Two‐Dimensional Full‐Wave Simulation of Whistler Mode Wave Propagation Near the Local Lower Hybrid Resonance Frequency in a Dipole Field

Abstract

AbstractWe investigate the propagation of whistler mode waves near the local lower hybrid resonance (LHR) frequency in a dipole field with a two‐dimensional full‐wave model. First, we run a simulation in which a parallel whistler with frequency above the local LHR frequency is launched at the equatorial region in electron plasma. We find that whistler emission propagates along the dipole field line to a high latitude, turns quasi‐electrostatic where the wave frequency is close to the local LHR frequency, and continues to propagate until being absorbed. Then, the proton response is considered. We find that (1) a quasi‐electrostatic whistler reflects where the wave frequency is below the local LHR frequency and propagates to a larger L‐shell and lower latitude, (2) a strong standing‐wave pattern is formed in the LHR reflection region, and (3) the whistler emission turns from right‐hand circularly polarized to linearly polarized near the reflection region. Finally, we run a simulation in which a quasi‐electrostatic whistler is launched at a high latitude with a small‐scale density irregularity added as a depletion to the background plasma. We find that a small portion of quasi‐electrostatic whistler energy can be coupled to a parallel whistler, which can propagate to a much lower altitude while most of the wave energy experiences LHR reflection. Moreover, the mode coupling depends on the transverse and longitudinal sizes of the density irregularity. This makes a possible explanation of ground observations of nonducted whistler emission, which could have been reflected in the high‐latitude ionosphere and magnetosphere.