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The nonlinear evolution of field line resonances in the Earth's magnetosphere

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  • Magnetohydrodynamic, field line resonances in the Earth's magnetosphere can have very large velocity shears and field-aligned currents. Auroral radar measurements of high-latitude resonances indicate that the velocities associated with the resonances in the E and F regions am often substantially greater than 1 km/s, and that the frequencies are in the interval from 1 to 4 mHz. Assuming that these resonances are oscillating at the fundamental mode frequency, and mapping these velocity fields along magnetic field lines to the equatorial plane shows that the velocity shears in the equatorial plane are of the order of 200 km/s over a radial distance of less than 2000 km (the amplitude of the velocity fluctuations is 100 km/s). Using a three-dimensional magnetohydrodynamic computer simulation code, we show that the resonances evolve through the development of Kelvin-Helmholtz instabilities near the equatorial plane. Within this framework, the instability is taking place on dipole magnetic field lines, and the resonances form a standing shear Alfven wave field due to the boundary conditions which must be satisfied at the polar ionospheres. We find that the nonlinear evolution of the Kelvin-Helmholtz instability leads to the propagation of vorticity from the equatorial plane to the polar ionosphere and thal the vorticity leads ultimately to the dissipation of the resonance. This occurs within a quarter wave period of the shear Alfven field associated with the resonances.

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    © 1993 American Geophysical Union. This version of this article is open access and can be downloaded and shared. The original author(s) and source must be cited.
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    • Rankin, Robert, Harrold, B. G., Samson, J. C., & Frycz, P. (1993). The nonlinear evolution of field line resonances in the Earth's magnetosphere. Journal of Geophysical Research: Space Physics, 98(A4), 5839-5853.
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