Observations of two types of Pc 1–2 pulsations in the outer dayside magnetosphere

Abstract

Analysis of high time resolution magnetometer data from the Polar satellite and from an array of high‐latitude ground stations in Antarctica has identified 20 simultaneously observed Pc 1–2 wave events in the outer dayside magnetosphere during the first 17 months of Polar operations. In contrast to most earlier satellite studies of Pc 1–2 waves, based on data from equatorial satellites near apogee which moved only slowly across L shells if at all, the initial orbit of Polar allowed it to rapidly cross outer magnetospheric L shells, but significantly away from the magnetic equator. Consistent with several previous studies of outer magnetospheric Pc 1–2 waves, the majority of these events (75%) were associated with significant compressions of the magnetosphere. Seven of the events occurred simultaneously in satellite and ground data, with wave bursts temporally associated with compressions. These events, most at L values > 9, confirm the suggestion of Anderson and Hamilton [1993] that the outer dayside magnetosphere is often near marginal stability for electromagnetic ion cyclotron (EMIC) wave generation, so waves can be stimulated by even modest magnetospheric compressions. However, 10 of the wave events (five associated with compressions, and five not) were highly localized in L shell. In these “spatially localized” cases, continuous wave emissions were seen on the ground for extended periods of time, while in space waves were observed for only a few minutes and occurred only in radially narrow regions. The noncompressional events, all spatially localized, appear to be the first examples identified in space of the class of wave events known as Pc 1–2, sustained narrowband emissions which have durations at high‐latitude ground stations of the order of 10 hours in the day and dusk local time sectors. All 10 of the spatially localized events, whether compressional or not, followed intervals of at least 12 hours of sustained very quiet magnetospheric conditions. Energetic ion observations from Polar confirm earlier suggestions that drifting plasma sheet ions are the source of dayside Pc 1–2 waves in the outer dayside magnetosphere, but they also show different particle configurations for the spatially localized and temporally ordered event categories. Events in both categories occurred within radially extended regions with ring‐like, moderately anisotropic distributions of ≥5 keV protons and with deep minima in the flux distributions at energies <5 keV. However, spatially localized wave events occurred only in association with radially localized regions that also contained highly anisotropic fluxes of ∼0.5–3 keV protons, at a considerable distance from the magnetospheric boundary. In contrast, no such radial structure was evident in any of the temporally ordered events, or in three “uncertain” events. The association of the spatially localized events with highly structured populations of plasma sheet protons of keV and higher energies indicates an important but unanticipated role for these protons in destabilizing plasma in the outer dayside magnetosphere, possibly through increasing the local plasma beta near the magnetic equator.