NBZ currents and their connection to polar cap aurora

The magnetospheric cusp is one of the most important areas of the Earth's magnetosphere, as it is the boundary region between the interplanetary magnetic field (IMF) as well as Earth's closed and open magnetospheric field lines. Through this region, the solar wind can directly penetrate the magnetosphere and reach the ionosphere, creating there the so-called cusp aurora. Understanding the interaction between solar wind, cusp aurora and connected auroral regions allows to better understand the solar wind-magnetosphere-ionosphere interaction.

 

During northward interplanetary magnetic field (IMF) conditions, auroral structures such as cusp aurora, High latitude dayside aurora (HiLDA) and transpolar arcs (TPAs) can appear in the polar cap. At the same time, an additional pair of field aligned currents (FAC), called NBZ currents, is present in the polar cap. In this study, using DMPS imaging and particle data as well as AMPERE FAC data, we investigate the connection of polar cap aurora with the NBZ currents.  As expected, we find that the ion cusp aurora is located between the downward and upward NBZ currents. We additionally show that an area of electron precipitation is connected to the ion cusp region, appearing dawnward of it. This electron aurora can exhibit different characteristics, such as broadband aurora (signalling wave-particle interactions), monoenergetic electron precipitation (HiLDA), or inverted-V structures (TPAs). We demonstrate that the electron aurora appearing in connection with cusp aurora during northward IMF is the visible signature of the upgoing NBZ current region. We further investigate with three detailed cases studies, as well as statistically, how the IMF orientation and especially the IMF By component influences the location of the cusp aurora and this electron precipitation region.