ANISOTROPY OF SOLAR WIND

Beyond about 0.1 a.u. from the Sun, the density of the solar wind is so low that Coulomb collisions cannot keep the velocity distribution isotropic and Maxwellian. Analysis of the particle dynamics in an ideal spiral interplanetary magnetic field shows that if there are no collisions beyond 0.1 a.u., then at the Earth the temperature of the wind is very anisotropic, with T∥/T⊥ = 35; this is much greater than is observed. To study the effects of interactions with magnetic irregularities we solved numerically Boltzmann's equation with Krook's collision term; this shows that the anisotropy observed by the Vela satellite requires each particle to make an average of 2 or 3 collisions between 0.1 and 1 a.u. The temperature averaged over direction roughly follows an adiabatic law, with γ = 3 2; γ tends to increase with distance. The theory predicts an excess of high-velocity particles, as is observedby Vela, even when the collision frequency is independent of velocity, but to produce an effect as strong as that observed requires a fairly strong velocity-dependence of the collision frequency. © 1969.