On the poleward boundary of the nightside auroral oval under northward interplanetary magnetic field conditions

Abstract

In this paper we examined the dependence of the poleward boundary of the nightside auroral oval on interplanetary magnetic field (IMF) conditions. In particular, we performed the study for periods after the IMF Bz becomes northward at or some time during the recovery phase of major storms. For the selected periods, we used the precipitating particle energy flux observed by the Defense Meteorological Satellite Program satellites to determine the poleward boundary of the nightside oval. The following results were obtained. (1) We confirmed the expected result that under a more strongly northward IMF Bz, the poleward boundary of the nightside oval tends to be located more poleward, implying a smaller polar cap. (2) However, it tends to take a very long time after the IMF Bz turns northward for the polar cap size to substantially reduce (e.g., ∼15 h or longer for a poleward movement by ∼5°). This is likely because the magnetosphere during the selected northward IMF periods was still under the substantial influence of the preceding southward IMF conditions associated with the storm's main phase and also because a finite IMF By can support dayside reconnection to sustain an open polar cap region of substantial size. (3) We found that for a given sign of the IMF By, the poleward boundary of the nightside oval is asymmetric between premidnight and postmidnight regions in a given hemisphere and between hemispheres at a given magnetic local time region. For example, for a positive IMF By, the Northern (Southern) Hemispheric oval boundary is more poleward (equatorward) in the premidnight region, and the situation is opposite in the postmidnight region and for a negative IMF By. We suggest a possible reason that may be responsible for the asymmetry. (4) A similar (in an opposite sense) asymmetric feature can be seen with the IMF Bx but only when the IMF happens to be of a Parker spiral angle type. We conclude that the magnetosphere during the recovery phase of major storms likely remains open even after the IMF turns northward and the polar cap field lines are still connected to the IMF. The role of the finite IMF By is prominent in determining both the size and shape of the polar cap, which can affect details of the dynamical evolution during the storm recovery phase.