into account (under some assumptions) by deriving electron densities from white‐light coronagraphic images. From this, one can derive HI outflow speed maps by again exploiting the Doppler dimming technique discussed above.
Figure 1.3 2D map of radial outflow velocity in the plane of the sky derived from the ratio between white‐light and UV coronal emissions. The outer white region corresponds to altitudes where the Doppler dimming technique with the Ly spectral line cannot be applied anymore.
(Source: Taken from Bemporad, 2017. © 2017, IOP Publishing.)
The solar wind acceleration has also been measured via oscillations of the coronal plasma, identified as Alfvén waves in coronagraphic observations. Alfvén waves transverse to the plane‐of‐sky have been observed to be omnipresent in coronal holes within the first 0.3R⊙ of the solar atmosphere by the Coronal Multichannel Polarimeter (Tomczyk et al., 2016). Detailed analysis of off‐limb Doppler shifts (Fe XIII) at high time cadence has furthermore revealed the presence of upward‐ and downward‐propagating waves and, by comparing their respective phase speeds, allowed for the determination of the bulk flow speed profile of nascent fast wind flows (Morton et al., 2015, 2016). These observations have furthermore shown that the spectra of the oscillations show a predominant 1/f slope, reminiscent of solar wind measurements made beyond 0.3 AU by past space probes (e.g., Bavassano, Dobrowolny, Fanfoni et al., 1982; M. L. Goldstein et al., 1995). This supports the idea that the 1/f component of the oscillation spectra observed in the solar wind are already set in the low solar atmosphere and are advected outward (see, for example, Verdini et al., 2012). The statistical properties of fluctuations at magnetohydrodynamic (MHD) and kinetic scales are discussed further in Section 1.4.
1.2.2. Transient Coronal Outflows in the Nascent Solar Wind
The formation of the background solar wind, introduced in the previous section, is continually perturbed by the ejection of jets and small transients that form in the corona. Direct observations of these transient outflows in EUV and white‐light images by the STEREO and SoHO spacecraft have provided new insights on the origin of mesoscale structures measured in situ in the solar wind.
Variable solar wind outflows in the form of plasmoids are continually released from helmet streamers in white‐light (i.e., electron density) observations (e.g., Harrison et al., 2009; Rouillard, Davies, et al., 2010; Rouillard, Lavraud, et al., 2010; Rouillard et al., 2009; Sheeley et al., 1997; Sheeley et al., 2007; Wang et al., 1998; Wang et al., 2000). These plasmoids have been tracked from the corona, through the inner heliosphere, and in some cases out to 1 AU using heliospheric imagers (Rouillard, Davies, et al., 2010; Rouillard et al., 2011; Sheeley & Rouillard, 2010), showing in a direct way that some of the helmet streamer structures produced in the corona result in density structures measured in the inner heliosphere. Plasmoids (or “blobs”) have been tracked from the tip of streamers where they typically form to several tens of solar radii, and analysis of their kinematic properties has confirmed that they are advected in the slow wind (Sheeley et al., 1997). In fact, this type of analysis has provided one of the rare kinematic measurements of the forming slow wind. It has revealed that a subset of the slow solar wind is released right above helmet streamers and accelerates over 20–30 solar radii to reach its terminal speed of about 300 km/s (this acceleration is shown as the gray area in Figure 1.2).
Helmet
