The exponential increase of the Alfv•n speed in the topside ionosphere leads to the formation of a resonant cavity (Lysak, 1988) which has been termed the ionospheric Alfv•n resonator by Trakhtengertz and Feldstein (1984). These authors primarily considered the situation where the ionospheric Pealersen conductivity is low, while Lysak (1988) considered the opposite limit of infinite ionospheric conductivity. These results have been extended to arbitrary ionospheric conductivity by performing a numerical solution of the cavity dispersion relation, which involves Bessel functions of complex argument and order. These restfits indicate that the damping of excitations of this resonant cavity is strongest when the ionospheric Pedersen and Alfv•n conductivities are comparable and that growth is possible for incoming wave boundary conditions. The existence of this cavity leads to a modification of the Alfv•n wave reflection coefficient at the ionosphere. While this reflection coefficient is independent of frequency at low frequencies, it exhibits structure due to the resonant cavity modes at frequencies around 0.1-1 Hz. These cavity modes can also be excited by feedback instabilities (Sato, 1978; Lysak, 1986), leading to growth rates which are enhanced over the case without the cavity. These waves have maximum growth at short wavelengths, particularly when the background Pedersen conductivity is large. The perturbations associated with these instabilities can lead to structuring of auroral currents during substorms, and may help explain the westward traveling surge. Maltsev et al., 1974Maltsev et al., , 1977Mallinckrodt and Carlson, 1978] that reflections of Alfv•n waves from the ionosphere are controlled by the ratio of the ionospheric height-integrated conductivity to an effective Alfv6n conductivity defined by Z a =c2/4•VA, where VA is the Alfv•n speed. The Alfv6n speed, however, is not constant above the ionosphere, but rather rises rapidly from its value in the ionosphere, reaches a peak in the lower magnetosphere (2-3 Re), and then gradually declines in the outer magnetosphere. For example, for an ionospheric mass density of 106rnv cm -3 and a magnetic field of 0.5 G, the Alfv6n speed is about 1000 kin/s, while at 2 Re, with a magnetic field of 0.05 G and a density of 1 ms, cm -3 such as may occur within an auroral density cavity, the Alfv•n speed can be 100 times as high. Even in a less extreme case, there is at least an increase by a factor of 10 in the Alfv6n speed between the ionosphere and the magnetosphere. Thus it is not clear what effective Alfv6n speed should be taken in computing the reflection coefficient of Alfv6n waves off the ionosphere. 1970;The reflection of Alfv•n waves is further complicated by the fact that there is a field-aligned current associated with the wave, and this field-aligned current can be associated with the precipitation of energetic particles, particularly for upward currents where electrons are flowing into the ionosphere. Fieldaligned potential drops can form in this region, and...

Kinetic Alfv6n waves have been invoked in association with auroral currents and parfide acceleration since the pioneering work of Hasegawa [1976]. However, to date, no work has considered the dispersion relation including the, full kinetic effects for both electrons and ions. Results fi'om such a calculation are presented, with emphasis on the role of Landau damping in dissipating Alfvtn waves which propagate fi'om the warm plasma of the outer magnetosphere to the cold plasma present in the ionosphere. It is found that the Landau damping is not important when the perpendicular wavelength is larger than the ion acoustic gyroradius and the electron inertial length. In addition, ion gyroradius effects lead to a reduction in the Landau damping by raising the parallel phase velocity of the wave above the dectron thermal speed in the short perpendicular wavelength regime. These results indicate that low-fi'equency Alfvtn waves with perpendicular wavelengths greater than the order of 10 km when mapped to the ionosphere will not be significantly affected by Landau damping. While these results, based on the local dispersion relation, are strictly valid only for short parallel wavelength Alfvtn waves, they do give an indication of the importance of Landau damping for longer parallel wavelength waves such as field line resonances.

