Speaker
Description
Narrowband (∆f < 0.1 fcp), high-frequency (0.9 fcp < f < fcp) electromagnetic ion cyclotron (EMIC) waves, or HFEMIC waves for short, are a new type of EMIC waves found in the Earth’s inner magnetosphere. Observations suggest that they can be excited by low energy (< ~100 eV), very anisotropic protons. Here, we explore the instability threshold condition and hybrid simulations of HFEMIC waves, and compare the results with the observations. Linear theory analysis at parallel propagation shows that the anisotropy-parallel beta relation at instability threshold approximately follows an inverse relation like the one for typical H-band EMIC waves. Unlike typical EMIC waves, however, the minimum temperature anisotropy required is very large (> 10), the convective growth rate at a fixed instability threshold is large because of small group velocity of HFEMIC waves, and heavy ions affect the instability only weakly, primarily through the introduction of stop bands. To investigate the quasilinear and nonlinear behavior, one-dimensional hybrid simulations are run in the dipole-like background magnetic field and with initial parameters constrained by observation. Despite a narrow source region (within about ±3◦ latitude due to a large anisotropy required for HFEMIC instability), HFEMIC waves in the simulations grow well above the thermal noise level, due in large part to to a small group velocity of HFEMIC waves at the equatorial source region. The saturation level is well within the range of observational amplitudes, and the wave evolution is primarily determined by the quasilinear process. We demonstrate that the present results compare favorably to the recent observational findings, thereby supporting anisotropic low-energy protons as free energy source for HFEMIC waves.
