The methods of an unambiguous determination of the parameters of the magnetospheric resonator (resonance frequency, its meridional gradient, and width of the resonance) by studying the spatial structure of ULF waves, in the Pc3-4 frequency range, are summarized and reviewed. The methods considered are the gradient technique (synchronous measurements of ULF field at two nearby stations) andthe polarization method (multicomponentobservations atone station). Bothmethods are experimentally tested using the data from an experiment at low latitude (L = 1.5). Taking into account modifications of the structure of the ULF magnetic field due to geoelectric inhomogeneities, both methods demonstrate consistent results and are in a qualitative agreement with theoretical predictions. These methods should provide a useful tool for monitoring resonant frequencies and the distribution of plasma in the magnetosphere.

CD‐ROMs containing data from a global network of geomagnetic observatories (Figure 1) are now available for 1991 through 1995 from the International Real‐time Magnetic Observatory Network (INTERMAGNET), a nonprofit association of world institutions that operate geomagnetic observatories. The data are 1‐min, hourly, and daily mean values of the vector components (X, Y, X or H, D, Z) and, for most stations, the total field, F. Resolution is 0.1 nanotesla or 0.1 min of arc for declination. Magnetic activity indices (K) are available for most stations, and baseline and absolute value plots are given for all observatories for the 1993, 1994, and 1995 CD‐ROMs.