Shear Alfvén waves in a magnetic beach and the roles of electron and ion damping

Experiments are performed in the Large Plasma Device ~LaPD! @Gekelman et al., Rev. Sci. Instrum. 62, 2875 ~1991!# at the University of California, Los Angeles to study the propagation of the shear Alfvén wave in a parallel gradient of the background magnetic field. The waves are excited by modulating a field-aligned electron current drawn to a disk antenna with a radius on the order of the electron skin-depth, d5c/vpe . The resulting shear waves have a nonzero parallel electric field and propagate both parallel and perpendicular to the background magnetic field. In this experiment, the wave is launched in a region where its frequency, v equals one-half the local ion-cyclotron frequency, vci and the local Alfvén speed, vA , is approximately equal to the electron thermal speed, v̄e . The wave propagates along a slowly decreasing background field to where v5vci and vA ' v̄e/2. The wave thus propagates from a region where Landau damping is significant to where ion-cyclotron damping dominates. Detailed two dimensional measurements of the wave magnetic field morphology are presented. The measured wavelength decreases in accord with WKB solutions of a modified wave equation. Wave damping is also observed and dissipation by both ions and electrons is required in the WKB model to fit the data. Suppression of the damping via electrons in the model results in a predicted wave magnetic field amplitude twenty times larger at the ion-cyclotron resonance point than observed. © 2001 American Institute of Physics. @DOI: 10.1063/1.1389092#