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The Forefront of Space Science

The Forefront of Space Science:New Aspects of Magnetosphere Dynamics
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Past and future in situ observation of magnetospheric plasma

The in situ observation of magnetospheric plasma by scientific satellite is to observe and measure plasma particles, electromagnetic fields, and plasma waves at the satellite’s location. Since MHD had been dominant before the 1990s, most research on ion-particle data was processedinto MHD parameters (analyzing hydrodynamic parameters such as density, flow velocity, and temperature) and the electron was treated only as a reference. Triggered by the joint U.S.-Japan GEOTAIL satellite launched in 1992, since the 1990s to date, the prevailing awareness in satellite data analysis is that ion and electron are different fluids, and that we should identify their kinetic effects. As a result, when talking about the magneto-tail reconnection, for example, we come to understand the electrical current structure at the ion-scale, ion/electron-particle acceleration, etc., around the magnetic reconnection region.We believe a factor of this success can be attributed to the use of higher-sensitivity particle-measurement instruments than before, to conduct a thorough study to investigate in detail the behavior (distribution function) of particles in velocity space, and to proceed with data analysis while referring to the results of large-scale simulation including kinetic effects of plasma.

The Mercury Magnetospheric Orbiter (MMO), an explorer on the joint Japan-Europe BepiColombo mission to explore Mercury, will investigate the magnetosphere of Mercury. The plasma-measuring instrument aboard the MMO has a phenomenal (one might say incredible) time-resolution capability, compared to that of past planetary explorers. This is because the goal of
the mission is not to observe the rough outline of Mercury's magnetosphere, but to provide data responding to the most advanced issues in space-plasma physics and to contribute to the formulation of space-plasma understanding using evidential findings from Mercury's magnetosphere that has different parameters from earth. In fact, we expect a variety of phenomena caused by Mercury's different parameters from earth. They are, for example: (1) the possibility of observing an interplanetary shock wave, caused by explosions on the solar surface, of far greater levels in Mercury's orbit than on
earth's; (2) the possibility that the turbulence effect in the boundary layer is intensified because of the smaller size of Mercury's magnetosphere; and (3) issues on the magnetic reconnection related to the fact that the tail current layer is always thin. Since the space-plasma world has shown us quite unforeseen mechanisms to date, though some were predicted, we expect that our knowledge will advance with evidential verification while enjoying the surprise of a new discovery.

We may compare in situ observation of space plasma by scientific satellites to weather observation from a boat in the Pacific Ocean. The data observed from the boat give us detailed information on the spot. Obtaining global pressure patterns from data from a single boat is difficult, however, and by the same token, with a single satellite observation we cannot predict the plasma's spatial structure. Nonetheless, we have achieved significant accomplishments by, for example, tying up with numerical calculations. The situation is getting to be more and more frustrating, however, as our desire to clarify the issue of "cross-scale coupling" increases. We are investigating in detail a small region that plays a key role in the whole,
but at the same time we want to elucidate how the dynamics on larger-scales develop in time.

Efforts toward understanding the whole picture were partially made in the International Solar-Terrestrial Physics (ISTP) program. This program coordinates international scientific satellites exploring various magnetospheric regions, to increase the simultaneous observation event of various regions and to capture the magnetospheric dynamics across regions
(Fig. 2). The Japanese GEOTAIL is one of the satellites in the ISTP. It played an important role in simultaneous observation together with European and US satellites to clarify large-scale features of magnetic reconnection at the daytime side of the magnetospheric boundary. In the ISTP, only a few satellites at most performed simultaneous observation, but nevertheless,
the effectiveness of simultaneous and multipoint observation is highly evaluated.

Figure 2. ISTP Program
Figure 2. ISTP Program

The accurate investigation of spatial structure of the key region can be achieved by flying several satellites in a compact formation to enclose the key region among the satellites. This was first achieved by European Cluster-II satellites. With four observation satellites flying in formation, while obtaining expertise on the ideal satellite separation distances and cross-referencing techniques with data from several satellites, etc., in each region of the magnetosphere, the scientific outcome is now coming to fruition. In addition, the US will launch THEMIS satellites in 2006. The five-satellite formation will take a close look at the origin of the auroral explosion, which is located in the magneto-tail.

The The on-going programs discussed above can only focus on ion-scale dynamics at best, because of limitations in time resolution in plasma observation. On the other hand, it is well known by the observation of plasma waves that interesting phenomena in the electron scale indeed develop with reaction to large-scale dynamics. A typical sample is the electrostatic solitary-wave structure generated with strong current. The strong current is locally driven by development of large-scale, dynamic phenomena, and this then induces connection with electron dynamics which in turn affects large-scale dynamics. This is a good proof indicating that the "cross-scale coupling" arises, extending over scales of several digits different. To truly understand the "cross-scale coupling", however, requires high time resolution capability to resolve the electron scale in the plasma observation.

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