FREQUENCY VARIATIONS OF QUASI-PERIODIC ELF-VLF EMISSIONS - A POSSIBLENEW GROUND-BASED DIAGNOSTIC OF THE OUTER HIGH-LATITUDE MAGNETOSPHERE

Magnetic pulsations and quasi-periodic (QP) amplitude modulations of E LF-VLF waves at Pc 3-4 frequencies (15-50 mHz) are commonly observed s imultaneously in cusp-latitude data. The naturally occurring ELF-VLF e missions are believed to be modulated within the magnetosphere by the compressional component of geomagnetic pulsations formed external to t he magnetosphere. We have examined data from South Pole Station (L sim ilar to 14) to determine the occurrence and characteristics of QP emis sions. On the basis of 14 months of data during 1987 and 1988 we found that QP emissions typically appeared in both the 0.5-1 kHz and 1-2 kH z receiver channels at South Pole Station and occasionally in the 2-4 kHz channel. The QP emission frequency appeared to depend on solar win d parameters and interplanetary magnetic field (IMF) direction, and th e months near fall equinox in both 1987 and 1988 showed a significant increase in the percentage of QP emissions only in the lowest-frequenc y channel. We present a model consistent with these variations in whic h high-latitude (nonequatorial) magnetic field minima near the magneto pause play a major role, because the field magnitude governs both the frequency of ELF-VLF emissions and the whistler mode propagation cutof fs. Because the field in these regions will be strongly influenced by solar wind and IMF parameters, variations in the frequency of such emi ssions may be useful in providing ground-based diagnostics of the oute r high-latitude magnetosphere.