Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsa44a..05v&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SA44A-05
Other
2411 Electric Fields (2712), 2431 Ionosphere/Magnetosphere Interactions (2736), 2736 Magnetosphere/Ionosphere Interactions (2431)
Scientific paper
At very low magnetic latitudes, plasma flows in the ionosphere and disturbances of the geomagnetic field are sometimes observed to undergo temporal variations closely correlated with changes in solar wind parameters and associated variations of magnetospheric convection at high latitudes. Conventionally, such events are interpreted as the result of an electric field imposed by the solar wind, penetrating from the polar cap down toward the equator; time scales and other properties have been calculated often from purely electromagnetic considerations, assuming that electric fields drive conduction currents in the ionosphere and produce plasma bulk flows by E × B drifts. It has been amply demonstrated, however, that in large-scale plasmas the electric field is produced by the flow rather than the other way around, and that ionospheric currents arise directly from deformation of the magnetic field as the result of plasma-neutral collisional drag. The penetration of magnetospheric convection to low latitudes must therefore be described directly in terms of flows and stresses; the conventional formulation in terms of electric fields, although it may offer some mathematical advantages, is not a viable description of the physical process. At low altitudes, where any change of the magnetic field is necessarily very small in comparison to the Earth's dipole field, the two-dimensional (height-averaged) flow is effectively incompressible, irrespective of any vertical flows or compressibility of the plasma, because the magnetic field cannot be appreciably compressed. We show that, as a consequence, any changes in magnetospheric convection flow over the polar cap are immediately propagated to all latitudes at the fast-mode speed. At any radial distance R, the time scale for penetration is ~ R/V_A, shortest just above the ionosphere (combination of enhanced V_A because of the low density and minimal R). This time scale differs from those given (under various approximations) by the purely electromagnetic approach; thus, although largely equivalent in the steady state, the two approaches do differ when time variations are involved.
Song Paul
Vasyliūnas Vytenis M.
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