Within the ecliptic plane the solar wind is dominated by low speed streams with typical velocities, proton densities, and temperatures (at 1 AU) of , , and , respectively, although high speed streams with lower densities and are not uncommon (see Feldman et al., 1977). At the maximum of the 11-year solar activity cycle, similar solar wind behavior is seen at almost all latitudes. However, Figure 1 shows that at solar minimum the wind above ecliptic latitude is uniformly high speed, low density wind with (McComas et al., 2000, 2002). This high speed wind originates from coronal holes, which are particularly prominent on the Sun during solar minimum conditions. These solar wind data imply a total mass loss rate for the Sun of . Although thermal temperatures for the wind at 1 AU are of order , densities are low enough that the wind cannot equilibrate to this temperature, and the ionization state of the wind is actually frozen in at coronal temperatures closer to . Hydrogen is fully ionized, meaning that protons are the dominant constituents of the wind by mass.
Perhaps the most fundamental question to ask about the solar wind is why it exists. Addressing this question is also necessary to assess whether similar winds should exist around other stars. Even before satellites proved the existence of a more-or-less steady solar wind, Parker (1958) predicted that such a wind should be present due to the existence of the corona surrounding the Sun. In Parker’s model, the solar wind exists because of thermal expansion from the hot corona. The predictions of this simple model agree remarkably well with the observed properties of the low speed wind that dominates in the ecliptic plane, although additional wind acceleration mechanisms invoking MHD waves have been proposed to explain the high speed streams (see MacGregor and Charbonneau, 1994; Cranmer, 2002). Thus, any star that has a hot corona analogous to that of the Sun should also have a wind analogous to that of the Sun. Observations with X-ray satellites such as Einstein and ROSAT clearly demonstrate that coronae are a ubiquitous phenomenon among cool main sequence stars (see Schmitt, 1997; Hünsch et al., 1999), so solar-like winds should be present around all solar-like stars. However, that does not mean that they are easy to detect (see Section 1).
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