The Global Pc 5 event of November 14–15, 1979

Abstract

Large‐amplitude Pc 5 waves with 7–8 min quasi‐periodic variations were observed almost continuously for the 48 hours of November 14‐15, 1979, by several spacecraft in or near geostationary orbit on the dayside portions of their orbits. The waves were observed as large modulations in both the electron and ion fluxes over a wide range of energies (∼1 to 500 keV) by the spacecraft 1976–059, 1977–007, 1979–053, P78‐2, and GEOS 2 and as magnetic field peak‐to‐peak modulations of 15–25 γ by the P78‐2 and GOES 2 and 3 magnetometers. The remarkably long persistence of these waves contrasts substantially with observations of typical flux modulation events which usually last less than 1 hour and which typically show little modulation of the ≥150 keV proton fluxes. Data taken concurrently by the ISEE spacecraft in the solar wind and outer magnetosphere indicate that the solar wind also had unusual properties. ISEE 3 measurements indicate that the solar wind velocity (∼350 km/s) and density (∼2 cm−3) were simultaneously very low for this period. The alpha‐to‐proton ratio for the solar wind plasma attained an extremely low value (<1%) early in the event. These solar wind properties imply such a much reduced dynamic pressure on the magnetosphere during this period. Consequently, the ISEE 1 and 2 spacecraft passed through the magnetopause at the uncommonly large radial distance of 18 RE at ∼0830 local time where the typical magnetopause geocentric distance is 12 RE. The exceptional solar wind and outer magnetospheric conditions may have determined the unusual properties of the ULF event observed near geostationary orbit. Some candidate mechanisms for producing these oscillations are presented, but no definitive explanation for this event can be given at present.