E€ect of the IMF By component on the ionospheric ¯ow overhead at EISCAT: observations and theory

• Main Authors: H. Khan, S. W. H. Cowley
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: The ''Y''; EISCAT
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.371.6025; http://hal.archives-ouvertes.fr/docs/00/31/68/20/PDF/angeo-18-1503-2000.pdf

Abstract. We have analysed a database of 300 h of tristatic ionospheric velocity measurements obtained overhead at Tromsù �66.3 ° magnetic latitude) by the EISCAT UHF radar system, for the presence of ¯ow e€ects associated with the y-component of the IMF. Since it is already known that the ¯ow depends upon IMF Bz, a least-squares multivariate analysis has been used to determine the ¯ow dependence on both IMF By and Bz simultaneously. It is found that signi®cant ¯ow variations with IMF By 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 ¯ows 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 signi®cant changes of ¯ow of order a few hundred m s)1 over the usual range of IMF By of about ‹5 nT. At other local times the IMF By-related perturbation ¯ows are much smaller, less than 5ms)1 nT)1, and consistent with zero within the uncertainty estimates. We have investigated whether these IMF By-dependent ¯ows can be accounted for quantitatively by a theoretical model in which the equatorial ¯ow in the inner magnetosphere is independent of IMF By, but where distortions of the magnetospheric magnetic ®eld associated with a ``penetrating'' component of the IMF By ®eld changes the mapping of the ®eld to the ionosphere, and hence the ionospheric ¯ow. We ®nd that the principal ¯ow perturbation produced by this e€ect is an east±west ¯ow whose sense is determined by the north±south component of the unperturbed ¯ow. Perturbations in the north±south ¯ow are typically smaller by more than an order of magnitude, and generally negligible in terms of observations. Using equatorial ¯ows which are determined from EISCAT data for zero IMF By, to which the corotation ¯ow has been added, the theory predicts the presence of zonal perturbation ¯ows which are generally directed eastward in the Northern Hemisphere for IMF By ...