[1] Measurements from the Cluster spacecraft of electric fields, magnetic fields, and ions are used to study the structure and dynamics of the reconnection region in the tail at distances of $18 R E near 22.4 MLT on 1 October 2001. This paper focuses on measurements of the large amplitude normal component of the electric field observed in the ion decoupling region near the reconnection x-line, the structure of the associated potential drops across the current sheet, and the role of the electrostatic potential structure in the ballistic acceleration of ions across the current sheet. The thinnest current sheet observed during this interval was bifurcated into a pair of current sheets and the measured width of the individual current sheet was 60-100 km (3-5 c/w pe ). Coinciding with the pair of thin current sheets is a large-amplitude (±60 mV/m) bipolar electric field structure directed normal to the current sheets toward the midplane of the plasma sheet. The potential drop between the outer boundary of the thin current sheet and the neutral sheet due to this electric field is 4-6 kV. This electric field structure produces a 4-6 kV electric potential well centered on the separatrix region. Measured H + velocity space distributions obtained inside the current layers provide evidence that the H + fluids from the northern and southern tail lobes are accelerated into the potential well, producing a pair of counterstreaming, monoenergetic H + beams. These beams are directed within 20 degrees of the normal direction with energies of 4-6 keV. The data also suggest there is ballistic acceleration of O + in a similar larger-scale potential well of 10-30 kV spatially coinciding with the larger scale size ($1000-3000 km) portions of current sheet surrounding the thin current sheet. Distribution functions show counterstreaming O + populations with energies of $20 keV accelerated along the average normal direction within this large-scale potential structure. The normal component of the electric field in the thin current sheet layer is large enough to drive an E Â B drift of the electrons $10,000 km/s (0.25 x electron Alfven velocity), which can account for the magnitude of the cross-tail current associated with the thin current sheet.Citation: Wygant, J. R., et al. (2005), Cluster observations of an intense normal component of the electric field at a thin reconnecting current sheet in the tail and its role in the shock-like acceleration of the ion fluid into the separatrix region,

[1] We present evidence based on measurements from the Polar spacecraft for the existence of small-scale, large-amplitude kinetic Alfvén waves/spikes at the plasma sheet boundary layer (PSBL) at altitudes of 4-6 R E . These structures coincide with larger-scale Alfvénic waves that carry a large net Poynting flux along magnetic field lines toward the Earth. Both structures are typically observed in the PSBL but have also been observed deeper in the plasma sheet. The small-scale spikes have electric field amplitudes up to 300 mV m À1 and associated magnetic field variations between 0.5 and 5 nT. Previous analysis has shown that the larger-scale Alfvén waves have periods of $20-60 s and carry enough Poynting flux to explain the generation of the most intense auroral structures observed in the Polar Ultraviolet Imager data set. In this paper it is shown that the smaller-scale waves have durations in the spacecraft frame of 250 ms to 1 s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is $4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfvén waves with scale sizes perpendicular to the magnetic field on the order of 20-120 km (with an electron inertial length c/w pe $10 km and an ion gyroradius $20 km). Theoretical analysis of the observed spikes suggests that these waves should be very efficient at accelerating electrons parallel to the magnetic field. Simultaneously measured electron velocity space distribution functions from the Polar Hydra instrument include parallel electron heating features and earthward electron beams, indicating strong parallel energization. The characteristic parallel energy is on the order of $1 keV, consistent with estimates of the parallel R Edl associated with small-scale kinetic Alfvén wave structures. The energy flux in the electron ''beams'' is $0.7 ergs cm À2 s À1 . These observations suggest that the small-scale kinetic Alfvén waves are generated from the larger-scale Alfvén waves through one or more of a variety of mechanisms that have been proposed to result in the filamentation of large-amplitude Alfvén waves. The observations presented herein provide strong evidence that in addition to the auroral particle energization processes known to occur at altitudes between 0.5 and 2 R E , there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.

The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1-2 orders of magnitude larger than that estimated from the large-scale steady state convection electric field and field-aligned current system.

The coupling of compressional and transverse hydromagnetic waves is studied in the cold and inhomogeneous outer magnetosphere. A general computer program has been developed for a dipole model. This model allows a realistic spatial variation of Alfven speed and includes dipole geometric effects. The propagation and the structure of each mode are analyzed on a two‐dimensional map of a meridian plane. The properties of coupling are also investigated through time histories and wave frequency spectra. The highly spatially structured form of transverse and compressional waves is shown on the meridian plane. The theory of global compressional mode damping is compared with the numerical results. We have found a set of global modes whose spatial structure is complicated due to the inhomogeneity of the dipole geometry. These global modes are strongly coupled to field line resonances when the global mode frequency is harmonically matched to the toroidal resonant frequency. The coupling of the two modes results in relatively large‐amplitude oscillations along the resonant field lines.

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The dynamics of magnetosphere‐ionosphere coupling has been investigated by means of a two‐dimensional two—fluid MHD model including anomalous resistivity. When field‐aligned current is generated on auroral field lines, the disturbance propagates toward the ionosphere in the form of a kinetic Alfvén wave. When the current exceeds a critical value, microscopic turbulence is produced, which modifies the propagation of the Alfvén wave. This process is modeled by a nonlinear collision frequency, which increases with the excess of the drift velocity over the critical value. The system evolves toward an electrostatic structure, with the perpendicular electric field having a shorter scale than the field‐aligned current. The approach to a steady state is strongly dependent on the presence or absence of the turbulence and on the boundary conditions imposed in the generator. As current is increased or scale size is decreased, the turbulent region reflects and absorbs most of the Alfvén wave energy, decoupling the generator from the ionosphere.