Dispersive Field Line Resonances on Auroral
Field Lines

A. V. Strteltsov and W. Lotko

The formation of dispersive Alfven resonance layers is investigated using a three-dimensional, two-fluid, magnetically incompressible model, including electron inertia and finite pressure. The equations are solved in ``box'' geometry with uniform magnetic field bounded by perfectly conducting ionospheres. Field line resonance (FLR) is stimulated within a density boundary layer with gradient transverse to ambient B; a parallel gradient in the \alf speed is also included. Numerical results show that the resonance amplitude is largest on the magnetic shell with eigenfrequency matching the frequency of the surface wave propagating on the density boundary layer. Efficient coupling between the resonant Alfven wave and surface wave produces a relatively narrow FLR spectrum, even when the driver is broadbanded. Effective coupling to the external driver occurs only for long-wavelength azimuthal modes. It is shown that the parallel inhomogeneity limits radiation of dispersive Alfven waves by the FLRThe results provide new insights into low-altitude satellite observations of auroral electromagnetic fields and the formation of discrete auroral arcs.

JGR, 100, 19,457-19,472, 1995

The Fine Structure of Dispersive, Nonradiative Field
Line Resonance Layers

A. V. Strteltsov and W. Lotko

The fine structure of dispersive Alfven wave resonance layers extending along magnetic field lines from northern to southern auroral ionospheres is investigated using a magnetically incompressible, linear, two-fluid model. The model includes effects of finite electron inertia (at low altitude) and finite electron pressure (at high altitude). Plasma parameters are chosen so that refraction by the parallel inhomogeneity causes the dispersive Alfven wave to become trapped in the resonance layer. The parallel and perpendicular structure of these nonradiative, dispersive resonance layers is computed for the first four odd harmonics. A significant enhancement of theperpendicular and parallel components of the electric field near the ionosphere is found. The instantaneous potential drop along themagnetic field is sufficient to accelerate electrons up to several keV. The thickness of FLR layers in the ionosphere is estimated to be less than 5 km.The results suggest that dispersive resonance layers produce sub-kilometer-scale, multiple, discrete auroral arcs.

JGR, 101, 5343-5358, 1996

Dispersive, Nondariative Field Line Resonance
in a Dipolar Magnetic Field Geometry

A. V. Strteltsov and W. Lotko

Results from a numerical study of field line resonances (FLRs), formed by dispersive Alfven waves standing between southern and northern ionospheres along auroral magnetic field lines, are presented. Dispersion of the Alfven wave is due to the finite electron inertia at low altitudes and the finite electron temperature at high altitudes. Previous numerical studies of the phenomenon based on linear, magnetically incompressible, two fluid MHD in a slab geometry (with constant ambient magnetic field and temperature - so called ``box'' model) are extended here to a dipolar magnetic field geometry. The new computations quantify earlier qualitative conclusions from the box model that the electric field at low altitudes is significantly amplified in nonradiative FLRs, with an associated field-aligned potential drop sufficient to accelerate electrons up to several hundreds of eV, leading to luminous auroral structures. The dipolar results also provide further insights into the role of the transverse plasma inhomogeneity in the formation of the dispersive, nonradiative FLRs: even a moderate transverse inhomogeneity of the background plasma, localized to the equatorial section of the flux tube, significantly increases the growth rate of resonances. Similarities and differences between results obtained from the new dipolar model and the previous box model are discussed, as well as the relation between the new results and observations.

JGR, 102, 27,121-27,135, 1997

Small-Scale, Dispersive Field Line Resonances in the Hot
Magnetospheric Plasma

A. V. Strteltsov, W. Lotko, J. R. Johnson, and C. Z. Cheng

The formation, temporal behavior, and spatial structure of field line resonance (FLR) layers formed by shear Alfven waves standing along auroral magnetic field lines between the ionospheres are investigated when the layer develops transverse structure on the scale of the ion Larmour radius. Using a new numerical model including full ion Larmour radius correction in dipole magnetic geometry with realistic distributions of background plasma temperature and density it is shown that: (1) Hot magnetospheric ions significantly retard the development of a parallel electric field in ion gyroscale dispersive Alfven waves. (2) A fundamental FLR forming near L = 7.5 can contract to a transverse scale-size of several hundred of meters in the direction perpendicular to the geomagnetic field at ionospheric altitudes, with a parallel electric field sufficient to produce a kV potential drop along the resonance field line from the ionosphere up to about 4 RE altitude, in the region where the wave dispersion is due to the finite electron inertia. (3) A plasma density depletion in the lower auroral magnetosphere (~2 - 5 RE geocentric distance) enables the formation of a nonradiative fundamental FLR. (4) Dispersive FLRs for the higher harmonics are more radiative at the equatorial magnetosphere than the fundamental mode.

