Observations of Pc 1–2 waves in the outer magnetosphere

Abstract

High time resolution measurements made inside the magnetopause from L=7 to L=14 by the University of California at Los Angeles fluxgate magnetometer aboard the Ogo 5 satellite demonstrate that Pc 1–2 magnetic field oscillations are occasionally present in portions of the outer magnetosphere. Eleven events have been studied so far, and all of them occurred in the afternoon sector at geomagnetic latitudes from 0° to ±30°. Both the solar wind dynamic pressure and the magnetospheric cold plasma density were enhanced during events for which the respective data were available. The IMF had a southward component approximately 2 hours before the events; however, there was no significant correlation between the time of the events and either storm recovery phase or substorm onset. The magnetic field perturbations in each event were primarily transverse to the ambient field with amplitudes ranging from 2 to 8 γ; however, small compressional oscillations, evidenced by 1‐ to 2‐γ fluctuations in the total field magnitude, were sometimes present. Most of the transverse perturbations exhibited left‐handed polarization. Brief excursions into the magnetosheath during two of the events revealed a highly disturbed magnetic field with significant power in frequencies of ≥0.1 Hz. Although the magnetosheath was a source of free energy for waves at Pc 1–2 frequencies, the data within the magnetosphere suggest that the observed pulsations were ion cyclotron waves generated at the geomagnetic equator at large radial distances.