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

• Main Authors: J. J. Carroll III, M. E. Koepke, M. W. Zintl, V. Gavrishchaka
• Format: Article
• Language: English
• Published: Copernicus Publications 2003-01-01
• Series: Nonlinear Processes in Geophysics
• Online Access: http://www.nonlin-processes-geophys.net/10/131/2003/npg-10-131-2003.pdf

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 <font face='Symbol'>v</font>_r in the lab frame within an order of magnitude of the ion gyrofrequency <font face='Symbol'>v</font>_ci, propagate nearly perpendicular to the magnetic field (k_z /k<font face='Symbol'>_^</font> << 1), and can be Landau resonant (0 < <font face='Symbol'>v</font>₁/k_z < <font face='Symbol'>n</font>_d) with a parallel drifting electron population (drift speed <font face='Symbol'>n</font>_d), where subscripts <i>1</i> and <i>r</i> 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 < <font face='Symbol'>v</font>₁/k_z < <font face='Symbol'>n</font>_d to 0 > <font face='Symbol'>v</font>₁/k_z > <font face='Symbol'>n</font>_d for a pair of IEDD eigenmodes is observed as the degree of in-homogeneity in the transverse <b><i>E × B</i></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 <font face='Symbol'>n</font>_d > 0. For stronger shear, both eigenmodes become reactive, with one's wavevector component k_z remaining parallel, but with <font face='Symbol'>v</font>₁/k_z > <font face='Symbol'>n</font>_d , and the other's wavevector component k_z becoming anti-parallel, so that 0 > <font face='Symbol'>v</font>₁/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 <font face='Symbol'>v</font>_r <font face='Symbol'>v</font>₁, both of which are previously unrecognized aspects of inhomogeneous energy-density driven waves.