The Johns Hopkins University/Applied Physics Laboratory HF radar at Goose Bay often sees F‐region drifts or electric fields which are associated with field line resonances in the Earth's magnetosphere. These resonances are seen in the interval from local midnight to morning, and have discrete, latitude‐dependent frequencies at approximately 1.3, 1.9, 2.6–2.7, and 3.2–3.4 mHz. We show that these frequencies are compatible with MHD waveguide modes, with antisunward propagation and reflection at the magnetopause and at turning points on dipolar field lines.

Field line resonances with frequencies in the range 1 to 4 mHz have recently been observed by the JHU/APL HF Doppler radar during quiescent geomagnetic conditions. These structures are observed to have stable frequencies for durations of several hours, leading us to the conclusion that they may be standing waves (in the radial direction, as opposed to standing waves along a field line) of the magnetosphere driven by the solar wind. Using this premise, the propagation of fast mode ULF waves in the magnetosheath and the near earth magnetosphere are examined in an ideal, linearized MHD context. A model is presented in which fast waves propagate in the equatorial plane between the flanks of the bow shock and a turning point deep within the magnetosphere. Due to the magnetic field gradient near the earth, a field line resonance develops between the turning point and the plasmapause. Using a realistic set of magnetospheric parameters, it is possible to reproduce the set of observed frequencies and the respective positions of their field line resonances within the ionosphere (assuming a dipole mapping). However, because the model cavity frequencies are sensitive to magnetosheath parameters, this model does not explain the extreme stability with respect to geomagnetic conditions of the observed frequencies.

Abstract. The power spectra of magnetometer data recorded at low latitudes show a remarkable similarity over the latitudinal range, 1.3

This manuscript presents a study of the polarizations of Pi 2's recorded by the magmetometer array of the University of Alberta. To simplify the interpretation, all the data are plotted in substorm‐centered coordinates (θs‐latitude, λs‐longitude). We have determined the center of the substorm current wedge (θs = 0, λs = 0) by using magnetometer data from midlatitude observatories to identify the ΔD crossover and magnetometer data from high latitude stations to identify the latitude of the onset of the substorm enhanced, westward electrojet. Within the region of the westward travelling surge, WTS, the polarizations in the horizontal H‐D plane are clockwise, CW (viewed downward), whereas equatorward of this region all polarizations are counterclockwise, CC, for all λs. Far to the east and west of the WTS, the polarizations are CW, poleward of θs = 0°, whereas poleward of the WTS the polarizations are CC. The orientations of the polarization ellipses suggest the presence of two distinct regions of field‐aligned current, FAC, associated with the Pi 2 oscillations. One region is centered near θs = 0°, λs = 5°E, and the other, possibly associated with the WTS, at θs = 0°, λ = −10°E. Many of the features of the polarizations of the Pi 2's suggest that the overall morphology of the Pi 2 FAC, and ionospheric currents, are similar to that of substorm current systems, and that the elliptical polarizations observed on the ground are caused by azimuthal expansion of the FAC associated with the Pi 2. When these azimuthal motions occur, the latitudinally integrated FAC at a given longitude is phase‐shifted with respect to the latitudinally integrated ionospheric Hall current.

Magnetohydrodynamic, field line resonances in the Earth's magnetosphere can have very large velocity shears and field‐aligned currents. Auroral radar measurements of high‐latitude resonances indicate that the velocities associated with the resonances in the E and F regions are often substantially greater than 1 km/s, and that the frequencies are in the interval from 1 to 4 mHz. Assuming that these resonances are oscillating at the fundamental mode frequency, and mapping these velocity fields along magnetic field lines to the equatorial plane shows that the velocity shears in the equatorial plane are of the order of 200 km/s over a radial distance of less than 2000 km (the amplitude of the velocity fluctuations is 100 km/s). Using a three‐dimensional magnetohydrodynamic computer simulation code, we show that the resonances evolve through the development of Kelvin‐Helmholtz instabilities near the equatorial plane. Within this framework, the instability is taking place on dipole magnetic field lines, and the resonances form a standing shear Alfvén wave field due to the boundary conditions which must be satisfied at the polar ionospheres. We find that the nonlinear evolution of the Kelvin‐Helmholtz instability leads to the propagation of vorticity from the equatorial plane to the polar ionosphere and that the vorticity leads ultimately to the dissipation of the resonance. This occurs within a quarter wave period of the shear Alfvén field associated with the resonances.