Although the term “coronal hole” was not first used until the middle of the 20th century, people have reported the existence of visible features associated with the Sun’s corona – seen during total eclipses – for centuries (see, e.g., Wang and Siscoe, 1980; Vaquero, 2003). A popular astronomy book from the first decade of the 20th century (Serviss, 1909) contained clear descriptions of coronal streamers, eruptive prominences, and polar plumes in coronal holes. The following description of the latter, from an eclipse in 1900, conveys that early speculation may sometimes be prescient:
“The sheaves of light emanating from the poles look precisely like the ‘lines of force’ surrounding the poles of a magnet. It will be noticed in this photograph that the corona appears to consist of two portions: one comprising the polar rays just spoken of, and the other consisting of the broader, longer, and less-defined masses of light extending out from the equatorial and middle-latitude zones. Yet even in this more diffuse part of the phenomenon one can detect the presence of submerged curves bearing more or less resemblance to those about the poles. Just what part electricity or electro-magnetism plays in the mechanism of solar radiation it is impossible to say, but on the assumption that it is a very important part is based the hypothesis that there exists a direct solar influence not only upon the magnetism, but upon the weather of the earth” (Serviss, 1909).
The first quantitative observations of coronal holes were made by Waldmeier (1956, 1957) at the Swiss Federal Observatory in Zürich. These features were identified as long-lived regions of negligible intensity in coronagraphic (off-limb) images of the 5303 Å green emission line (see also Waldmeier, 1975, 1981). Waldmeier called the features that appeared more-or-less circular when projected onto the solar disk Löcher (holes), and the more elongated features were called Kanal (channels) or Rinne (grooves).
In off-limb eclipse and coronagraph images, the darkest coronal hole regions are surrounded by brighter and more complex streamers. These wispy structures appear to be connected to closed magnetic field lines at the solar surface, but they are often stretched upwards to an elongated cusp-like point, with thin “stalks” of radial rays at the top. For this reason their appearance was likened to a pointed German helmet (or a brush-topped Greek or Roman helmet), and the common phrase helmet streamers is often seen. The earliest studies of coronal morphology tended to concentrate more on streamers than coronal holes because the former are significantly easier to see than the latter (see, e.g., Miller, 1908; Mitchell, 1932; Newkirk Jr, 1967; Pneuman, 1968). Piddington (1972) outlined some early ideas about the global structure of the “quiet” (i.e., solar minimum) corona. Figure 1 compares an adaptation of J. H. Piddington’s sketch of the quiet corona to a more recent photograph from another eclipse around solar minimum.
As the quality of the observations improved, coronal holes became objects of study in their own right. The largest coronal holes were observed to contain fine thread-like polar plumes that appear to follow the superradially expanding open magnetic field lines above the solar limb (Saito, 1958; Stoddard et al., 1966; Newkirk Jr and Harvey, 1968). These elongated structures were found to correlate with bright chromospheric faculae on the surface (e.g., Harvey, 1965) and with longer extensions for the small jet-like spicules that continually rise and fall above the limb (Lippincott, 1957; Beckers, 1968).
Coronal holes were essentially re-discovered in the late 1960s and early 1970s as discrete dark patches on the X-ray and ultraviolet solar disk. Newkirk Jr (1967) reviewed some of the earliest rocket-based measurements in the extreme UV, and Tousey et al. (1968) discussed how the UV emission was “usually weaker over the poles” in images from a series of rocket flights between 1963 and 1967 (around solar minimum). These regions on the solar disk came to be called coronal holes in parallel with the earlier off-limb usage. Munro and Withbroe (1972) analyzed OSO–4 observations to conclude that both the density and electron temperature were lower in these dark regions. In 1973 and 1974, solar instruments on the Apollo telescope mount (ATM) on Skylab confirmed many earlier ideas about coronal holes with data significantly better in quantity and quality (Huber et al., 1974; Kahler, 2000).
In addition to the large north and south polar holes, there were also found to be smaller coronal holes that exist at lower latitudes (often at times other than solar minimum). Sometimes the largest coronal holes can exhibit thin “peninsulas” that jut out from the main regions. Harvey and Recely (2002) called these regions “polar lobes.” Notable examples have been the so-called “Boot of Italy” seen by Skylab in 1974 (e.g., Zirker, 1977) and the “Elephant’s Trunk” seen by SOHO in 1996 (Del Zanna and Bromage, 1999). A third example, from December 2000, is shown in Figure 2.
Additional insights came from the fusion of spectroscopy and coronagraphic occultation. Inspired by rocket-borne UV observations of the extended corona during a solar eclipse in March 1970, Kohl et al. (1978) developed a UV coronagraph spectrometer to measure the profile shape of the bright H i Ly emission line at 1216 Å. This line is sensitive to several key properties of the velocity distribution of coronal protons, and thus these observations could be used to begin distinguishing proton temperatures from electron temperatures in the collisionless outer regions of coronal holes (see Section 4.3). The rocket-borne UV coronagraph spectrometer was launched three times (in 1979, 1980, and 1982), and the results included the first direct evidence for proton heating and supersonic outflow in coronal holes (Kohl et al., 1980; Strachan et al., 1993; Kohl et al., 2006).
The fact that coronal holes coincide with regions of open magnetic field that expands out into interplanetary space was realized during the first decade of in situ solar wind observations (e.g., Wilcox, 1968; Altschuler et al., 1972; Hundhausen, 1972). Noci (1973) made a theoretical argument, on the basis of measured wave fluxes and heat conduction, that coronal holes should have the largest solar wind kinetic energy fluxes (i.e., the highest speeds). Pneuman (1973) argued that coronal holes need not have lower energy deposition than closed-field regions (as is suggested by the lower intensities of coronal holes) if the solar wind carries away much of that energy. Krieger et al. (1973) utilized X-ray sounding rocket images to identify a large coronal hole as the solar source of a strong high-speed stream as measured by the Pioneer 6 and Vela spacecraft. Around this time it was also realized that coronal holes and high-speed wind streams are also responsible for a fraction of the geomagnetic storms seen at 1 AU (Bell and Noci, 1973; Neupert and Pizzo, 1974; Bell and Noci, 1976; see also Tanskanen et al., 2005 and Zhang et al., 2007). Although there is still no complete understanding of which types of solar wind flow are connected with which types of coronal structures, the causal link between the largest coronal holes and high-speed streams remains firm (see also Section 4.1).
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