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

摘要

We 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.