Multi-component ground-based observation of ULF waves: goals and methods

A revival of the combined magnetic and telluric electric measurements at magnetic observatories is suggested.A number of problems, where such observations might be very helpful, are outlined: 1) the account for the resonance structure of the ULF field during the magnetotelluric probing of low-conductive geoelectrical structures; 2) the hydromagnetic diagnostics of the magnetospheric plasma distribution; 3) the discrimination of ionospheric and seismic contributions in anomalous ULF signals possibly related with earthquakes. The experimental apparatus for telluric current measurements, which has recently been installed at the observatories of Borok (Russia) and L'Aquila (Italy), is described.