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
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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