要旨
The near-Earth space environment is also known as the geospace, and it consists of the Earth’s magnetosphere, ionosphere, thermosphere, and nearby interplanetary space. The geospace is not as calm and peaceful as it looks. A disturbance phenomenon called a “substorm” (Figure 1) can be frequently observed in the geospace. These events can have a great impact on elements in the humanospheric environment, such as spacecraft charging, GPS positioning error due to ionospheric perturbation, and disasters in power transmission networks caused by telluric current. The substorm is a global phenomenon, whose signature can be traced in the generation of high-energy particles in the magnetosphere, in aurora brightening and enhancement of the electro-jet current in the ionosphere, and in heating of the thermosphere. Therefore, understanding the substorm could be a breakthrough in systematic understanding on the near-Earth space environment from a global perspective. However, one-point satellite observation, or even multiple-point observations (for example, THEMIS mission) only reveal local features of substorms.
To understand the physical process of the substorm, we use a global magnetohydrodynamics (MHD) simulation (Figure 2) together with THEMIS satellite observations (Figure 3). At the present stage, we are focusing on a key question?what is the triggering mechanism of the substorm? Two major models have been proposed, one is the near-Earth neutral line (NENL) model [e.g., Baker et al., 1996], and the other is the current disruption (CD) model [e.g., Lui, 1996]. These two models can partially explain the observed localized phenomena; however, it is difficult for either model to fully describe the substorm signatures, such as aurora brightening and ionospheric electro-jet currents, which have several ten-minute scales. From the viewpoint of MHD simulation [Tanaka et al., 2010], the formation of a high-pressure region (HPR) in the night-side magnetosphere is considered to be a potent candidate. By comparing results of the MHD simulation to the THEMIS observations, we investigated the formation and evolution of the HPR, and its relationship to the substorm onset. In this seminar, I will introduce the comparison results of the MHD simulation to the THEMIS observations on the plasma pressure variation, describe the formation and evolution processes of the HPR, and discuss associated relationship to the substorm expansion onset.
Figure 1. Three steps of a whole substorm, (a) growth, (b) expansion, (c) recovery phases.
Figure 2. THEMIS mission.
Figure 3. Global MHD simulation.
参考文献
1) Baker, D. N., T. I. Pulkkinen, V. Angelopoulos, W. Baumjohann, R. L. McPherron (1996), Neutral line model of substorms: Past results and present view, J. Geophys. Res., 101, 12975–13010.
2) Lui, A. T. Y. (1996), Current disruption in the Earth’s magnetosphere: Observations and models, J. Geophys. Res., 101(A6), 13067–13088, doi:10.1029/96JA00079.
3) Tanaka, T., et al. (2010), Substorm convection and current system deduced from the global simulation, J. Geophys. Res., 115, A05220, doi:10.1029/2009JA014676.
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2015年6月16日作成
