Electrostatic ion cyclotron wave
In plasma physics, an electrostatic ion cyclotron wave is a longitudinal oscillation of the ions (and electrons) in a magnetized plasma, propagating nearly (but not exactly) perpendicular to the magnetic field. The angle (in radians) between the direction of propagation and the direction perpendicular to the magnetic field must be greater than about the square root of the mass ratio,
where Ωc is the ion cyclotron frequency and vs is the ion sound speed. This relation is the result of restoring forces due to the Lorentz force (see Upper hybrid oscillation for more details), the electrostatic force (the Te term in vs), and the ion pressure (the Ti term in vs).