Laboratory measurements of shear Alfven wave fields, ion drifts, and density perturbations

Alfven waves in the auroral ionosphere are frequently observed in correlation with accelerated electrons and regions of density depletion, but the connections between these phenomena are not yet well understood. In the Large Plasma Device at UCLA, the fields, ion drifts, and density perturbations of large-amplitude shear Alfven waves (Bwave/B o ∼ 10-3) have been studied in detail using a diverse set of diagnostics. These include laser-induced fluorescence (LIF), ∂B/∂t, and Langmuir probe measurements of ne, Te, and ion saturation current. The waves propagate along the axis of a 9 m long, 40 cm diameter cylindrical argon plasma in the kinetic regime (beta e ≈ 6.0 · (me/mi)), with fwave/fci ≈ 0.8. Care was taken to record the measurements from each diagnostic at the same spatial positions on four cross-sectional planes along the length of the plasma. Two-dimensional LIF was used to measure Ti and the E x B and polarization drifts, which were as large as 14% of the ion thermal speed. The two drifts were distinguished by their phase relation to Bwave, allowing E⊥ (the transverse component of E wave) to be computed. E|| was estimated from E⊥ based on kinetic regime Alfven wave theory, and found to have a maximum value of ∼0.5 mV/cm. The waves' interaction with the plasma resulted in two types of density perturbations. The first was an oscillating flow across Bo at the frequency of the wave, with &dgr;n/n as large as 12% in regions of small-scale transverse density gradients. The second effect was the gradual smoothing of those density gradients over several wave periods. Detailed measurements and data visualization are presented.