Lobe cell convection and field‐aligned currents poleward of the region 1 current system

Abstract

We present a case and statistical study of plasma convection in the Northern Hemisphere during summer conditions using electric field, magnetic field, and particle data taken during dawn‐dusk directed orbits of the FAST satellite. To our knowledge, this set provides the most comprehensive combination of data as yet presented in support of lobe cell convection from an ionospheric perspective this far from the noon sector. In particular, we study the current systems and convection patterns for all passes in July 1997 that show evidence for six large‐scale field‐aligned currents (FACs) rather than the usual system of four FACs associated with the region 1/region 2 current systems. A total of 71 passes out of 232 in the study had the extra pair of FACs. The extra pair of FACs in 30 of the 71 cases lies either on the dawnside or on the duskside of the noon‐midnight meridian, and their position is strongly correlated with the polarity of the IMF By (negative and positive, respectively). This is consistent with the IMF dependence of a three‐cell convection pattern of coexisting merging, viscous, and lobe‐type convection cells. The occurrence of the asymmetric FAC pair was also strongly linked to conditions of IMF |By/Bz| > 1. The extra pair of FACs in these cases was clearly associated with the lobe cell of the three‐cell convection system. The remaining 41 cases had the pair of FACs straddling the noon‐midnight meridian. The extra pair of FACs was often (20 cases out of 30) observed at magnetic local times more than three hours away from noon, rather than being confined to regions near noon and the typical location of the cusp. Such a current system consisting of a pair of FACs poleward of the nearest region 1 current is consistent with the IMF By‐dependent global MHD model developed by Ogino et al. [1986] for southward IMF conditions, as well as with other magnetospheric and ionospheric convection models that include the effects of merging occuring simultaneously at both low‐latitude dayside and high‐latitude lobe and flank magnetopause reconnection sites. Finally, the presence of the additional FACs and three‐cell convection well away from noon show that the entire dayside ionosphere is affected by IMF‐dependent processes, rather than only a limited region around noon.