The ionosphere and neutral atmosphere are strongly coupled, dynamically as well as chemically. At low and middle latitudes on the Earth's day side, for example, thermospheric neutral winds move the conducting plasma of the ionosphere across geomagnetic field lines, driving an atmospheric dynamo that generates the Sq (solar quiet) current systems and the equatorial electrojet, a powerful eastward current that flows in the E region along the geomagnetic equator. In the polar regions, on the other hand, it is the ions drifting over the polar cap, in response to the imposed magnetospheric convection electric field, that "drag" the neutrals and thus generate neutral winds with speeds sometimes exceeding 3600 km per hour in the high-latitude F-region thermosphere (see the figure at the bottom of this page).

The ionosphere also interacts strongly with the magnetosphere. A central aspect of this interaction is the electrodynamic coupling effected by electric currents flowing along the geomagnetic field lines that connect the ionosphere to the plasma sheet and the magnetospheric boundary layers. These "field-aligned" or "Birkeland" currents produce an electric field across the polar cap, which generates the horizontal currents in the polar ionosphere responsible for the convective ion flow referred to above. The field-aligned currents are carried both by auroral electrons precipitating downward along the field lines and by upward-flowing ionospheric electrons. The former deposit a substantial amount of energy into the upper atmosphere, with profound effects on both the ionosphere and the neutral thermosphere. In addition to exciting auroral emissions, auroral electron precipitation enhances the plasma density and conductivity of the high-latitude ionosphere and supplies as much as a trillion watts of heat to the upper atmosphere, dramatically altering global thermospheric wind patterns at times of strong geomagnetic activity.

A view of the Earth from the Dynamics Explorer (DE) 1 spacecraft showing the northern auroral oval. The Sun is to the right. The yellow arrows superimposed on the image indicate the direction and speed of thermospheric neutral winds created through ion-neutral drag by the movement of ionospheric plasma in the polar region. The flow of both neutrals and ions is strongly antisunward in the polar cap; a smaller sunward flow occurs in the dusk sector of the auroral oval. Neutral wind data were obtained with the Fabry-Perot interferometer on the DE 2 spacecraft. (Figure from: Killeen, T. L., et al., On the relationship between dynamics of the polar thermosphere and morphology of the aurora: Global-scale observations from Dynamics Explorers 1 and 2, J. Geophys. Res., 93, 2675, 1988.)

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