HIGH-LATITUDE GROUND OBSERVATIONS OF PC 1 2 MICROPULSATIONS/

A ground-based survey of Pc 1/2 (0.1-0.4 Hz) and Pc 1 micropulsations throughout 1986 has provided evidence for the location of the Pc 1/2 s ource region. Data where taken from three high-latitude stations, loca ted at South Pole (-75-degrees geomagnetic latitude; 1530 UT local noo n), Sondre Stromfjord (+74-degrees, 1330 UT LN) and Siple (-61-degrees , 1700 UT LN). The study revealed a diurnal occurrence pattern for wav es in the 0.1-0.4 Hz band (Pc 1/2) and showed that the pattern was not due to the effects of sunlight on the ionosphere but instead from a p ostnoon magnetospheric source region. On the basis of the latitudinal occurrence patterns of the waves above and below 0.4 Hz, it is conclud ed that the waves observed on the ground above 0.4 Hz come primarily f rom plasmapause latitudes, while the source of the Pc 1/2 lies between the plasmapause and the magnetopause. The estimate of source location s for waves above and below this frequency, combined with the typicall y sharp upper frequency limit of waves in the 0. 1-0.4 Hz band (Pc 1/2 ) are interpreted as evidence that He+ ions in the outer magnetosphere influence propagation and possibly wave growth. These results are com pared with those of Anderson et al. [1990, 1992a,b], who showed with a spacecraft study that Pc 1 are more commonly observed beyond L = 7 th an in regions closer to the Earth. It is concluded that many of the wa ves above the He+ gyrofrequency from the outer magnetosphere do not al ways reach the ground. An extensive search for correlations between Pc 1/2 occurrence and solar wind pressure and magnetic field orientation showed no direct connection between solar wind parameters and Pc 1/2 generation. They may instead be amplified by plasma sheet ions that dr ift sunward on the dusk side of the magnetosphere [Kaye and Kivelson, 1979; Anderson and Hamilton, 1993] and undergo ion-cyclotron resonance in the afternoon sector. This mechanism is consistent with the diurna l pattern and apparent source location of the Pc 1/2.