Compressional ULF waves in the outer magnetosphere: 1. Statistical study

Using 14 months of magnetic field and plasma data observed by the ISEE 1 and 2 spacecraft, ULF waves of period 2–20 min in the outer magnetosphere were studied. Statistical properties of the ULF waves can be summarized as follows: (1) Intense compressional waves are a persistent feature near the two flanks of the magnetosphere. They are mainly polarized in a meridian plane with comparable compressional and transverse amplitudes and have larger amplitude at magnetic latitudes below 20° than at higher latitudes. The magnetic pressure perturbations for the waves are in antiphase with the plasma pressure perturbations; (2) Transverse waves polarized in the azimuthal direction (azimuthal waves) are found to be mainly a nightside phenomenon. Their appearances on the nightside magnetosphere seem to be associated with substorm activity; (3) Compressional wave power and plasma β, the ratio between the plasma pressure and the magnetic field pressure, are correlated, but ULF wave power in the above noted period range is not significantly correlated with AE , plasma pressure or V A ; (4) The compressional waves are most likely to be generated internally in the regions where plasma β and field line curvature are large.