JapaneseGEMSIS-Magnetosphere
Study of substorm triggering mechanism
Substorms involve the dissipation of the energy stored in the magnetotail, causing, for example, violent auroral activities in the nightside polar regions. Determining which process in the magnetotail initially triggers a substorm expansion onset has been one of the most controversial issues in magnetospheric research for decades. To solve this issue, we are studying the dynamics of the Earth's magnetotail around substorm expansion onsets.
Pressure changes associated with dipolarization in the magnetotail
In the present study, we focused our examination on pressure changes associated with the dipolarization in the plasma sheet. We performed statistical analyses and case studies using Geotail data. Unlike previously reported results, we found that the plasma pressure does increase in association with the initial dipolarization. Principal candidate models have been proposed for the substorm triggering mechanism. Among them, the current disruption model proposes a scenario in which tailward propagating rarefaction waves are generated by a pressure decrease associated with dipolarization, leading to magnetic reconnection in the midtail at a later time. However, this rarefaction wave scenario is not consistent with our observational results.
Reference: Miyashita et al., JGR, 2010
Energy balance and transport in the near-Earth magnetotail at substorm expansion onsets
Our previous study [Miyashita et al., 2009] showed that the energy release associated with onsets is the most significant midway between the magnetic reconnection and initial dipolarization regions in the magnetotail. In the present study, we have statistically studied the substorm-associated energy balance and transport in the magnetotail using Geotail data to understand where and how the energy released in the midway region is transported and spent. We found that a large amount of energy is released in the midway region, associated with onsets, but only a part of this energy is transported to the near-Earth initial dipolarization region mainly in the form of the thermal flux and the wave Poynting flux. It is possible that the energy carried by fast earthward flows and waves from the reconnection region is not sufficient for the thermal energy increase and the outward transported energy in the initial dipolarization region. A considerably large amount of the magnetic energy comes from the lobes in the form of the Poynting flux also in the initial dipolarization region. These results are important for understanding the causal relationship between the magnetic reconnection and the dipolarization as well as their physical precesses.
Reference: Miyashita et al., JGR, 2012