ULF pulsation evidence of the plasmapause: 1. Spectral studies of Pc 3 and Pc 4 pulsations nearL= 4

Power spectra and digital dynamic power spectra of geomagnetic field fluctuations observed at a latitudinal network of stations near L = 4 were computed for the entire day of January 5, 1972, in order to study magnetospheric ULF pulsations on a nonstatistical basis. It was found that there are three dominant spectral peaks in the Pc 3–4 frequency range and another peak in the Pc 5 range. The peaks in the Pc 3–4 range are approximately higher harmonics of the spectral peak in the Pc 5 range. The contours of the peak intensities in the spectral bands have been determined as a function of geomagnetic latitude and universal time. These contour maps clearly show that the locations of the intensity maximums of the Pc 3 and Pc 4 peaks gradually change equatorward and poleward in parallel throughout the day and maintain the relationship that the intensity maximum of the Pc 4 peak is always located at a lower latitude than the intensity maximum of the Pc 3 peak. From these observational results it is suggested that the harmonic relationship results from the spectrum of the driving source and that the difference in the location of the intensity maximums of the Pc 3 and Pc 4 frequency bands arises from a sharp decrease in the phase velocity of the Alfven waves just inside the plasmapause. It is pointed out that these dynamic spectral analyses of Pc 3–4 pulsations near L = 4 open the possibility of studying the dynamics of the plasmapause by ground-based ULF techniques.