Resonant-to-nonresonant transition in electrostatic ion-cyclotron wave phase velocity

Abstract

Because of the implications for plasmas in the laboratory and in space, attention has been drawn to inhomogeneous energy-density driven (IEDD) waves that are sustained by velocity-shear-induced inhomogeneity in cross-field plasma flow. These waves have a frequency vr in the lab frame within an order of magnitude of the ion gyrofrequency vci, propagate nearly perpendicular to the magnetic field (kz/k^ \u3c\u3c 1), and can be Landau resonant (0 \u3c v1/kz \u3c nd) with a parallel drifting electron population (drift speed nd), where subscripts 1 and r indicate frequency in the frame of flowing ions and in the lab frame, respectively, and kz is the parallel component of the wavevector. A transition in phase velocity from 0 \u3c v1/kz \u3c nd to 0 \u3e v1/kz \u3e nd for a pair of IEDD eigenmodes is observed as the degree of in-homogeneity in the transverse E × B flow is increased in a magnetized plasma column. For weaker velocity shear, both eigenmodes are dissipative, i.e. in Landau resonance, with kz nd \u3e 0. For stronger shear, both eigenmodes become reactive, with one\u27s wavevector component kz remaining parallel, but with v1/kz \u3e nd , and the other\u27s wavevector component kz becoming anti-parallel, so that 0 \u3e v1/kz . For both eigenmodes, the transition (1) involves a small frequency shift and (2) does not involve a sign change in the wave energy density, which is proportional to vr v1, both of which are previously unrecognized aspects of inhomogeneous energy-density driven waves

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