Global Simulation of EMIC waves at Earth: Generation and Application of Linearly Polarized EMIC waves

Abstract:

We have developed a two-dimensional, finite element code that solves the electromagnetic full wave equations in global magnetospheric geometry. The code produces the three-dimensional wave structure, including mode conversion effects, for plasma waves launched in a two-dimensional axisymmetric background plasma with general magnetic field topology. Using this code, we have examined how EMIC waves are generated and propagated along the magnetic field line. While left-handed polarized EMIC waves are known to be excited by the cyclotron instability associated with hot and anisotropic ion distributions in the equatorial region of the magnetosphere, the generation mechanism of linear and right-handed polarized EMIC waves, which are often observed near the magnetic equator, remains as one of the unsolved scientific questions. In this presentation, we show the linear polarization of the EMIC waves can be explained by mode conversion at the ion-ion hybrid (IIH) resonance (an analogue of the field-line resonance when the resonance frequency is on the order of the heavy ion cyclotron frequency) when externally driven compressional waves propagate into an increasing/decreasing heavy ion concentration or inhomogeneous magnetic field. Since these mode-converted waves depend sensitively on the heavy ion concentration, it possible to estimate the heavy ion concentration ratio from the wave propagation characteristics. We also evaluated the absorption coefficients at the IIH resonance at Earth’s geosynchronous orbit for variable concentrations of He⁺ and wave frequencies and have found that the resonance only occurs for a limited range of wave frequencies, defined such that the IIH resonance frequency is close to, but not exactly the same as the crossover frequency. Using the wave absorption and observed EMIC waves from the GOES-12 satellite, we demonstrate how this technique can be used to estimate that the He+ concentration is around 4% near L = 6.6.