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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.

Abstract. Particle and field data from a 4100-km-altitude satellite pass through a 1.3-mHz field line resonance, identified by ground-based optical, magnetic and radar signatures, are compared with results from a two-fluid MHI)-gyrokinetic simulation, including dispersively and resistively sustained parallel electric fields. It is shown that the resonance powers spatially adjacent up-and down-going suprathermal-electron fluxes, a 10-km-scale auroral arc and an imbedded electrostatic shock. Alfv6n wave dispersion and anomalous plasma resistivity are key elements in the interpretation of the event.

Abstract.The formation, temporal behavior, and spatiM structure of field line resonance (

[1] We report properties of substorm-related, globally excited Alfvén waves on a temporal scale of 6 to 300 s (3.3 to 167 mHz) at geocentric distances between 5 and 6 R E . The waves were observed in the tail lobes and the plasma sheet boundary layer (PSBL) by the Polar satellite. In each region we made the following observations: (1) The tail lobe Alfvén waves started at substorm onset as determined from ground magnetometer data. Hence these ULF lobe waves can possibly be used as a new substorm indicator. Although on open field lines, they often showed local standing wave signatures with a large perpendicular scale size and a near-zero net Poynting flux. We do not classify those waves as FLR but interpret them as the superposition of incident and reflected waves. The same oscillations were simultaneously recorded in ground magnetometer data. Immediately poleward of the PSBL, the lobe Alfvén waves traveled earthward (no reflection), suggesting their dissipation in the ionosphere. The lobe waves were superimposed on the signature of a field-aligned current (FAC). The onset of this FAC was simultaneous to the onset of the magnetic substorm bay. (2) The substorm-related PSBL Alfvén waves carried two to three orders of magnitude larger Poynting flux ($1 erg cm À2 s À1) than the lobe Alfvén waves. These PSBL waves were a mixture of standing and traveling Alfvén waves for different frequency ranges. Most Poynting flux was carried in large-scale earthward traveling waves (40-300 s). For one event, we also measured large standing wave components (>0.5 erg cm À2 s À1 ), but such events are rare. In the intermediate range (40-67 s), which overlaps with the Pi2 range, some waves showed clear standing wave signatures. At smaller periods (6-24 s), noninterfering earthward and tailward traveling waves were present with small Poynting fluxes (<0.05 erg cm À2 s À1). A trend for increasing E to B ratios with increasing wave frequency was observed. The PSBL waves were left-hand elliptically polarized. The wave vector was within 35°of the background magnetic field direction, suggesting that the waves were phase-mixed. The largeamplitude, substorm-related PSBL Alfvén wave events ($1 erg cm À2 s À1 ) were found in regions of upward currents.Citation: Keiling, A., G. K. Parks, J. R. Wygant, J. Dombeck, F. S. Mozer, C. T. Russell, A. V. Streltsov, and W. Lotko (2005), Some properties of Alfvén waves: Observations in the tail lobes and the plasma sheet boundary layer,