Auroral System Science - Multifluid Simulations of Auroral Arc Ionospheric Current Closure

The auroral zone, a band of enhanced auroral activity located ~10-20° from either pole, houses complex and highly structured electrodynamics within the ionosphere. The study of auroral arc systems often requires heavy simplification such as assuming minimal longitudinal structure, referred to hereafter as “sheetlike”. With the help of state-of-the-art 3D modeling, we investigate the breakdown of this assumption by progressively introducing structure and dynamics to a typical inverted-V sheetlike arc. Furthermore, we aim to better understand the associated current system and the role Hall current plays with increasingly less sheetlike and/or more energetic precipitation.

The distributed map of DC field-aligned current (FAC) in the auroral ionosphere is fed by ExB flow shear and gradients in Pedersen (∇ΣP) and Hall conductances (∇ΣH) (current continuity equation, see Kelley, 2009), yet for sheetlike arcs the Hall term is often ignored. This is because only the along-arc component of ∇ΣH contributes to the FAC and for sheetlike systems the gradients are across-arc. We look at the limitations of this assumption by using multifluid electrostatic model runs provided by GEMINI (Zettergren, 2012, 2015) of systems with imposed along-arc structure as well as east-west motion (appropriate to that of a westward traveling surge). With the help of 3D visualization, we examine how this additional FAC affects current closure, especially in cases of high energy precipitation.

We investigate auroral systems with and without a steep along-arc bend which can provide enough disturbance in both the E field and ∇ΣH directions for the Hall term in the current continuity equation to become non-negligible. We systematically look at the effects of the westward motion and how it changes the trailing conductance in the wake. We look at the effects precipitation energy and flux, i.e. the impact ionization altitude profile, has on the role of Hall current in closing the FAC. We determine whether the degree of bend, or arc morphology in general, has any effect on the enhancement of along-arc structure. The aim of this study is to explore simulation-realized relationships between current, flow, and auroral brightness consistent with the current continuity equation.