A statistical study of narrow bandwidth, high-latitude geomagnetic pulsations.

摘要

This study is a survey of micropulsations at three high-latitude ground stations. Results concerning the source region of Pc1/2 waves are discussed. The study revealed a diurnal occurrence pattern for waves in the 0.1-0.4 Hz band (Pc1/2), and showed that the pattern was not due to the effects of sunlight on the ionosphere, but instead from a postnoon magnetospheric source region. The importance of He+ ions in the magnetosphere to wave growth is also indicated. Based on the diurnal and seasonal occurrence patterns of the waves above and below 0.4 Hz, it is concluded that the waves observed on the ground above 0.4 Hz come primarily from plasmapause latitudes. Those below 0.4 Hz, in the Pc1/2 band, come from farther out. The different source locations for waves above and below 0.4 Hz, combined with the typically sharp, approximately 0.4 Hz upper frequency limit of Pc1/2 spectra, suggest strongly that He+ ions in the outer magnetosphere influence wavegrowth and propagation. This fits well with the observations of Anderson, et al. (1990), who showed with a spacecraft study that waves above the He+ gyrofrequency are more commonly observed beyond L = 7 than in regions closer to the Earth. This ground study shows that waves above the He+ gyrofrequency from the outer magnetosphere do not readily reach the ground. The organization of ground observations by He+ has never been shown before in previous ground studies. An extensive search for correlations between Pc1/2 occurrence and solar wind pressure and magnetic field orientation rules out the solar wind as a direct source of Pc1/2 generation. They may instead be amplified by plasma sheet ions that drift sunward on the dusk side of the magnetosphere (Kaye & Kivelson, 1979; Anderson et al., 1991) and undergo ion-cyclotron resonance in the afternoon sector. This mechanism is consistent with the diurnal pattern and apparent source location of the Pc1/2.