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[1] We present a numerical study of the propagation of VLF whistler waves in the magnetospheric plasma. In this study the plasma is considered to be homogeneous in the direction along the ambient magnetic field and strongly inhomogeneous across it. The goal of this investigation is to understand whistler propagation in magnetic-field-aligned channels (also called ducts) with either enhanced or depleted plasma density. In particular, the paper is focused on situations where the transverse scale size of the duct is comparable to or smaller than the perpendicular wavelength of the whistler. In this case, classical analysis of the whistler dynamics based on the geometrical optic approximation becomes invalid, and numerical solutions of the full wave equations should be performed. Our simulations extend the earlier analysis based on the ray-tracing technique and analytical studies of the very low frequency wave equations. We show that high-density ducts are inherently leaky and this leakage depends on the perpendicular wavelength of the wave inside the duct. We also show that whistler trapping occurs not only at density maxima and minima but also at critical points along a density gradient. This effect can explain whistler guiding along strong transverse plasma density gradients at the plasmapause.

[1] 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.Citation: Streltsov, A. V., and W. Lotko (2008), Coupling between density structures, electromagnetic waves and ionospheric feedback in the auroral zone,

[1] In this paper we investigate how the parameters of the ionosphere and the low-altitude magnetosphere mediate the formation and spatiotemporal properties of small-scale, intense electromagnetic structures commonly observed by low-altitude satellites in the auroral and subauroral magnetosphere. The study is based on numerical modeling of a time-evolving, nonlinear system that describes multiscale electrodynamics of the magnetosphere-ionosphere coupled system in terms of field-aligned currents, both quasistatic and Alfvénic. Simulations show that intense electric fields and currents with a perpendicular size of 10-20 km at 120 km altitude can be generated by a large-scale, slowly evolving current system interacting with a weakly conducting ionosphere, even without a resonant cavity in the magnetosphere. These structures form in the strong gradient in the ionospheric conductivity that develops at the boundary between the largescale upward and downward currents when the background ionospheric Pedersen conductivity, S P , is low but higher than the Alfvén conductivity, S A = 1/m 0 v A , above the ionosphere. When S P % S A the ionosphere can generate electromagnetic waves with perpendicular sizes less than 10 km. These waves can be trapped inside the cavity of the classical ionospheric Alfvén resonator, and their amplitude can be significantly amplified there by the ionospheric feedback instability.

Satellite (IC-B-1300) data on the electromagnetic structures in the high-latitude ionosphere are presented. One can observe three kinds of vortices, namely vortex chains as well as solitary dipolar and monopolar vortex structures. The theoretical treatment that is carried out in the present paper is in reasonable agreement with the observations.