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We have analysed a database of &#x223C;300 h oftristatic ionospheric velocity measurements obtained overhead at Tromsø (66.3°magnetic latitude) by the EISCAT UHF radar system, for the presence of floweffects associated with the y-component of the IMF. Since it is alreadyknown that the flow depends upon IMF Bz, a least-squaresmultivariate analysis has been used to determine the flow dependence on both IMFBy and Bz simultaneously. It is found thatsignificant flow variations with IMF By occur, predominantlyin the midnight sector (&#x223C;2100–0300 MLT), but also pre-dusk (&#x223C;1600–1700 MLT),which are directed eastward for IMF By positive and westwardfor IMF By negative. The flows are of magnitude 20–30 m s–1nT–1 in the midnight sector, and smaller, 10–20 m s–1 nT–1,pre-dusk, and are thus associated with significant changes of flow of order afew hundred m s–1 over the usual range of IMF By ofabout ±5 nT. At other local times the IMF By-relatedperturbation flows are much smaller, less than &#x223C;5 m s–1 nT–1,and consistent with zero within the uncertainty estimates. We have investigatedwhether these IMF By-dependent flows can be accounted forquantitatively by a theoretical model in which the equatorial flow in the innermagnetosphere is independent of IMF By, but where distortionsof the magnetospheric magnetic field associated with a "penetrating"component of the IMF By field changes the mapping of the fieldto the ionosphere, and hence the ionospheric flow. We find that the principalflow perturbation produced by this effect is an east-west flow whose sense isdetermined by the north-south component of the unperturbed flow. Perturbationsin the north-south flow are typically smaller by more than an order ofmagnitude, and generally negligible in terms of observations. Using equatorialflows which are determined from EISCAT data for zero IMF By,to which the corotation flow has been added, the theory predicts the presence ofzonal perturbation flows which are generally directed eastward in the NorthernHemisphere for IMF By positive and westward for IMF Bynegative at all local times. However, although the day and night effects aretherefore similar in principle, the model perturbation flows are much larger onthe nightside than on the dayside, as observed, due to the day-night asymmetryin the unperturbed magnetospheric magnetic field. Overall, the model results arefound to account well for the observed IMF By-related flowperturbations in the midnight sector, in terms of the sense and direction of theflow, the local time of their occurrence, as well as the magnitude of the flows(provided the magnetic model employed is not too distorted from dipolar form).At other local times the model predicts much smaller IMF By-relatedflow perturbations, and thus does not account for the effects observed in thepre-dusk sector. Key words: Magnetospheric physics (magnetosphere ·ionosphere interactions) – Ionosphere (plasma convection; auroral ionosphere

annales geophysicae

Effect of the IMF &amp;lt;i&amp;gt;B&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;&amp;lt;/i&amp;gt; component on the ionospheric flow overhead at EISCAT: observations and theory