A statistical study of Pc 1–2 magnetic pulsations in the equatorial magnetosphere: 1. Equatorial occurrence distributions

Abstract

A study of AMPTE CCE magnetic field data covering the frequency range 0.1–4.0 Hz using all data obtained during the first complete local time precession of the satellite orbit major axis (7500 hours of observations) has been made to evaluate the occurrence of transverse, narrowband Pc 1–2 emissions, identified as electromagnetic ion cyclotron (EMIC) waves, in the equatorial magnetosphere from L = 3.5 to L = 9 at all local times. We report this work in a pair of papers of which this is the first. A set of example events illustrates the pattern of Pc 1–2 occurrence: events occur primarily for L > 7, and a radial separation of several RE between low (<6) and high L (>6) pulsations is observed. Statistically, the highest concentration of events occurred near apogee in the afternoon. The L > 6 and L < 6 event populations appear to be radially separated in the morning but merge together in the afternoon. We construct a normalized occurrence distribution throughout the equatorial plane from L = 3.5 to L = 9 of Pc 1–2 with peak to peak amplitudes greater than 0.8 nT. The occurrence distribution exhibits a number of properties: for L > 7, Pc 1–2 occur at any given place in the early afternoon (1200–1500 MLT) with 10–20% probability and in the morning (0300–0900 MLT) with ≈3% probability; the L = 6–7 local time distribution reproduces results obtained previously from data at geostationary orbit; L < 5 events occur with a probability of ≤1% and a relatively uniform local time distribution. The predominance of L > 7 events implies that plasma sheet ions develop sufficient temperature anisotropy to generate EMIC waves on a routine basis in their drift from the nightside to the dayside and that plasma sheet ions on open drift paths rather than ring current ions on closed drift paths present the greatest source of equatorially generated EMIC waves. Consideration of the maximum convective growth rate of EMIC waves in a pure proton plasma versus cold plasma density and magnetic field strength illustrates how the predominance of L > 7 events and radial gap structure can be understood in terms of radial profiles of linear convective growth rates.