Recent multiple satellite observations of compressional Pc 5 waves indicate for the first time that the field‐aligned mode structure of the compressional magnetic field was antisymmetric with respect to the earth's magnetic equator. To understand the mode structures of low frequency compressional waves, a comprehensive eigenmode analysis of compressional Alfven instabilities was performed for a two component anisotropic plasma in a dipole magnetic field. The eigenmode equations derived from the gyrokinetic equations are solved analytically and numerically for both the drift mirror and the drift compressional instabilities. The solutions indicate that the drift mirror mode has an antisymmetric compressional component with respect to the magnetic equator, whereas the drift compressional mode has a symmetric one. For typical storm time plasma parameters near geosynchronous orbit, it is suggested that the recently observed compressional Pc 5 wave was in a nonlinearly saturated state of the drift mirror instability.
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