Effect of the IMF B y component on the ionospheric flow overhead at EISCAT: observations and theory

• Published in: Annales Geophysicae
• Main Authors: H. Khan, S. W. H. Cowley
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2000
• Subjects: Science; Q; Physics; QC1-999; Geophysics. Cosmic physics; QC801-809; The ''Y''; Tromsø; EISCAT
• Online Access: https://doi.org/10.1007/s00585-001-1503-6; https://doaj.org/article/40c3a34e7eaf44f6baee1926a607950e

We have analysed a database of ∼300 h of tristatic ionospheric velocity measurements obtained overhead at Tromsø (66.3° magnetic latitude) by the EISCAT UHF radar system, for the presence of flow effects associated with the y -component of the IMF. Since it is already known that the flow depends upon IMF B z , a least-squares multivariate analysis has been used to determine the flow dependence on both IMF B y and B z simultaneously. It is found that significant flow variations with IMF B y occur, predominantly in the midnight sector (∼2100–0300 MLT), but also pre-dusk (∼1600–1700 MLT), which are directed eastward for IMF B y positive and westward for IMF B y negative. The flows are of magnitude 20–30 m s –1 nT –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 a few hundred m s –1 over the usual range of IMF B y of about ±5 nT. At other local times the IMF B y -related perturbation flows are much smaller, less than ∼5 m s –1 nT –1 , and consistent with zero within the uncertainty estimates. We have investigated whether these IMF B y -dependent flows can be accounted for quantitatively by a theoretical model in which the equatorial flow in the inner magnetosphere is independent of IMF B y , but where distortions of the magnetospheric magnetic field associated with a "penetrating" component of the IMF B y field changes the mapping of the field to the ionosphere, and hence the ionospheric flow. We find that the principal flow perturbation produced by this effect is an east-west flow whose sense is determined by the north-south component of the unperturbed flow. Perturbations in the north-south flow are typically smaller by more than an order of magnitude, and generally negligible in terms of observations. Using equatorial flows which are determined from EISCAT data for zero IMF B y , to which the corotation flow has been added, the theory predicts the presence of zonal perturbation flows ...