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The Forefront of Space Science

Space Gas Vortices Carrying Source Particles of Aurora
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Towards next-generation of multi-satellite observation

The discovery of vortices rolling up on the flank side of the magnetosphere is a significant finding in magnetospheric physics in that we were able to identify the physical process leading to the supply of plasmas into the magnetosphere when the solar wind magnetic field is northward. The discovery is also a significant achievement in cosmic plasma physics in that we were able to prove observationally the role of vortices in plasma mixing. However, it does not mean that we were able to elucidate the microscopic mixing process that must take place inside the vortices. The CLUSTER Observations indicate that there exist regions near the vortices where the earth’s and the solar wind magnetic fields are twisted and, accordingly, have oppositely-oriented components. However, it is still unclear whether or not magnetic reconnection actually occurs in these regions. Furthermore, there is another suggestion that the mixing in vortices is caused by other mechanisms such as diffusion triggered by electromagnetic waves. Unless we can understand the essence of this microscopic process, it remains impossible for us to assess correctly the efficiency of the solar wind intrusion through vortices.

To understand the precise microscopic process in the vortices, we must not only have a grasp of their entire structure and phase of development, but also simultaneously identify, in which part of the structure and phase, what is happening. In other words, we need a set of satellites flying in formation that can resolve the macro-scale vortex structure and, at the same time, another set of formation satellites that can resolve the micro-scale structure. Further, in order to look into the mixing site, it is essential to conduct plasma observations with very high temporal resolution.

Figure 3
Figure 3. The CROSS-SCALE project

Since the researchers in Europe, who have promoted the CLUSTER project, too now recognize the necessity to observe both large and small spatial-temporal scales simultaneously, we are now, jointly with ESA, planning the “CROSS-SCALE” mission (Fig. 3), which will observe the magnetosphere by a constellation of more than 10 satellites. The main objective of the mission is to understand the essence of elementary physical processes common to cosmic plasmas, such as magnetic reconnection and particle acceleration by shock waves. After the launch of the CROSS-SCALE satellites planned for the coming ten years, we may be solving the mystery of plasma mixing in vortices as well as understand the elementary physical processes mentioned above.

(Hiroshi HASEGAWA)

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