GLOBAL STRUCTURE OF LOW-LATITUDE AND EQUATORIAL GEOMAGNETIC-PULSATIONS ASSOCIATED WITH STORM SUDDEN COMMENCEMENTS

Using geomagnetic data from a global network equipped with fluxgate magnetometers and data loggers by Kyushu University, two moderate storm sudden commencement(SSC) events which occurred on Jun. 10 and Aug. 13, 1992 were analyzed. The network consists of about 30 stations, the main portion of which are arrayed nearly along three magnetic meridians (Japanese, Brazilian and African meridians) in low-latitude and equatorial regions. As for low-latitude and equatorial SSC-associated geomagnetic pulsations (Psc's), the following results were obtained. (1) The shape of the waveform is consistently ordered by local time for all low-latitude and equatorial stations; it is steeper near noon (or near the source) and has a more moderate slope near midnight (far from the source). This indicates that the Psc's are not cavity-mode oscillations but compressional hydromagnetic (HM) waves propagating from dayside to nightside two-dimensionally along the equatorial plane. (2) The Psc's have globally almost identical dominant frequencies which agree well with those of Pi2's. This implies that the frequencies of low-latitude and equatorial pulsations may be determined by the global structure of the magnetosphere or the plasmasphere independently of source mechanisms. During its passage through the plasmapause the compressional HM wave generated in the outer magnetosphere might be regulated in such a waveform as could be expressed as a superposition of exponentially damped sinusoids at ground stations, whose frequencies are globally almost identical.