The Gravitational Interchange Mode or g-Mode

We now consider the case where the magneto-fluid system is subject to a gravitational force F _g = ρ g, where is a constant gravitational acceleration vector. In a system with straight field lines, the equilibrium condition is Recall that in a system with curved field lines, but no gravity, the equilibrium condition is where κ = ${\rm{\hat b}}$ · ∇${\rm{\hat b}}$ is the field-line curvature. Therefore, by using gravity as a proxy force, it is possible to study the stability properties of systems with curved field lines while using Cartesian geometry with straight field lines. This is a great simplification. This accounts for both the importance of the gravitational problem in MHD and the richness of its solutions.

We can lick gravity, but sometimes the paperwork is overwhelming.

Wernher Von Braun