This oscillation is closely related to the plasma oscillation found in unmagnetized plasmas or parallel to the magnetic field, where the ω pe term arises from the electrostatic Coulomb restoring force and the 3 k ² v e,th ² term arises from the restoring force of electron pressure. In the upper hybrid oscillation there is an additional restoring force due to the Lorentz force. Consider a plane wave where all perturbed quantities vary as exp(i (kx -ω t)). If the displacement in the direction of propagation is δ x, then