B(Y) FLUCTUATIONS IN THE MAGNETOSHEATH AND AZIMUTHAL FLOW VELOCITY TRANSIENTS IN THE DAYSIDE IONOSPHERE

Most researchers agree that ionospheric convection is controlled by the magnetosheath B(y) and B(z) components and by sheath flows. However transient changes in ionospheric flows are nonetheless generally attributed to transient changes in the merging rate. In particular, bursts in the azimuthal flow velocity and associated auroral transients are often associated with Flux Transfer Events (FTEs). We propose a simple but apparently overlooked alternative: ionospheric flow transients are directly driven by changes -- abrupt but not necessarily transient -- in the magnetosheath flows and fields. For example, an increase in the sheath B(y) component drives faster flows eastward or westward; while a decrease in the magnitude of B(y) leads to a transient propagating largely poleward. It would not be surprising if changing magnetosheath conditions led to changes in merging behavior; but we emphasize that azimuthal flow velocity changes in the ionosphere do not intrinsically imply any change in the merging rate. Changing the merging rate simply alters the number of field lines which are executing the same sequence of motions. In our model, auroral transients are caused by the velocity shears associated with changing convection patterns imposed on the ionosphere by an altered set of magnetosheath conditions. We point out also that many observational difficulties are resolved if merging occurs continuously, and sheath transients directly drive ionospheric transients. For example, auroral transients frequently convect through the pre-existing optical cusp; whereas if there has been no change in the sheath parameters any new merging should not move relative to the equally open cusp field lines. It is because of the variability in the sheath parameters that variability in ionospheric convection is imposed. The sources of such sheath field variability include intrinsic IMF variations; variations created at the bowshock; and compression of the sheath field by a variable plasma pressure.