Hybrid simulations of plasma expansion

We study effects of radial expansion of solar-wind plasma on temperatures of protons and alphas, and on a drift velocity between the two populations. The double adiabatic approximation predicts that the radial expansion leads to an important ion temperature anisotropies, since the perpendicular temperature decreases and the parallel one is constant. The adiabatic approximation also predicts that the ratio between the drift velocity and the local Alfven velocity increases during the expansion. Such phenomena are not observed in the solar wind and there are different physical mechanisms that can explain this effect. One of the mechanisms is a local generation of instabilities driven by a temperature anisotropy and/or by a drift velocity. We use a modified version of one and two dimensional hybrid code, a Hybrid Expanding Box (HEB) code. We study the evolution of an expanding plasma consisting of protons and alphas with a drift between them in two case: for cold protons and alphas and for hot protons and alphas. The HEB simulations show that the locally driven instabilities are able to destroy the adiabaticity in the two cases. We discuss the results and compare them with linear theory and observations.