Harmonic structure of compressional Pc5 pulsations at synchronous orbit

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Abstract
A harmonic structure of compressional Pc5 oscillations during geomagnetically disturbed conditions is found in magnetic field data from the geostationary satellites, GOES 2 and 3. Compressional Pc5 pulsations sometimes show an interesting property that the compressional component oscillates with a frequency twice as high as that of the transverse component. In addition to these second harmonic events, waves with a transitional character between normal Pc5 waves and second harmonic events are also observed. In the normal Pc5 waves, the compressional component changes out of phase with the radial component in the northern equatorial region. In a transitional event, a small hump appears at the maximum phase of the radial component. These Pc5 oscillations with second harmonics dominant in the compressional component were observed mainly at GOES 3 and rarely at GOES 2 in the winter season. This occurrence pattern suggests that compressional Pc5 with harmonic structure is spatially confined to the equatorial region. It is also suggested that the particular modulation dominant in the compressional component reflects a finite amplitude effect in the inhomogeneity of the hot ring current plasma near the magnetic equator.

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