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Analysis of Pc3 observational data along the 210° magnetic meridian showed a complicated frequency-latitude structure at middle latitudes. The observed period-latitude distributions vary between events with a "noisy source": the D component has a colored-noise spectrum, while the spectrum of <i>H</i> component exhibits regular peaks that vary with latitude, and events with a "band-limited source": the spectral power density of the <i>D</i> component is enhanced at certain frequencies throughout the network. For most ULF events a local gap of the <i>H</i> component amplitude has been exhibited at both conjugate stations at <i>L </i><u>~</u> 2.1. A quantitative interpretation has been given assuming that band-limited MHD emission from an extra-magnetospheric source is distorted by local field line resonances. Resonant frequencies had been singled out with the use of the asymmetry between spectra of <i>H</i> and <i>D</i> components. Additionally, a local resonant frequency at <i>L </i><u>~</u> 1.6 was determined by the quasi-gradient method using the data from nearly conjugate stations. The experimentally determined local resonance frequencies agree satisfactorily with those obtained from a numerical model of the Alfven resonator with the equatorial plasma density taken by extrapolation of Carpenter-Anderson model. We demonstrate how simple methods of hydromagnetic spectroscopy enable us to monitor simultaneously both the magnitude of the IMF and the magnetospheric plasma density from ULF data.<br><br><b>Key words. </b>Magnetospheric physics (Magnetosphere-ionosphere interactions; MHD waves and instabilities; plasmasphere)