Substorm simulation: Insight into the mechanisms of initial brightening

2017

Self Cite

At substorm expansion onset, upward field‐aligned currents (FACs) increase abruptly, and a large amount of electromagnetic energy starts to consume in the polar ionosphere. A question arises as to where the energy comes from. Based on the results obtained by the global magnetohydrodynamics simulation, we present energy flow and energy conversion associated with the upward FACs that manifest the onset. Our simulations show that the cusp/mantle region transmits electromagnetic energy to almost the entire region of the magnetosphere when the interplanetary magnetic field is southward. Integral curve of the Poynting flux shows a spiral moving toward the ionosphere, probably suggesting the pathway of electromagnetic energy from the cusp/mantle dynamo to the ionosphere. The near‐Earth reconnection initiates three‐dimensional redistribution of the magnetosphere. Flow shear in the near‐Earth region results in the generation of the near‐Earth dynamo and the onset FACs. The onset FACs are responsible to transport the electromagnetic energy toward the Earth. In the near‐Earth region, the electromagnetic energy coming from the cusp/mantle dynamo is converted to the kinetic energy (known as bursty bulk flow) and the thermal energy (associated with high‐pressure region in the inner magnetosphere). Then, they are converted to the electromagnetic energy associated with the onset FACs. A part of electromagnetic energy is stored in the lobe region during the growth phase. The release of the stored energy, together with the continuously supplied energy from the cusp/mantle dynamo, contributes to the energy supply to the ionosphere during the expansion phase.

Section: Introductionmentioning

confidence: 99%

Energy Flow Exciting Field‐Aligned Current at Substorm Expansion Onset

2017

Self Cite

“…Ebihara and Tanaka [], hereinafter referred to as Paper 1, dealt with rapid intensification of upward FAC at the beginning of the substorm expansion phase (hereinafter, simply referred to as an initial brightening). Based on the global MHD simulation, they explained the initial brightening as follows: (1) Associated with the development of the convection, a near‐Earth neutral line (NENL) [ Nishida and Nagayama , ; Russell and McPherron , ] releases magnetic tension to compress and accelerate plasma earthward.…”

Section: Introductionmentioning

confidence: 99%

Substorm simulation: Formation of westward traveling surge

2015

Self Cite

Auroral substorm expansion is characterized by initial brightening of aurora, followed by a bulge expanding in all directions, and a westward traveling surge (WTS). On the basis of the result obtained by a global magnetohydrodynamic simulation, we propose a scenario for the onset and the subsequent formation of WTS. (1) Near‐Earth neutral line releases magnetic tension in the near‐Earth plasma sheet to compress plasma and accelerate it earthward. (2) Earthward, perpendicular flow is converted to parallel flow in the near‐Earth tail region. (3) Plasma moves earthward parallel to a field line. The plasma pressure is additionally enhanced at off‐equator with an expanding slow‐mode variation. (4) Flow vorticities coexist near the off‐equatorial high‐pressure region. Resultant field‐aligned current (FAC) is connected to the ionosphere, which may manifest initial brightening. (5) Due to continued earthward flow, the high‐plasma pressure region continues to expand to the east and west. (6) The ionospheric conductivity continues to increase in the upward FAC region, and the conductivity gradient becomes steeper. (7) The convergence of the Hall current gives rise to divergent electric field near the steep gradient of the conductivity. (8) Due to the divergent electric field, magnetospheric plasma moves counterclockwise at low altitude (in the Northern Hemisphere). (9) The additional flow vorticity generates a localized upward FAC at low altitudes, which may manifest WTS, and redistributes the ionospheric current and conductivity. Thus, WTS may be maintained in a self‐consistent manner, and be a natural consequence of the overflow of the Hall current.

“…Recently, it has been presented that the finer the spatial resolution of the global magnetohydrodynamic (MHD) simulations, which consider the effects of the magnetosphere‐ionosphere (M‐I) coupling, the more realistic the features of the auroral or polar substorms that can be reproduced [ Tanaka et al ., ; Ebihara et al ., ; Tanaka , , hereinafter referred to as Paper 1; Ebihara and Tanaka , , Paper 2; Ebihara and Tanaka , , Paper 3]. In these precise simulations, ionospheric elements of substorms, such as the sudden decrease in the Auroral Lower ( AL ) index, the quiet arc, the dipolarization front, the initial brightening, the westward traveling surge (WTS), the double oval, and the overshielding, are reproduced.…”

Section: Introductionmentioning

confidence: 99%

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

2016

Self Cite

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.