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.

Formation of the Sun‐aligned arc region and the void (polar slot) under the null‐separator structure

“…The protruding plasma sheet structures contain relatively irregular structures. Similar protruding structure into the lobe has been reported by Ebihara and Tanaka [], in relation to the preonset N‐S arc.…”

“…As an explanation for the similar structure of shear flow in the lobe, there is a theory for the growth phase N‐S arc. In this case, an instability has been proposed for the development of the branching structure into the lobe [ Ebihara and Tanaka , ]. This instability develops in the following cycle: variation of pressure → variation of field‐perpendicular currents → FACs → ionosphere electric fields → change of flow → advection of pressure.…”

Reproduction of Ground Magnetic Variations During the SC and the Substorm From the Global Simulation and Biot‐Savart's Law

“…The global simulation reproduces a series of auroral phenomena during the substorm, such as the quiet arc (Tanaka, ; Tanaka, Ebihara, et al, ) and the preonset N‐S arc (Ebihara & Tanaka, ) during the growth phase, the initial brightening at the onset (Ebihara & Tanaka, ; Tanaka, Ebihara, et al, ; Tanaka, Ebihara, et al, ), and the westward traveling surge (WTS) during the expansion phase (Ebihara & Tanaka, ). The global simulation is verified by comparing these results with observations.…”

“…The global simulation is verified by comparing these results with observations. It was discovered from these studies that the quiet arc is projection of convective shear on the plasma sheet‐lobe boundary (Tanaka, Ebihara, et al, ), that the preonset N‐S arc is a structure in the lobe (Ebihara & Tanaka, ), and that the onset is due to the formation of the near‐earth dynamo (Ebihara & Tanaka, ). Ebihara and Tanaka () have proposed a highly reliable mechanism (ionospheric polarization) of the WTS based on calculation results.…”

Auroral streamer and its role in driving wave-like pre-onset aurora

“…In the recent years, as a major benefit from the development of hightemporal resolution ASIs, details of auroral evolution have been dramatically improved. The relation between auroral streamer and substorm onset also becomes a hot research topic for the past few years, and most likely the auroral streamer evolution is consistent with NENL model (Ebihara and Tanaka 2016;Nishimura et al 2011). However, we need to point out that even though the streamer-like aurora is usually observed ahead of a substorm expansion onset, it does not mean that the streamer triggers the substorm expansion onset.…”