Entropy modes in multi-component plasmas confined by a dipole field

Abstract

In this paper, the collisionless electrostatic instability of the entropy mode is investigated linearly in multi-component plasmas confined by a dipole magnetic field, which commonly exists in space environments, as well as laboratory plasmas, such as Collisionless Terrella eXperiment, Levitated Dipole eXperiment, and Dipole Research EXperiment. We focus on characteristics of the entropy mode driven by the plasma density or/and the temperature gradient at low plasma beta (=8πP0/B02). The theoretical analysis of this work agrees with the calculated results qualitatively. It is indicated that the peak growth rate of the instability is in the regime of k⊥ρi ∼ 1, and the entropy mode tends to be more stable as the percentage of the heavy ion increases. For multi-component plasmas, each component has the entropy mode feature of its own, instead of simply averaging all the components. While for the electron with an isotropic temperature, the use of weighted harmonic average can be a good approximation for simplification.