Gyrokinetic perpendicular dynamics

Gyrokinetic perpendicular dynamics, an important component not systematical ly considered in previous gyrokinetic theories, is identified and developed . A "distribution function'' S and its governing gyrokinetic equation are i ntroduced to describe the gyrokinetic perpendicular dynamics. The complete treatment of the perpendicular current rendered by the gyrokinetic perpendi cular dynamics enables one to recover the compressional Alfven wave from th e gyrokinetic model. From the viewpoint of gyrokinetic theory, the physics of the compressional Alfven wave is the polarization current at second orde r. Therefore, in a low frequency gyrokinetic system, the compressional Alfv en wave is naturally decoupled from the shear Alfven wave and drift wave. I n the gyrocenter coordinates, the gyrophase dependent parts of the distribu tion function S and (f) over tilde are decoupled from the gyrophase indepen dent part f. Introducing the gyrokinetic perpendicular dynamics also extend s the gyrokinetic model to arbitrary frequency modes. As an example, the Be rnstein wave is recovered from the gyrokinetic model. The gyrokinetic perpe ndicular dynamics uncovered here emphasizes that the spirit of gyrokinetic reduction is to decouple the gyromotion from the particle's gyrocenter orbi t motion, instead of averaging out the gyromotion. (C) 1999 American Instit ute of Physics. [S1070-664X(99)02705-6].