Counter equatorial electrojet and overshielding after substorm onset: Global MHD simulation study

Abstract

By performing a global magnetohydrodynamic (MHD) simulation, we have demonstrated for the first time that an electrojet at the dayside magnetic equator can be reversed and an overshielding condition can be established in the inner magnetosphere after substorm onset without northward turning of the interplanetary magnetic field. Near the substorm onset, the plasma pressure is highly enhanced in the inner magnetosphere on the nightside. The Region 2 field-aligned current diverges from the diamagnetic current on the surface of the dayside extension of the high-pressure region, which is connected to the ionosphere in the relatively low-conductivity region a few degrees equatorward of the main auroral oval that is formed as the projection of the plasma sheet. The separation of the equatorward boundary of the auroral region and the equatorward boundary of the Region 2 current results in dusk-dawn electric fields that generate a counter electrojet (CEJ) at the dayside magnetic equator. Poleward electric fields in a narrow latitudinal width, which may be regarded as subauroral ion drift and subauroral polarization stream, are simultaneously intensified. The dusk-dawn electric fields may propagate to the inner magnetosphere along a field line as shear Alfvén waves. Then, the inner magnetosphere is completely constrained by the overshielding condition. The intensity and polarity of the CEJ depend largely on at least the ionospheric conductivity that is related to the plasma pressure (probably associated with diffuse aurora). This may explain the observational fact that overshielding does not always occur after onset.

Substorm simulation: Insight into the mechanisms of initial brightening

“…However, a MHD simulation provides a solution that fully satisfies continuity of mass, momentum, energy, force balance, and Faraday's and Ampère's laws throughout the domain from the solar wind to the inner magnetosphere. Recently obtained results using a high-resolution grid system show that many of the striking features associated with substorms are global in nature and can be represented by MHD equations [Tanaka et al, 2010;Ebihara and Tanaka, 2013;Ebihara et al, 2014].…”

“…Instead, we introduced empirical equations that relate the FAC and plasma pressure to the height-integrated conductivity. The ionospheric conductivity used in this study is described by Ebihara et al [2014] in detail.…”

“…The solar wind density and Y component of the IMF were kept constant at 5 cm À3 and 2.5 nT, respectively. All simulation settings, grid system, and ionospheric conductivities are the same as those used by Ebihara et al [2014]. The grid system is based on a dodecahedron [Moriguchi et al, 2008].…”

“…The grid spacing in the outward direction is 0.044 Re at 2.6 Re, and 0.22 Re at 12 Re at midnight in the equatorial plane. Readers may refer to Ebihara et al [2014] and Tanaka (Substorm auroral dynamics reproduced by the advanced global M-I coupling simulation, in Auroral Dynamics and Space Weather, AGU Monogr., ed. Y. Zhang, in press) for more detailed information about the simulation settings, grid system, and ionospheric conductivities.…”

Theory, modeling, and integrated studies in the Arase (ERG) project

“…In this subsection, we show an example of the simulation result of CIMI with the electric and magnetic fields provided by the MHD simulation REProduce Plasma Universe (REPPU) (Tanaka 2015). REPPU is capable of reproducing important features that manifest a substorm including quiet arcs, north-south-/east-west-aligned arcs and equatorward movement of the auroral oval during the growth phase, generation of FACs and westward traveling surge during the expansion phase, and overshielding (in which the polarity of the convection electric field in the inner magnetosphere is reversed) during the recovery phase (Ebihara et al 2014;Tanaka 2015a, b, 2016;Tanaka 2015;Tanaka et al 2017). In order to construct a steady-state magnetosphere, we imposed the following parameters on the upstream of the solar wind: solar wind speed of 372 km/s, solar wind density of 5 cm…”