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 v_r in the lab frame within an order of magnitude of the ion gyrofrequency v_ci, propagate nearly perpendicular to the magnetic field (k_z /k_^ << 1), and can be Landau resonant (0 < v₁/k_z < n_d) with a parallel drifting electron population (drift speed n_d), where subscripts 1 and r indicate frequency in the frame of flowing ions and in the lab frame, respectively, and k_z is the parallel component of the wavevector. A transition in phase velocity from 0 < v₁/k_z < n_d to 0 > v₁/k_z > n_d 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 k_z n_d > 0. For stronger shear, both eigenmodes become reactive, with one's wavevector component k_z remaining parallel, but with v₁/k_z > n_d , and the other's wavevector component k_z becoming anti-parallel, so that 0 > v₁/k_z . 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 v_r v₁, both of which are previously unrecognized aspects of inhomogeneous energy-density driven waves.