Roles of the M‐I Coupling and Plasma Sheet Dissipation on the Growth‐Phase Thinning and Subsequent Transition to the Onset

Abstract

The plasma-sheet dynamics leading to the substorm onset is analyzed from a global simulation.We calculated three cases, the normal (case 1), reduced magnetosphere-ionosphere (M-I) coupling (case 2) and reduced tail dissipation (case 3). The normal solution reproduces the thinning, reconnection, tail flow, and the dipolarization in the magnetosphere, and the AU/AL, the initial brightening, the beads, the pre-onset arc, and the poleward expansion in the ionosphere. Case 2 shows that the M-I coupling promotes the thinning by evacuating magnetic field from the plasma sheet. Case 3 shows that it is implausible to reproduce the AU/ AL of the expansion phase only with numerical dissipation. The projection area of the growth phase plasma sheet is so narrow that the quiet arcs on the high-latitude side are projected to the lobe. The key point to understand such narrow projection is to consider the origin of the field-aligned current (FAC). The FAC occurs to transmit motion, and particle precipitation is the consequence. The upward FACs for the quiet arc and initial brightening, which produce the substorm sequence, are generated to transmit enhanced convection from the magnetosphere to the ionosphere. The substorm onset is the transmission of the convection transient. The onset FAC is generated by the near-earth dynamo. Before the onset, a two-stage reconnection occurs at the mediumtail (MTNL) and the near-earth (NENL) neutral lines through retreating nulls. The accompanying flow explains the model that assumes two causal areas for the onset in the near-earth and middle tails.

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

“…Furthermore, only the MHD can transmit convection motion to the ionosphere by exchanging net electric charge. However, the M‐I coupling is not a one‐way effect in which the FAC is supplied from the magnetosphere to the ionosphere (Tanaka et al., 2021c). The reproduction of the arc aurora is possible only when all conditions are satisfied such as the dissipation model is somewhat accurate, the M‐I coupling is working sufficiently, and the ionospheric electric conductivity is modeled to some extent correctly (Tanaka et al., 2021c).…”

“…In the geospace, the ionosphere is the only domain where in situ measurements approaching global scales are possible. Seen from the reproducibility of aurora and magnetic field variations in the ionosphere (Tanaka et al., 2021c), it is reasonable to assume that the onset is caused by the near‐Earth dynamo and narrow shear. Traditional onset models seem not yet precise enough in triggering stage of the onset.…”

“…The local model for the substorm is limiting the ionospheric solutions from the beginning by initial‐boundary conditions (Birn & Hesse, 2013). Even in the global MHD, the final FAC structure also seems to be incomplete if the coupling between the magnetosphere and the ionosphere is numerically insufficient (Tanaka et al., 2021c; Xi et al., 2016).…”

“…A weak point of the local model is the decline of ionospheric comparison, due to the lack of the M-I coupling. Tanaka et al (2021c) showed that lack of precise ionospheric closure of the FAC also prevents reproduction of the correct onset. Local models belong to such a deficient model.…”

Interpretation of the Theta Aurora Based on the Null‐Separator Structure

“…It will be shown in the following part of this paper that each element seen in Figure 1 can be reproduced even in the global simulation. The global simulation in the present paper is calculated adopting the REPPU (REProduce Plasma Universe) code (Tanaka, Ebihara, et al., 2017; Tanaka et al., 2021b). The REPPU code is a parallelized high‐resolution 3D magnetosphere‐ionosphere (M‐I) coupling simulation.…”