Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsm24b..01k&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #SM24B-01
Physics
[2704] Magnetospheric Physics / Auroral Phenomena, [2721] Magnetospheric Physics / Field-Aligned Currents And Current Systems, [2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2756] Magnetospheric Physics / Planetary Magnetospheres
Scientific paper
The phrase “magnetosphere-ionosphere coupling” has become almost hackneyed in the terrestrial context, but plays an important role in the terrestrial system and must also be emphasized in the context of planetary- and moon-magnetospheres because the underlying principles are similar in all systems. This talk will introduce only two intriguing aspects of the coupling problem for planets and moons. In describing the first topic, we note that, especially for the gas giants Jupiter and Saturn, much of the evidence of magnetosphere-ionosphere coupling is obtained from auroral imaging. In images of Jupiter’s polar ionosphere, bright auroral spots are found to link magnetically to the moons Io, Europa and Ganymede. The spots give evidence of intense field-aligned currents generated near the equator in the interaction between the moons and the flowing plasma of Jupiter’s magnetosphere. The currents must penetrate through regions of impedance mismatch near the upper and lower boundaries of Jupiter’s equatorial plasma torus in order to close in the planetary ionosphere. There is some evidence that the signal propagates through the strong gradient of plasma density at the boundary of the plasma torus by converting into a striated structure that guides high frequency waves. As well, at Io, the interaction has been found to generate localized intense electron fluxes observed to flow along and antiparallel to the magnetic field near the equator. These bidirectional beams are probably accelerated by parallel electric fields near the ionospheric ends of the flux tube, but how the accelerated electrons reach the equator has not been explained. It seems likely that their presence there requires that the (parallel) electric fields in the Jovian ionosphere vary either temporally at high frequency or spatially on short transverse length scales. The full explanation has not yet been developed. As a second example of the role of magnetosphere-ionosphere coupling in planetary systems, we turn to an MHD simulation of the mini-magnetosphere of Ganymede carried out by X. Jia (2010). The significance of the ionosphere in the simulation is that, as the inner plasma boundary, it affects the flow and the dynamics of the entire system. The mathematical reason for the result is evident: differential equations have different solutions for different boundary conditions, but the dramatic changes that arise throughout the entire volume of the magnetosphere as the inner boundary condition is slightly modified may be both surprising and illuminating.
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