Wave and plasma observations during a compressional Pc 5 wave event August 10, 1982

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Abstract
Magnetometer and thermal plasma instruments on the polar‐orbiting Dynamics Explorer 1 satellite observed a small‐amplitude ultralow frequency (ULF) pulsation event at the outer edge of the plasmapause near the geomagnetic equator in the midafternoon sector on August 10, 1982, during the recovery phase of a magnetic storm. Transverse pulsations of 30–50 s period were observed throughout the event, and a 270‐s period, purely compressional Pc 5 pulsation with several shifts in phase occurred within ±5° of the geomagnetic equator. Electric fields and the motion of thermal ions appeared to be in quadrature with pulsations in magnetic field magnitude throughout the event. This suggests that the net Poynting flux for the compressional waves was zero, consistent with their being standing waves. Large fluxes of trapped 90° pitch angle 10‐eV protons, also symmetric about the geomagnetic equator, were observed in conjunction with the waves. These may serve as a source of free energy for the pulsations. Our observations lend support to recent studies suggesting that many dayside compressional wave events are related to localized field line resonance near plasmapauselike boundaries, but also include features that cannot be explained by existing theories.