Spatially Resolved Spectral Measurements of EIH Waves Generated by Plasma Shear Flow Layer*

Waves and instabilities generated by flows in plasmas due to the presence of mutually perpendicular electric and magnetic fields are of general relevance to the study of multiscale plasma dynamics and space physics. In particular, the waves generated by the shear in such flow profiles can have both stabilizing and destabilizing effects on the plasma depending on the shear's magnitude and scale length and may be the trigger of energy cascades. Over the years, laboratory experiments have been performed to simulate the broadband emissions from dipolarization fronts (highly compressed, Earth-ward propagating plasma sheets resulting from reconnection events) that energize the near-earth plasma environment, resulting in magnetospheric substorms. In this experiment, spatially scanned floating potential probes are used to conduct temporal crosscorrelation measurements of fluctuations generated by the shear at the boundary of a cylindrical flow layer in a magnetized plasma column. This 3D data set is analyzed using discrete Fourier transforms to isolate the 2D spatial structure of each mode and to reconstruct the azimuthal dispersion relation of the excited waves. Since the shear layer length scale is subLarmor radius and the minimum observed mode wavelength is greater than the plasma skin depth, these results are compared to the non-local electrostatic theory for Electron-Ion Hybrid (EIH) waves. It is shown that, in accordance with theory, the modes excited are broadly distributed around m=3 and that the waves propagate in the direction of the cross-field flow.