Physics – Plasma Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm22a..08v&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM22A-08
Physics
Plasma Physics
[2756] Magnetospheric Physics / Planetary Magnetospheres, [2772] Magnetospheric Physics / Plasma Waves And Instabilities, [7831] Space Plasma Physics / Laboratory Studies And Experimental Techniques
Scientific paper
In a magnetized plasma consisting of two ion species, the perpendicular dielectric coefficient vanishes at the ion-ion hybrid frequency, where shear Alfven waves have zero parallel group velocity and experience a cut-off. Since the ion-ion hybrid frequency is proportional to the magnetic field, it is possible for propagating shear waves to be reflected in regions of increasing magnetic field, i.e., at a magnetic step. Thus, in principle, it is possible for a magnetic well configuration in a two ion plasma to behave as an Alfven wave resonator. It has been proposed* that such resonators occur naturally in the equatorial regions of planetary magnetospheres. This study explores the possibility of establishing such a resonator in the linear plasma column generated in the Large Plasma Device (LAPD) at UCLA using Hydrogen-Helium and Helium-Neon mixtures. The resonator response is investigated by launching monochromatic waves or sharp tone bursts from a magnetic loop antenna. With a single magnetic step, it is found that the waves are unable to penetrate into the high-field region beyond the ion-ion hybrid layer. Wave patterns in the high-field region completely lack the cone-like features of the waves launched from the low-field region. In the low-field region the radial profiles of the wave amplitude broaden, as is characteristic of a reflection. When a magnetic well is established by adding a second step, the radial profiles, in the well, further broaden and a line structure begins to emerge in the power spectrum of waves trapped in the well. The radial profiles of the lines in the spectra begin to resemble global radial modes rather than waves launched from a localized source. The observations are compared to predictions of a square wave model of the well and the Budden transmission model. *Guglielmi, A. V, Potapov, A. S, Russell, C. T, “The ion cyclotron resonator in the magnetosphere,” JETP Lett., v72, n6, 2000.
Farmer W.
Maggs James
Morales G. J.
Vincena S. T.
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