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, 1983), however never Maxwellian but often fairly isotropic electrons (Pilipp et al., 1987a,b).
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