Effects of diverging coronal fields on the solar wind expansion

Abstract

The expansion of the solar wind in divergent flux tubes is calculated by taking into account a ‘magnetic acceleration’ of the particles, analogous to the magnetic mirror effect.

The resulting force term included in the magnetohydrodynamical equations describes a conversion of thermal into kinetic energy. This causes an additional acceleration of the solar wind plasma which has never been taken into account before. The force is directed opposite to the magnetic field gradient. Consequently, in this case the solar wind velocity increases faster to its asymptotic value than it does for corresponding nonmagnetic solutions. Therefore inside and close to the solar corona markedly higher velocities are found. Compared to strictly hydrodynamical models, the critical point is shifted towards the Sun, and the radial decrease of the ratio of thermal to kinetic energy is faster.

The necessary prerequisites for these calculations are (a) that the gyroperoid τ _g of the plasma particles is much shorter than the Coulomb collision time τ _c , and (b) that the collision time τ _c is shorter than the characteristic time τ _d in which an appreciable amount of thermal anisotropy is built up. Thus it is (a) insured that the particles have established magnetic moments and follow the guiding center approximation, and (b) an almost isotropic velocity distribution function is maintained which, in this first approximation of a purely radial expansion, justifies the use of isotropic pressures and temperatures.

Both (a) and (b) are shown to be fulfilled in a region around the Sun out to about 20R _⊙, and thermal anisotropies developing outside of this region could explain the observed magnetically aligned anisotropies at 1 AU.