Plasma Flow in the North‐South Aligned Discrete Aurora Equatorward of the Cusp

Abstract

On the equatorward side of the dayside cusp there often appear diffuse auroras. In this paper, we report north-south aligned discrete aurora events and show the spatial relationships between the auroras and the plasma flow. Several events of the north-south aligned discrete auroral structures were identified using an all-sky imager at Longyearbyen, Svalbard, during the recovery of a moderately disturbed period on 8 December 2013. During a brief interval of the moderately disturbed period, the cusp shifted to higher latitudes; as a result, the field-aligned beam of the EISCAT Svalbard Radar (ESR) passed through the northern portion of the north-south aligned auroral structures. Simultaneous observations from the all-sky imager and ESR reveal that enhancements in ion temperature (caused by fast ion flow) occurred near the eastward and westward boundaries of the north-south aligned auroral structures. However, within the region of the most enhanced aurora, the ion temperature enhancements were moderately suppressed. These features indicate that the ion flow slows down in the region of electron precipitation responsible for north-south aligned auroral structures. We can quantitatively interpret the slowdown of the flow as the reduction of the electric field due to the polarization effect in the north-south aligned region of the increased Pedersen conductivity. It thus appears that the magnetospheric source of the north-south aligned discrete auroras is a limited area embedded in the region of plasma flow toward the dayside magnetopause.