Substorm simulation: Quiet and N-S arcs preceding auroral breakup

Abstract

Auroral breakup at the onset of substorm expansion is sometimes preceded by auroral forms known as quiet arcs and N-S arcs. Observations have shown that both the auroral forms tend to move equatorward, and the initial brightening takes place in or near one of the quiet arcs. The auroral forms attract great attention, but generation of auroral forms and their association with the initial brightening are poorly understood. Recent global magnetohydrodynamic simulations are capable of producing upward field-aligned currents (FACs) that resemble the auroral forms in both shape and temporal evolution. Based on the simulation results, we propose the following scenarios: (1) When the convection electric field is weak (northward interplanetary magnetic field (IMF)), the high-pressure region is elongated from the plasma sheet toward higher latitudes and is structured by a coupling between the magnetosphere and the ionosphere (interchange-like instabilities). (2) When the convection electric field is strong (southward IMF), the structured high-pressure region moves equatorward (toward the plasma sheet). Upward currents are generated around it, which can be observed as arcs in the ionosphere. The upward current can be tentatively intensified in the course of the equatorward movement before the formation of a near-Earth neutral line (NENL). (3) The NENL releases magnetic tension and results in the enhancement of plasma pressure at off-equator in the near-Earth region. Sudden formation of the off-equatorial high-pressure region generates the onset current system that manifests initial brightening. Our scenario can explain the observational fact that poleward arcs remained undisturbed at the onset.