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Fig. 1: Earthward and tailward plasma flows (left), the deviation of the north-south
magnetic field (middle), and the normalized deviation of the total pressure (right).
The substorm onset (auroral breakup) is defined as t=0. The baseline of the deviations
is a value around 10 min before onset. The origin is the center of the Earth.
The X axis is directed toward the Earth and the Sun (sunward),
while the Y axis is directed toward dusk.
The unit of the position is the Earth's radius (RE).
Fig. 2: Summary of the results.
A State-of-the-Art Picture of Substorm-Associated Evolution of the Near-Earth Magnetotail
A substorm is a dissipation process of the energy stored in the magnetotail, causing, for example, very active auroras in the nightside polar regions. What processes in the magnetotail trigger the substorm is a major issue in magnetospheric research and has been under very intense debate for decades.
To understand the substorm triggering mechanism, Miyashita et al. (2009) statistically studied substorm-associated evolution of the near-Earth magnetotail and the inner magnetosphere, using plasma, and magnetic and electric field data from the Geotail, GOES, and Polar spacecraft. They also utilized 3787 substorm (auroral breakup) events which were selected from auroral images from the Polar or IMAGE spacecraft. It is with ten years of observations (particularly by Geotail) that the present statistical study was accomplished.
Fig. 1 shows one of the main results of the present study. Fig. 2 shows a summary of the results. The present study provides a state-of-the-art framework of magnetotail evolution associated with substorm onset. In particular, the magnetic reconnection and the dipolarization begin almost simultaneously (within 2 min); energy release is more significant between the regions of the magnetic reconnection and the initial dipolarization. The present results will be helpful as a reference guide to developing the overall picture of magnetotail evolution and studying the causal relationship between the magnetic reconnection and the dipolarization as well as detailed mechanisms of each of the two processes on the basis of multispacecraft observations.
References: Miyashita et al. (2009), J. Geophys. Res., doi:10.1029/2008JA013225.
Yukinaga Miyashita (STEL, Nagoya University)
April 2009