MODELING THE HIGH-LATITUDE EQUINOCTIAL ASYMMETRY

Fabry Perot Interferometer measurements of neutral winds and European Incoherent SCATter radar measurements of plasma velocities have shown a significant equinoctial asymmetry in the average behavior of the the rmosphere and ionosphere above northern Scandinavia. Existing standard models of the upper atmosphere use forcing functions that are symmetr ic about. the solstices, therefore these observations are unexpected. It is suggested that the asymmetry arises from the diurnal variation i n the cross polar cap potential difference (CPCPD) because there is a 12 hour phase difference between the variations at the March and Septe mber equinoxes. The variation in the CPCPD is caused by an annual and diurnal variation in the orientation of the magnetosphere with respect to the interplanetary magnetic field. This is known as the Russell-Mc Pherron (R-M) effect. The plausibility of this explanation of the equi noctial asymmetry in thermospheric winds is supported by investigation of the effect of their geomagnetic history, i.e., the repercussions o n the winds of the activity levels in the few hours prior to the obser vation. The consequences of the R-M effect have been simulated in the University College London/Sheffield/Space Environment Laboratory coupl ed thermosphere-ionosphere model by imposing a diurnally varying high- latitude electric field pattern. The results are used to test the pred ictions, given in an earlier paper, of the average behavior expected a t other high-latitude sites. A corollary to the study is that the evid ence presented here implies that the auroral oval may be smaller at so lar minimum, which is also unexpected.