motions in comet tails. Parker (1958, 1963) combined these empirical clues with the earlier discovery of a hot corona and postulated a theoretical model of a steady-state outward expansion of gas from the solar surface. Figure 1 shows analytic solutions to Parker’s isothermal solar wind equation, as well as other solutions including the case of spherical accretion found earlier by Bondi (1952). Traditionally this equation was believed to be unsolvable explicitly for the outflow speed, but Cranmer (2004) showed how a new transcendental function (the Lambert $W$ function) can be used to produce exact closed-form solutions of this equation.

Parker’s key insight was that the high temperature of the corona provides enough energy per particle to overcome gravity and produce a natural transition from a subsonic (bound, negative total energy) state near the Sun to a supersonic (outflowing, positive total energy) state in interplanetary space. An initial controversy over whether Parker’s “transonic” solution or Chamberlain’s always-subsonic “breeze” solutions were most physically relevant was dispelled when [*Mariner 2*]{} confirmed the existence of a continuous, supersonic solar wind in interplanetary space (Snyder and Neugebauer 1964; see also Neugebauer 1997). Also, Velli (2001) showed that the subsonic breeze solutions are formally unstable, and that the truly stable solution for the solar wind case is the one that goes through the sonic point.

In the years since [*Mariner 2,*]{} many other deep-space missions have added to our understanding of the solar wind. The turbulent inner heliosphere was probed by the two [*Helios*]{} spacecraft, which measured particle and field properties between 0.29 and 1 AU (Marsch 1991). In the 1990s, [*Ulysses*]{} became the first probe to venture far from the ecliptic plane and soar over the solar poles to measure the solar wind in three dimensions (Marsden 2001). The [*Voyager*]{} probes are still sending back data on the outer reaches of the solar wind, and one of t