Statistical characteristics of Pc5 waves at geostationary orbit.

Statistical analyses were made to clarify the latitude dependence of occurrence and other characteristics of Pc5 waves near L=6.6, using magnetic field data obtained with GOES 2 and 3 during 1978-1984. It is found that the occurrence probability of Pc5 waves with azimuthal polarization (A-class) is small at GOES 3 (4.7°N in magnetic latitude), as compared with that at GOES 2 (9.3-11.4°N). Most of A-class waves observed at GOES 2 are simultaneously detected near the subsatellite point of GOES 2. A typical amplitude ratio of ground Pc5 pulsations to A-class Pc5 waves at synchronous orbit is about 5 (50nT/10nT). In contrast to the A-class waves, compressional waves with radial polarization, observed mostly in the afternoon around 15h LT during magnetically disturbed conditions, have a larger occurrence probability at GOES 3 than at GOES 2. Amplitudes of the compressional Pc5 are larger at GOES 3 than at GOES 2, when the wave simultaneously occurs at both the satellites. The difference of Pc5 characteristics at GOES 2 and 3 suggests that the compressional Pc5 wave is confined to a latitudinally narrow region near the magnetic equator. The compressional Pc5 can be further divided into two types. One is of long duration and mostly occurs around noon in the recovery phase of magnetic storm. The other is of short duration and is observed on the afternoon-dusk side. The occurrence of short-duration events is well correlated with the individual substorm onset. A systematic delay of Pc5 occurrence behind the substorm onset is statistically obtained as a function of the local time of the satellite position. This delay suggests that the source region of the short-duration compressional Pc5 drifts westward with a speed of the gradient drift of 50-60keV protons.