An extensive survey of dayside diffuse aurora based on optical observations at Yellow River Station

TL;DR

This extensive 7-year optical survey at Yellow River Station classifies dayside diffuse auroras into unstructured and structured types, revealing their distribution, properties, and possible formation mechanisms, including a newly identified auroral form called throat aurora.

Contribution

The paper provides the first comprehensive classification and analysis of dayside diffuse auroras based on long-term optical observations, introducing the concept of throat aurora and linking auroral forms to ionospheric convection.

Findings

Unstructured DDAs are mainly in morning and afternoon, with different properties.

Structured DDAs often occur near MLN and show pulsating, drifting behaviors.

Stripy DDAs align with ionospheric convection and are associated with throat aurora.

Abstract

By using 7 years optical auroral observations obtained at Yellow River Station (magnetic latitude $76.24\,^{\circ}{\rm C}$N) at Ny-Alesund, Svalbard, we performed the first extensive survey for the dayside diffuse auroras (DDAs) and acquired observational results as follows. (1) The DDAs can be classified into two broad categories, i.e., unstructured and structured DDAs. The unstructured DDAs are mainly distributed in the morning and afternoon, but the structured DDAs predominantly occurred around the magnetic local noon (MLN). (2) The unstructured DDAs observed in morning and afternoon present obviously different properties. The afternoon ones are much stable and seldom show pulsating property. (3) The DDAs are more easily observed under geomagnetically quiet times. (4) The structured DDAsmainly show patchy, stripy, and irregular forms and are often pulsating and drifting. The drifting directions are mostly westward (with speed $\sim$5km/s), but there are cases showing eastward or poleward drifting. (5) The stripy DDAs are exclusively observed near theMLN and,most importantly, their alignments are confirmed to be consistent with the direction of ionospheric convection near the MLN. (6) A new auroral form, called throat aurora, is found to be developed from the stripy DDAs. Based on the observational results and previous studies, we proposed our explanations to the DDAs. We suggest that the unstructured DDAs observed in the morning are extensions of the nightside diffuse aurora to the dayside, but that observed in the afternoon are predominantly caused by proton precipitations. $\textit{(Abstract continues in PDF).}$