Simulation study of near-Earth space disturbances: 2. Auroral substorms

Abstract

A substorm is a transient phenomenon that lasts for only 1-2 h. One significant manifestation of the substorm is a sudden brightening of the aurora on the nightside. Simultaneously, the auroral electrojets are abruptly intensified in the ionosphere, disturbing the geomagnetic field in the polar region. The near-Earth space environment is highly disturbed, which manifests as an earthward fast flow of plasma, a sudden change in the magnetic field, or a sudden increase in hot plasma. Such disturbances are known to severely affect human society. The ultimate cause of the disturbances is the solar wind, but an explanation of the chain of processes leading from solar wind to the ionosphere is problematic. Here, the evolution of the auroral substorm is reviewed based on the results of a global magnetohydrodynamics (MHD) simulation, called a REProduce Plasma Universe (REPPU) code. The REPPU code is shown to reproduce many aspects of the auroral substorms and to be useful for understanding the primary chain processes from the solar wind to the ionosphere.

Terrestrial Energetic Neutral Atom Emissions and the Ground‐Based Geomagnetic Indices: Implications From IBEX Observations

“…In the near-Earth dynamo, the magnetic energy from the lobe region is converted to the internal energy as a high plasma pressure, followed by the kinetic energy, then the magnetic energy (i.e., field-aligned current). This energy conversion and generation of the FACs are shown by Ebihara and Tanaka (2015), Ebihara et al (2019), and Ebihara (2019). Thus, the increase of the plasma population (as observed by IBEX) described in this study may be a proxy of the compression of the ions and the generation of field-aligned current.…”

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

“…Off-equatorial path Equatorial path Onset Fig. 5 Paths of earthward plasma flow in the meridional plane during the expansion phase (Ebihara 2019). The greenish area represents the region where the plasma pressure is high.…”

“…The dawnward and the duskward flows generate two pairs of FACs. (Ebihara andTanaka 2015a, 2017). At high-latitude magnetosphere (and relatively low altitude), the plasma beta is low, and the diamagnetic current is relatively small.…”

“…The brightening of the aurora, however, starts to appear at ~ 69.7° MLAT, which is located near the equatorward Fig. 6 Paths of plasma (blue line) and current lines (red lines) during the expansion phase (Ebihara 2019). The shaded area represents the region of closed field lines border of the preexisting auroral arc, rather than the open-closed boundary.…”

“…In the simulation, the WTS appears as a consequence of the ionosphere-magnetosphere coupling. The possible explanation can be summarized as follows (Ebihara andTanaka 2015b, 2018): (1) The conductivity increases first in a part of the ionosphere at the early stage of the expansion phase. (2) The ionospheric Hall current (= Σ H B × E/B, where Σ H is the Hall conductivity) overflows near the poleward, or westward edge of the high-conductivity region.…”