2 The model

Details of the simulation model have been given in two previous papers (Landi & Pantellini, 2001; Pantellini, 2000) and shall not be repeated here to full extent. The model is spatially one dimensional, i.e. all fields depend on the heliocentric distance $r$ only. An equal number of protons and electrons are allowed to move freely in the domain $r_0<r<r_{\rm max}$, where $r_0$ is the solar radius and $r_{\rm max}$ is the outer boundary of the system located several solar radii beyond the sonic point. The equations of motion are those of a particle of mass $m$ and charge $q$ in a central gravitational field produced by a star of mass $M$ and a radial, charge neutralizing electric field, $\mathcal{E}(r)$, i.e.

$${{d^2r}\over{dt^2}} = -{{G M}\over r^2} + {{L^2}\over{m^2 r^3}} +{q\over m} \mathcal{E}(r).$$ (1)

$$\vec{L} \equiv m \vec{r}\times\vec{v}_\perp ={\rm constant}$$ (2)

where $G$ is gravitational constant, $\vec{L}$ the angular momentum of the particle and $\vec{v}_\perp$ its velocity component perpendicular to the radial direction. Two particles finding themselves simultaneously at the same radial distance $r$ do either make an isotropic elastic collision with a probability $\propto u^{-4} r^{-2}$ or just go through each other as if they were transparent. The $u^{-4}$ dependence of the collision probability mimics velocity dependence of the scattering cross section for Coulomb collisions whereas the $r^{-2}$ dependence accounts for the spherical geometry of the problem. The transport properties of such a plasma have been shown to be very much the same as those of a Fokker-Planck plasma (Landi & Pantellini, 2001; Pantellini & Landi, 2001).