JGR, 103, 26,559-26,572, 1998

Small-scale, ``Electrostatic'' Auroral Structures and
Alfven Waves

A. V. Strteltsov and W. Lotko

This paper explains some small-scale, large amplitude disturbances of the perpendicular electric field measured over decades by low-altitude satellites in the auroral magnetosphere in terms of ultra-low-frequency Alfen waves. The reason why these disturbances of the perpendicular electric field are not accompanied by the disturbances of the transverse magnetic field, matching them in structure and amplitude, is a reflection of the wave from a micro-turbulence layer, occurring at low-altitudes due to the plasma anomalous resistivity (AR). Without investigating what exactly causes the micro-turbulence, the plasma AR in the simplest form is included in the linear, reduced, two-fluid MHD/kinetic model, developed earlier to describe alfvenic field line resonances (FLRs) in a dipole magnetic geometry. Computations show that: (1) A fundamental FLR forming near L = 7.5 in the plasma with AR produces small-scale, intense disturbances of the perpendicular electric field at low-altitude, just above the micro-turbulence layer, without any significant disturbances of the perpendicular magnetic field with the same spatial structure. (2) Plasma AR causes a parallel electric field inside the micro-turbulence layer, sufficient to accelerate electrons up to 1 kV energy over a distance ~1000 km along the magnetic field line. (3) A numerical model of the alfvenic FLR, including the effect of the plasma AR even in the simplest form, reproduces the main features of the data measured by FAST satellite in the nightside auroral magnetosphere with a remarkable exactness.

JGR, 104, 4411-4426, 1999

Dispersive width of the Alfvenic field line resonance

A. V. Strteltsov

The results from a numerical investigation of the dependence of the eigenfrequency and structure of Alfvenic field line resonance (FLR) layers on the dispersion of the resonant Alfven waves are presented. The investigation is based on the linear, reduced, two-fluid MHD-kinetic model considered in the dipole magnetic field geometry with a realistic distribution of the background plasma parameters along the entire auroral flux tube. It is shown that (1) dispersion of the small-scale Alfven wave due to the finite electron inertia (inertial dispersions) decreases the eigenfrequency of the Alfvenic FLR and broadens the FLR layer in the direction along the transverse gradient in the background Alfven speed; (2) dispersion of the wave due to the finite plasma temperature (kinetic dispersion) increases the FLR eigenfrequency and broadens the resonance layer in the opposite direction; and (3) on the field line where the effect of the kinetic dispersion balances the effect of the inertial dispersion, the eigenfrequency becomes independent of the transverse wavelength of the resonant wave. As a result, a FLR occurring on such a particular field line develops fields and currents with smaller transverse scale sizes and larger amplitudes than FLRs on field lines where inertial and kinetic dispersions are not in a balance.

JGR, 104, 22,657-22,666, 1999

Numerical modelling of Alfven waves observed by the Polar spacecraft in the nightside plasma sheet boundary layer

A. V. Strteltsov, W. Lotko, A. Keiling, J. R. Wygant

Results from a numerical study of localized, large-amplitude electromagnetic disturbances measured by the Polar satellite in the nightside plasma sheet are presented. It is shown that these disturbances can be explained in terms of shear Alfven waves generated in the magnetotail and interacting with a microturbulent layer at low altitudes. The plasma microturbulence/anomalous resistivity self-consistently included in the non-linear, two-fluid MHD model, causes the wave reflection and defines its structure and amplitude in the lower magnetosphere. It also causes the dissipation of the wave energy inside the turbulent/resistive layer which leads to creation of parallel electric field and parallel electron acceleration. Thus plasma turbulence connects electromagnetic structures measured by Polar at radial distance 4-6 RE with bright aurora simultaneously observed by the Polar Ultraviolet Imager at the ionospheric footprint of the corresponding magnetic field lines.

JGR, 107, 10.1029/2001JA000233, 2002