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 duri ng periods of B-y positive (negative), as well as an increase in the s ize of that cell relative to its dawn (dusk) counterpart. The effect o f B-y on neutral winds and thermospheric composition equatorward of th e polar cap and auroral oval is less well established in observations, although it has been investigated through sophisticated numerical mod eling. A consequence of the compositional variations is that the spect rum of the dayglow emissions in the sunlit hemisphere is altered. We d escribe 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) daygl ow that have been obtained with the Dynamics Explorer-1 (DE-1) FUV ima ging photometer following auroral substorms. A fortuitous pair of DE-1 observations at similar to 1700 UT on days 267 and 279, 1981, suggest s 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 th e OI dayglow brightness on the sign of B-y. Hence, IMF orientation mus t be taken into consideration when interpreting FUV images of the dayg low and studying the underlying variations in thermospheric compositio n. (C) 1997 Elsevier Science Ltd.