The nature of the spatial structure of resonant ULF waves at low latitudes has been studied as part of a joint project between the U.S. Geological Survey, the Institute of Physics of the Earth, Moscow, and the Kyrgyzian Institute of Seismology. Gradient analysis of data taken at a meridional array of three stations in Soviet Central Asia showed that Alfven field line resonances, in the Pc 3 bandwidth, do exist at L = 1.5. Resonant frequencies of 66‐84 mHz (12‐15 s) were measured. Resonance width and the radial gradient of Alfven frequency were determined from our experimental data. When compared with previous published determinations of the resonance width, the resonance width is observed to increase at lower latitudes. This is the result of an increase in ionospheric damping at lower latitudes. Ionospheric damping significantly effects both resonant frequencies and resonance widths. Initial analysis of the data showed that effects of geologic inhomogeneities between two stations can obscure resonant effects that are observed in ground‐based magnetometer data. A method was developed to address these geologic effects so that the response of the resonator can be seen in both amplitude and phase calculations. The cross‐phase spectrum was determined to be the most useful technique to identify the resonant frequency of the field line between two ground stations. The diurnal behavior of resonant frequency was examined using a cross‐phase analysis technique and is shown to agree with theoretical predictions at low latitudes. We can conclude that diurnal variations in resonant frequency are mainly due to diurnal changes in plasma density along the oscillating field line.

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Abstract. By comparing data from a globally distributed set of high-latitude magnetic observatories with optical data in the nightside auroral ionosphere, we studied the features of short-term activations (_<10-15 min) during periods of steady magnetospheric convection (SMC) observed between two substorms. We found that the occurrence of local auroral and current structures stretched along the meridian to several hundreds of kilometers is the characteristic feature of activations in the nightside ionosphere. At premidnight and night hours (2100-0100 LT) we observed auroral structures that expand rapidly equatorward (V>_ 1 km/s) from the discrete auroral oval and become north-south aligned (N-S aurora or auroral streamers). In the night-morning hours (0100-0500 LT) torch-like structures form at the poleward

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The theoretical justification of the basic principles of Alfvén field line resonance in two-dimensional inhomogeneous plasma is given. The problem has recently been discussed by Hansen and Goertz [Phys. Fluids B 4, 2713 (1992)]. They suggested that the qualitative nature of magnetohydro- dynamic wave transformation in the region of field line resonance must be radically modified in multidimensional systems. A more correct treatment of the problem with the use of the Frobenius method proves the inconsistency of the criticism put forward by Hansen and Goertz. The singular behavior of the wave field near a resonant field line is maintained for two-dimensionally inhomogeneous configurations as well.