Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsm13a1654w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SM13A-1654
Physics
2736 Magnetosphere/Ionosphere Interactions (2431), 2752 Mhd Waves And Instabilities (2149, 6050, 7836), 2753 Numerical Modeling
Scientific paper
The interaction of field-aligned current with the auroral ionosphere is investigated using a 2-d (no azimuthal variation), non-linear, reduced-MHD model for a low-β plasma in dipole coordinates. It is shown that a magnetosphere-ionosphere resonant cavity (MIAR), similar in nature to the well-studied ionospheric Alfven resonator (IAR), exists at auroral latitudes. To characterize the role of parallel inhomogeneities in supporting an MIAR, the profile of the parallel wave conductivity, ΣA=1/μ0vA, which is proportional to the Alfven wave refractive index, is examined. At low-altitude, the negative gradient in ΣA defines the upper boundary of the IAR, whereas, the upper boundary of the MIAR is defined by a weaker, positive gradient at geocentric distances of about 7 RE. Excitation of the MIAR by ionospheric feedback instability resembles that of the IAR; however, the characteristic resonances of the MIAR are quite different. Numerically simulated oscillations of the simultaneously excited resonators are shown to originate from standing quarter-wavelength modes. The driven MIAR oscillates in the Pi2 range at a frequency much lower than that of the quarter-wave mode, and its threshold for instability is lower than that of the IAR. Perturbation analysis of a simple dispersion model is used to the explain this frequency shift. It is shown that the downshift in frequency occurs when the upper boundary of the resonator is formed by an upward gradient in ΣA. In contrast, the negative gradient in ΣA in the topside ionosphere gives rise to an upshift in frequency of the IAR oscillation relative to the quarter-wave mode. These characteristics may be understood in terms of the non-ideal boundary conditions on the standing wave modes at the ionospheric end of the field line.
Lotko William
Streltsov Anatoly V.
Watts Christopher J.
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