Satellite observations of Pi 2 activity at synchronous orbit

Abstract

Pi 2 magnetic pulsations are frequently observed at synchronous orbit by the UCLA fluxgate magnetometer on ATS 6. Events that occurred in September 1974 have been studied by using digital power spectra and coherency analysis to determine wave characteristics. From examination of wave form and application of spectral analysis, these Pi 2 events can be divided into three types. The first is a superposition of a 100‐s oscillation and a large‐amplitude, higher‐frequency Pi 1 activity. The second is a 100‐s wave unaccompanied by Pi 1. Both types have a significant compressional component. The third is a pure transverse wave in the azimuthal component. The pure transverse waves are quite rare, while the compressional waves occur during almost every substorm. In general, if the satellite is in the local time sector 1900–0300 LT, a Pi 2 burst accompanies every onset. In addition, onsets at the satellite are associated with ground Pi 2 bursts. The peak occurrence time of the satellite Pi 2 is 2100 LT. An examination of Pi 2 polarization at the satellite suggests that a polarization reversal occurs around midnight during quiet magnetic conditions (Kp ≤ 3+), left‐handed premidnight and right‐handed postmidnight. This result is similar to that obtained from ground‐based studies at stations equatorward of the auroral electrojet. The initial perturbation in the azimuthal component of a Pi 2 event is in the same sense as the perturbations caused by substorm‐associated, field‐aligned currents, positive (eastward) premidnight, negative (westward) postmidnight. This observation indicates that there may be a very close association between their causative mechanisms.