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Mariner-10 Mercury fly-bys, most aspects of Mercury remain unknown.
Two missions to explore Mercury are currently in progress: MESSENGER
(MErcury Surface, Space ENvironment, GEochemistry, and Ranging) and
BepiColombo. MESSENGER is on its way to Mercury after a successful
launch on August 3, 2004. BepiColombo is a joint mission between ESA
and ISAS/JAXA. Two separate spacecraft, the Mercury planetary orbiter
(MPO), and the Mercury magnetospheric orbiter (MMO) are planned to
be launched in the timeframe between 2012 and 2013. The MPO will study
the surface and internal composition of the planet, and the MMO will study
the detailed plasma structure and dynamics around Mercury. The combi-
nation between MMO and MPO will provide us with significant scientific
discoveries on Mercury.
Although the plasma scientific payload of Mariner 10 was very limited,
it made a very important discovery that Mercury possesses an intrinsic
magnetic field, whose intensity was in a very intriguing range in terms of
comparative planetary magnetospheres. 1 The dominance of the dipole term
in the spherical harmonic expansion of Mercury's magnetic field suggests
that the interaction between the solar wind and Mercury's magnetosphere
should be “Earth-like”, in contrast to the cases of Mars and Venus where
the planetary magnetic fields have only local effects on the interaction. On
the other hand, because of its small size and gravity, Mercury has very
different environmental characteristics compared to the Earth. The tenu-
ous atmosphere and the possible dominance of the heavy elements supplied
from the surface 2 , 3 may provide a unique opportunity to assess the rela-
tive importance of the solar wind source and the mixing mechanisms of
magnetospheric plasma. This is di cult to do at the Earth where the pro-
ton component of the ionospheric plasma cannot be uniquely distinguished
from the solar wind. The tiny nature of Mercury's magnetosphere requires
that totally new ideas be applied to the basic concepts that have been
applied to the Earth's magnetosphere. The large gyro-radii of energetic and
thermal plasma components (relative to the magnetospheric size) suggests
that ideal MHD may no longer apply; we anticipate that the scale-coupling
between the microscopic and macroscopic plasma processes, which has been
highlighted by recent theoretical work, has a global and dominant impor-
tance in Mercury's magnetosphere. As Mariner 10 observations showed,
Mercury's magnetosphere is rich in energetic particles of the keV order and
higher. Exploration of the particle environment of Mercury is very promis-
ing since the spatial distribution and temporal evolution of charged particles
within the magnetosphere provide important information on the dynamics
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