Azimuthal propagation and frequency characteristic of compressional Pc 5 waves observed at geostationary orbit

Energetic particle data from the 1977–007 and 1979–053 satellites and magnetic field data from the GOES 2 and 3 satellites have been used to study eight compressional Pc 5 wave events observed at geostationary orbit during 1979. All the events occurred on the dayside, and most of them were observed during the recovery phase of a geomagnetic storm. By using the data from two of the satellites which were close to each other, we measured the azimuthal phase velocity Vф and azimuthal wave number m for selected intervals. For all these intervals the waves propagated westward in the spacecraft frame, and we obtained |Vф| = 4–14 km/s and |m| = 40–120. In addition, harmonics of a local standing Alfvén wave were often present simultaneously with a compressional Pc 5 wave. The frequency of the compressional wave was typically 25% of that of the second harmonic of the Alfvén wave. These observed features are discussed in the light of existing theories of instabilities in the ring current plasma.