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

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 crossfield plasma flow. These waves have a frequency ωr in the lab frame within an order of magnitude of the ion gyrofrequency ωci , propagate nearly perpendicular to the magnetic field (kz/k⊥ 1), and can be Landau resonant (0 < ω1/kz < νd ) with a parallel drifting electron population (drift speed νd ), 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 < ω1/kz < νd to 0 > ω1/kz > νd for a pair of IEDD eigenmodes is observed as the degree of inhomogeneity 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 kzνd > 0. For stronger shear, both eigenmodes become reactive, with one’s wavevector component kz remaining parallel, but with ω1/kz > νd , and the other’s wavevector component kz becoming anti-parallel, so that 0 > ω1/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 ωrω1, both of which are previously unrecognized aspects of inhomogeneous energydensity driven waves.