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.

Analysis of magnetospheric Pc 1 and Pc 2 (in brief Pc 1–2) pulsations at a network of six northern hemisphere stations at latitudes 65°–76° CGM shows that their average frequency decreases with increasing latitude of the station. Their maximum occurrence is at L=6–9 and 1200–1600 LT. The ionospheric ducting is weak for f < 0.3 Hz. Occasionally the Pc 1 intensity at a station is modulated by a simultaneously arriving quasi‐periodic Pc 4–5 signal. A mechanism for generating micropulsations which is common to both high‐frequency and low‐frequency waves is presented and discussed, the proposed mechansim being that a broadband signal from the solar wind is amplified in the magnetosphere in the frequency bands of the field line resonance (Pc 4–5) and in the He+ cyclotron frequency (Pc 1–2) range.