Two‐dimensional hybrid code simulation of electromagnetic ion cyclotron waves in a dipole magnetic field

Abstract

Proton temperature anisotropy (T > 1) with sufficiently high will drive the electromagnetic ion cyclotron (EMIC) instability. A two‐dimensional hybrid code is employed to simulate the EMIC waves in a dipole magnetic field. We initialize the electron‐proton plasma such that the MHD equilibrium J × B = − · is satisfied for the parallel force, while it is only satisfied along the magnetic equator for the perpendicular force. A high temperature ratio T = 9 and low plasma beta β = 0.1 are used to lessen the effects of nonequilibrium off the equator. The plasma is more unstable to the EMIC waves along the magnetic equator where the temperature ratio is high and the plasma β is low. There, waves are generated with dominant left‐hand polarization with the expected EMIC wave frequencies based on linear theory. These waves propagate along the magnetic field toward the ionosphere; and the inhomogeneity of the magnetic field causes the waves in the low L shell region to travel faster than those in the high L shell, so that the wavefronts turn oblique to the local magnetic field. When the wave vector turns perpendicular to the magnetic field at high magnetic latitudes, the initially left‐hand waves become linearly polarized. The linear polarization of the EMIC waves is consistent with some observations and with ray tracing results for reflected waves. We also introduce resistive layers to damp the waves near the boundaries to reduce the waves that are reflected by the boundary condition. The power spectra indicate that there is a radial propagation of wave energy with a group speed equal to about 0.1 times the equatorial Alfvén speed. Additionally, we find that there is a radial coherence length equal to about 10 associated with the EMIC wave structure.