Influence of IMF B-y on large-scale decreases of O column density at middle latitudes.

The interplanetary magnetic field (IMF) y component, B-y, has a strong influence on ion convection and, hence, neutral circulation patterns at polar latitudes. This is observed in the Northern Hemisphere as an enhancement in ion and neutral velocities in the dusk (dawn) cell during periods of B-y positive (negative), as well as an increase in the size of that cell relative to its dawn (dusk) counterpart. The effect of B-y on neutral winds and thermospheric composition equatorward of the polar cap and auroral oval is less well established in observations, although it has been investigated through sophisticated numerical modeling. A consequence of the compositional variations is that the spectrum of the dayglow emissions in the sunlit hemisphere is altered. We describe observations of the influence of B-y on the brightness of the 130.4- and 135.6-nm emissions of OI in the far-ultraviolet (FUV) dayglow that have been obtained with the Dynamics Explorer-1 (DE-1) FUV imaging photometer following auroral substorms. A fortuitous pair of DE-1 observations at similar to 1700 UT on days 267 and 279, 1981, suggests that the sign of B-y influences the brightness and spatial extent of the OI emissions and, hence, the oxygen column density in the morning sector at subauroral latitudes. A series of images taken during four consecutive DE-1 orbits more clearly demonstrates the dependence of the OI dayglow brightness on the sign of B-y. Hence, IMF orientation must be taken into consideration when interpreting FUV images of the dayglow and studying the underlying variations in thermospheric composition. (C) 1997 Elsevier Science Ltd.