Evolution of auroral substorm as viewed from MHD simulations: dynamics, energy transfer and energy conversion
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Evolution of auroral substorm as viewed from MHD simulations: dynamics, energy transfer and energy conversion Yusuke Ebihara1 · Takashi Tanaka2 Received: 17 June 2019 / Accepted: 22 December 2019 © The Author(s) 2020
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. Keywords Magnetosphere · Auroral substorm · Simulation · Solar wind– magnetosphere–ionosphere coupling · MHD simulation
* Yusuke Ebihara [email protected]‑u.ac.jp 1
Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611‑0011, Japan
2
Kyushu University, Motooka 744, Nishi‑ku, Fukuoka 819‑0395, Japan
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Reviews of Modern Plasma Physics
(2020) 4:2
1 Introduction An auroral substorm is a noticeable phenomenon in the polar region on the nightside. Suddenly, bright aurorae appear on a pre-existing (faint) auroral oval, and expand quickly (Akasofu 1964). An example of the auroral substorm is shown in Fig. 1. Simultaneously, a large amount of electric current flows horizontally in the ionosphere at ~ 100-km altitude, which is called an auroral electrojet. The auroral electrojet causes magnetic disturbances on the ground in the polar region, and gives rise to Joule dissipation reaching ~ 1011 W (Ahn et al. 1983; Kamide et al. 1996; Østgaard 2002; Palmroth et al. 2005; Richmond et al. 1990; Rostoker et al. 1980; Tanskanen et al. 2002; Zhou et al. 2011). Observations have shown that the electrojet is connected to the field-aligned currents (FACs) intensified during the auroral expansion (Atkinson 1967; Boström 1964; Connors et al. 2014; Frank et al. 1982; Kamide et al. 1981; Kamide and Akasofu 1975; Rostoker et al. 1975). There are two important aspects of the FACs. First, they efficiently supply magne
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