IMF By and the spatio-temporal structure of the dayside aurora

With dayside auroral observations we may probe the spatial and temporal structure of solar wind-magnetosphere coupling during intervals when the IMF has a substantial By component. This work captures some of the magnetosphere-ionosphere features expected to be present when the Earth is embedded in corotating solar wind streams. Specifically, we examine auroral observations made at 76 degrees magnetic latitude to distinguish spatial from temporal magnetosphere features. To this end, we examine how the aurora changes as the station rotates with the Earth under a stationary magnetosphere structure during intervals of quasi-steady IMF orientation. Characterisitic features of the dayside aurora as a function of IMF orientation may be grouped as follows. (I) Strongly northward IMF: elongated cusp arc within 75-80 degrees. associated with sunward convection in the polar cap. (II) Strongly southward IMF: elongated, narrow, and stable cusp arc within 70-75 degrees, and an ionospheric convection symmetric about noon. (III) Southwest(east) IMF with vertical bar B-y/B-z vertical bar >= 1: (i) near-simultaneous auroral brightenings on field lines convecting noonward in the pre- and postnoon sectors. These are separated by a stable, but strongly attenuated, cusp aurora near noon. The polar convection is asymmetric; (ii) pre- and postnoon brightenings followed by noonward expansions accompanied by poleward moving auroral forms; (iii) strong auroral forms at the polar cap boundary on old, open field lines associated with a newly-discovered flow channel. Deflections in local ground magnetometers are seen whose cause is this flow channel. The phenomena under (iii) are presumably related to dynamo action in the high-latitude boundary layer downstream of the cusp.