Evolution of auroral substorm as viewed from MHD simulations: dynamics, energy transfer and energy conversion

Abstract

An auroral substorm is a visual manifestation of large-scale, transient disturbances taking place in space surrounding the Earth, and is one of the central issues in the space plasma physics. While a number of studies have been conducted, a unified picture of the overall evolution of the auroral substorm has not been drawn. This paper is aimed to overview the recently obtained results of global magnetohydrodynamics (MHD) simulations in a context of a priori presence of anomalous resistivity leading to magnetic reconnection, and to illuminate what the global MHD simulation can sufficiently reproduce the auroral transients during the auroral substorm. Some auroral transients are found to be seamlessly reproduced by the MHD simulation, including complicated auroral structures moving equatorward during the growth phase, auroral brightening starting to appear near the equatorward border of the preexisting auroral arc, and an auroral surge traveling westward. Possible energy transfer and conversion from the solar wind to the Earth are also overviewed on the basis of the MHD simulation. At least, 4 dynamo regions appear sequentially in the course of the development of the auroral substorm. Although the MHD simulation reproduces some transients, further studies are needed to investigate the role of kinetic processes.

Global Modeling of the Inner Magnetosphere Under the Influence of a Magnetic Cloud Associated With an Interplanetary Coronal Mass Ejection: Energy Conversion and Ultra‐Low Frequency Wave Activity

“…However, if we extract a time series, in the LT and R mentioned above we should get, after IMF orientation change, spikes in energy conversion when this setting is oscillating around position 7.64. In Figure 11b, after ∼77 min we observe a peak in the rate of conversion of magnetic energy into kinetic and internal energy, characteristic and also consistent with Load (Birn & Hesse, 2005; Ebihara & Tanaka, 2020). In this way, we find that the median of the energy conversion components, Figure 11a around the position R = 7.64 R E , is a representative amount of the energy conversion during the phase in which B z changes orientation.…”

“…Three types of energy densities which correspond to the terms to the right‐hand side of Equations in the MHD approximation are analyzed here, namely, magnetic ( E m ), internal ( E i ) and kinetic ( E k ) energies (Birn & Hesse, 2005; Ebihara & Tanaka, 2017, 2020), and their equations are defined in units of (nW/m 3 ) as follows: $$${E}_{m}=-\vec{V}\cdot \left(\vec{J}\times \vec{B}\right),$$$ $$${E}_{i}=\vec{V}\cdot \nabla P,$$$ $$${E}_{k}=\vec{V}\cdot \left(\vec{J}\times \vec{B}-\nabla P\right),\ \ $$$ where $$\vec{V}$$ is the plasma bulk velocity, $$\vec{B}$$ is the magnetic field vector, $$\vec{J}$$ is the plasma current density as derived from Ampère's law, and P is the plasma pressure. Notice that Equations has dimensions of energy density flux, that is, Watt/m 3 , but hereafter we will refer to them only as either of the three energy forms mentioned above, that is, magnetic energy, kinetic energy, and internal energy.…”

Unified Theory of the Arc Auroras: Formation Mechanism of the Arc Auroras Conforming General Principles of Convection and FAC Generation

“…The S‐M interactions are indispensable to generate the plasma regimes inside the magnetosphere. The energy source of the FAC is inside the magnetosphere (Ebihara & Tanaka, 2017, 2020). The thermal energy in the plasma regimes becomes energy of Poynting flux associated with the FAC.…”

“…In addition, field‐parallel flow (squeezing flow) from the NENL reaches the inner magnetosphere away from the equatorial plane, earlier than flow in the equatorial plane (Tanaka, Ebihara, et al., 2017). The near‐Earth dynamo forms following the occurrence of these flows (Ebihara and Tanaka, 2015, 2020). Figure 14 schematically draws the generation mechanism of the near‐Earth dynamos.…”