Coupling between density structures, electromagnetic waves and ionospheric feedback in the auroral zone

摘要

This paper presents results from a numerical study of nonlinear interactions between ultra-low-frequency (ULF) electromagnetic waves and the magnetospheric-ionospheric plasma at high latitudes. The study is motivated by observations of density cavities in the ionosphere in regions of downward field-aligned current adjacent to auroral arcs. The role of active ionospheric feedback in the development of intense, small-scale electromagnetic waves with frequencies of 0.1–1 Hz is considered, together with the effects of the waves on the ion dynamics. The numerical results are based on a reduced two-fluid MHD model that self-consistently describes shear Alfvén waves, ion parallel dynamics, and effects of ionospheric E-region activity and magnetosphere-ionosphere feedback instability. Numerical simulations performed in dipole magnetic field geometry with realistic parameters of the ambient plasma show that, under some conditions, ionospheric feedback gives rise to intense ULF electromagnetic waves, which, via the ponderomotive force, produce density cavities in the bottomside ionosphere (between the E- and F-region peaks) and an associated upwelling of the topside ionosphere. The simulated magnitude and spatial and temporal scales of the cavities match the corresponding parameters of cavities observed with ground-based radars.