A multipoint case study of high‐latitude daytime ULF pulsations

Abstract

We use a multipoint case study approach to delineate significant morphological differences between, and distinct sources for, two kinds of high‐latitude dayside ULF wave activity in the Pc 3 to Pi 1 frequency range. Ground data obtained during April 26 and 27, 1986, from search coil magnetometers at South Pole Station (invariant latitude −74°) and McMurdo (invariant latitude −79°), located at nominal dayside auroral oval/polar cleft latitude and in the polar cap, respectively, are compared with interplanetary magnetic field data from IMP 8 in the upstream solar wind, magnetic field data from Active Magnetosphere Partice Tracer Explorers (AMPTE) CCE in the prenoon equatorial outer magnetosphere, and precipitating energetic particle data from Defense Meteorological Satellite Program (DMSP) F7 satellite in the dayside ionosphere. Large‐amplitude, broad‐band Pi 1 activity (which was shown in an earlier study to be statistically dominant at South Pole) is usually weaker or not detectable at McMurdo or at AMPTE CCE. Its occurrence is shown to be related to precipitation of energetic plasma sheet electrons, as established earlier for auroral and subauroral Pi 1 activity. In the cases shown, cusp/cleft latitude Pi 1 activity appears in response to southward turnings of the IMF, with a time delay characteristic of substorm processes. On the other hand, narrow‐band Pc 3 activity on the ground occurred at the same time it was observed at AMPTE CCE, and was controlled by the cone angle of the IMF. The intensity of Pc 3 activity decreased roughly with the distance of each ground station from the nominal dayside cleft position, but was clearly evident at McMurdo even in predawn local times. Observed amplitude and local time variations of Pc 3 activity at these very high latitude stations and at AMPTE CCE support recent suggestions of a cusp/cleft‐related entry mechanism for these pulsations.