Microinstabilities in space plasmas: Anisotropy bounds by wave-particle scattering

If a large-scale computer model fails to provide an accurate description of a space plasma, the discrepancy is often blamed on computational limitations, for example, the boundary conditions may not be predicted with sufficient accuracy, or the computational mesh may not be sufficiently fine. However, another possible source of inaccuracy in such models may be the physics; if the fluid equations used to represent a plasma are obtained by means of inappropriate assumptions, they cannot provide a good description of the system. In a relatively dense, relatively cold plasma, particle/particle collisions are strong. Such collisions drive the species velocity distributions toward local thermodynamic equilibrium. By considering distributions to have only small perturbations about such an equilibrium state, a set of fluid equations can be derived which is well-posed theoretically and which provides a useful description of such plasmas. Many space plasmas are relatively tenuous and relatively hot so that particle/particle interactions are weak; such plasmas are called open-quotes collisionless.close quotes In such plasmas, interactions between particles are mediated by electromagnetic fields, including both the slowly varying electric and magnetic fields which are well represented by large-scale models and the rapidly varying, short wavelength fields which are not