Excitation of low‐frequency fluctuations at the magnetopause by intermittent broadband magnetosheath waves

Abstract

We study the excitation of magnetic field fluctuations at the magnetopause by incident low‐frequency waves (frequencies below the ion gyrofrequency) that are highly intermittent and that have a broad frequency spectrum, like those observed in the magnetosheath. We present simulations of the transient response of the subsolar magnetopause to such magnetosheath waves in the context of linear resistive magnetohydrodynamics (MHD) in low‐beta plasmas. The incident waves excite local Alfvén waves in resonant sheets within the magnetopause transition region wherever the condition for resonant MHD wave mode conversion is satisfied. Resonant sheet locations depend on the dominant frequencies in the incident wave spectrum. Because of the intermittency of the incident waves, the resonant sheets are rarely driven to saturation. The number of resonances, their position, their thickness, and the resonant wave amplitude all vary with time in response to the temporal changes in the incident wave spectra. The net result is that the low‐frequency fluctuation level at the magnetopause is enhanced throughout the transition region rather than in a single or in a few resonant sheets, while the resonant amplitudes never reach excessively high values. This also implies that a fraction of the incident wave energy is resonantly absorbed throughout the magnetopause layer, instead of the very local absorption associated with an incident monochromatic wave. Resonances never develop into very narrow length scale structures, so that the MHD approach remains justified.