3.2 Collisional conditions in the corona and solar wind

The corona still is weakly collisional but strongly magnetised, which means that the particle gyroradius is much smaller than the collisional free path, ri,e < < λi,e, and the gyrofrequency much larger than the collision frequency, Ω > > ν i,e i,e. Numerical values of the Coulomb collision rate, ν i,e, of the electrons and protons can, for example, be found in Braginskii (1965Jump To The Next Citation Point).

In dilute space plasmas, such as the solar corona and solar wind, collisions are generally rare (see Table 1). Therefore, solar wind electrons and ions strongly violate the requirements of classical transport, which is to say that their collisional free paths are large against any fluid scale, or their collision time much longer than their transit time through 1 AU. To determine the collisionality of the interplanetary medium, Livi et al. (1986) investigated different types of solar wind streams and empirically defined the collisional domains. In a simple black-and-white picture one may say that the fast wind from coronal holes is collisionless and the slow wind from the streamer belt and transiently open coronal loops or small holes is weakly collisional. It is only in the dense and cold heliospheric current sheet, where even at 1 AU sometimes collisions may suffice to equilibrate particle temperatures or ion differential speeds (Borrini et al., 1981), or even produce Maxwellian protons (Marsch and Goldstein, 1983Jump To The Next Citation Point), however never Maxwellian but often fairly isotropic electrons (Pilipp et al., 1987aJump To The Next Citation Point,bJump To The Next Citation Point).

Table 1: Varying Coulomb collisions
Parameter Chromosphere Corona Solar wind
(1.01 R ) ⊙ (1.3 R ) ⊙ (1 AU )
n (cm–3) 1010 107 10
T (K) 103 1 – 2 × 106 105
λc (km) 1 103 107

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