Diagnostics of the Solar Wind Plasma

The solar wind is a fully ionized plasma, coming from the outer atmosphere of the Sun, the so-called solar corona, which expands as a supersonic flow into the interplanetary medium [55]. The first observations indicating that the Sun might be emitting a wind were made by Biermann in 1946 of comet tails [1], which are observed to point away from the Sun. Comets usually exhibit two tails: a dust tail driven by the radiation pressure and a plasma tail, which points in slightly different directions pushed by the “solar corpuscular radiation” of the Sun. In 1958, E.N. Parker explained theoretically this “particle radiation” using a simple fluid model [55], showing that the solar atmosphere is not in hydrostatic equilibrium but must expand into the interplanetary medium as a wind. The existence of this solar wind was debated until it was indeed confirmed by spacecraft Lunik 2 and 3 [16] and continuously observed by Mariner 2 [53]. The Parker theory is discussed fully in Chap. 7 (Velli).