New Criterion for Fluid Model Applicability in Space Plasmas

Fluid models, in theory, should not be applied to describe space plasmas because there are insufficient particle–particle collisions in those thin media to transfer momentum between adjacent plasma parcels. However, in practice, fluid description often works well: countless studies successfully employ magnetohydrodynamics and other fluid models to describe space plasma phenomena. We resolve this controversy by demonstrating that in magnetized plasmas, particle–field interactions can effectively substitute for collisions in keeping the velocity distribution compatible with the fluid picture. Careful investigation of the specifics of the local momentum and energy transfer mechanism led to the introduction of a new criterion for determining when fluid models can be applied to magnetized collisionless plasma, moving beyond the traditional criteria based on collision frequency and the Knudsen number. According to the new criterion, fluid description can be applied to a significantly wider range of astrophysical plasmas than previously thought.