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.

A Physical Explanation for the Formation of Auroras

“…The difference between diffuse auroras and discrete auroras is the intensity of light, and its physical nature may be the difference in energy density as described from Figure 1. On the equator-ward side of the dayside cusp there often appear diffuse auroras [48]. Thus, the diffuse aurora defines the extent of the auroral zone while the discrete auroras are sharply defined brightness features within the diffuse aurora.…”

Dayside Cusp Aurorae and Ionospheric Convection Under Radial Interplanetary Magnetic Fields

“…The I 557.7 /I 630.0 ratio is approximately 0.2 in the magnetic zenith (∼75.1° MLAT, Figures 3h and 4h). This kind of low ratio can be explained as being a result of electrons precipitating along open field lines in the cusp (e.g., Lorentzen & Moen, 2000;Taguchi et al, 2019). The equatorward motion of the open/closed boundary location and PMAF could be linked to magnetic reconnection or FTE that occurred on the magnetopause (Sandholt et al, 1986(Sandholt et al, , 1990.…”