AMPTE CCE observations of Pc 3–4 pulsations at L = 2–6

Abstract

Very few spacecraft observations of magnetic Pc 3–4 pulsations at L < 4 have been reported in the past despite the fact that Pc 3–4 pulsations are a common occurrence on the ground at L < 4. In order to fill this observational gap in the magnetosphere, we have initiated an analysis of magnetic field data acquired by the AMPTE CCE spacecraft. A procedure has been developed to reduce the artificial modulation and noise in magnetic field measurements, with the resultant data useful for studying magnetic pulsations with amplitude less than 1 nT in the 1000 nT (L ∼ 3) background and periods longer than 12 s. From a survey of 41 CCE passes in L = 2–6 and in the postnoon sector, we find that the most commonly observed pulsations in this region are the east‐west oscillations of magnetic field with clearly L‐dependent frequencies. These are identified to be the toroidal mode standing Alfvén waves. From a comparison of CCE and GOES 5 magnetic field measurements there is evidence that the resonances can be excited by the same mechanism as those previously found at geostationary orbit or beyond. Abrupt changes in resonance frequency, as a function of L, are not usually seen, supporting previous reports that the mass or number density profile on the dayside does not have a steep plasmapause structure.