Space relevant laboratory studies of ion-acoustic and ion-cyclotron waves driven by parallel-velocity shear

Results from a Q-machine experiment are used to illustrate the dispersion-relation regimes associated with ion-acoustic and ion-cyclotron waves modified by the presence of shear in the magnetic-field-aligned (parallel) ion flow. Identifying the regimes requires knowledge of the sign of the ratio of wavevector components and the sign of parallel-velocity shear. Ion-temperature anisotropy is shown to influence significantly the propagation direction of oblique ion-acoustic waves. The necessity of documenting the instability mechanism with measurements of the electron and ion distribution functions, the electron and ion parallel-velocity shear, the flow-frame Doppler shifts, both small and large values of propagation angle, and the growth rate is illustrated. The existence of these shear-modified, low-frequency waves at values of parallel electron-drift velocity substantially smaller than the excitation thresholds predicted by homogeneous-plasma theory demonstrates the suitability of parallel-velocity shear for playing a role in the mechanism responsible for broadband extremely-low-frequency waves observed in the auroral ionosphere where there exists spatial variations and filamentation in the parallel plasma flow. This article was scheduled to appear in issue 5 of Plasma Phys. Control. Fusion. To access this Special issue please follow this link: http://www.iop.org/EJ/toc/0741-3335/45/5