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.

Global characteristics of Pc1 magnetic pulsations during solar cycle 23 deduced from CHAMP data

“…The MLT distribution clearly peaks around noon with sizable occurrence rates confined to the 08:00-14:00 MLT sector. This result is in line with magnetospheric observations by Anderson et al (1992a) on L shells beyond 7. It is just this subgroup that is responsible for the minor peak at noon in Fig.…”

“…Anderson et al, 1992aAnderson et al, ,b, 1996Posch et al, 2010;Min et al, 2012); mid-altitude satellites, e.g. Viking (e.g.…”

“…In the outer magnetosphere near the geomagnetic equator, Pc1 pulsations generally occur in the earlyafternoon sector (Anderson et al, 1992a,b;Erlandson and Ukhorskiy, 2001;Halford et al, 2010;Clausen et al, 2011;Keika et al, 2013). They are dominantly transverse (Anderson et al, 1992a). The polarization is mostly linear in the dawn/morning sector and left-handed (LH) in the afternoon, while right-handed (RH) Pc1 pulsations are relatively rare (Anderson et al, 1992b, Fig.…”

Modeling the electromagnetic ion cyclotron wave‐induced formation of detached subauroral proton arcs

“…The equatorial mapping of the detached arc to the plasmaspheric drainage plume is suggestive of a mechanism involving the interaction of hot ring current ions with cold plasmaspheric material. This is an environment suitable for the amplification of EMIC waves as first predicted by Cornwall et al [1970] and subsequently investigated by many others [e.g., Anderson et al, 1992;Gary et al, 1995]. Scattering by EMIC waves will lead to the precipitation of ring current protons and the excitation of subauroral arcs Spasojevic et al, 2004].…”

Study of Electromagnetic Ion-Cyclotron Instability in a Magnetoplasma

“…Observations by means of the satellites, especially the Active Magnetospheric Particle Tracer Explorers (AMPTE)/CCE, have indicated the frequent presence of electromagnetic Ion-cyclotron (EMIC) waves in the outer magnetosphere beyond L = 7 [1][2][3][4]. The wave spectral properties and frequency of occurrence were found to be dependent on local time and could be explained on the basis of resonant instability of anisotropic ring current H + ions in the background plasma.…”