Wave Activity Stimulated By Perpendicular Currents

The frequent occurrence of particle drifts perpendicular to the ambient magnetic field in space plasmas in general, and the geoplasma in particular, warrants the study of their stability. The adopted model uses a perpendicular gravity field to generate the drifts, albeit the results can be applied to other environments supporting perpendicular currents: it is the free energy of the cross-field motion that feeds wave growth. Solu- tion of the appropriate dispersion equation for arbitrary arbitrary orientations of the wavevector shows the existence of a bewildering variety of instabilities that basically arise from (i) a (zero-drift threshold) purely growing, nonoscillatory mode propagat- ing perpendicularly to the drift direction and (ii) beam-like resonant wave-particle interactions with the drifting populations. Emphasis of the analysis lies in a cold, ho- mogeneous medium but the effects of both finite temperatures and the (neglected, inhomogeneous) magnetic field created by the perpendicular currents are assessed, and potential applications to the geoplasma are discussed.