Geoscience Reference
In-Depth Information
of Mercury's magnetosphere. Mariner 10 observations suggested that accel-
eration events reminiscent of the Earth's “substorms” occur in Mercury's
magnetosphere.
4
Detailed analyses of particle acceleration events by MMO
will place significant constraints on some of the acceleration mechanisms
discussed so far to explain particle acceleration in Earth's magnetosphere,
particularly in terms of magnetic reconnection. In order to fulfill the science
objectives, a comprehensive instrument package was proposed for plasma,
high energy particle and energetic neutral atom measurements with su
-
ciently high time resolution, wide energy range, wide dynamic range, wide
angle coverage, and high mass resolution under restricted thermal, weight,
and power consumption conditions. In this paper, we will briefly introduce
the Mercury ion analyzer (MIA), one of the low energy ion sensors.
2. Mercury Ion Analyzer
The scientific objectives of low-energy ion measurement on Mercury orbit
are to understand: (1) structure of the Mercury magnetosphere, (2) plasma
dynamics of the Mercury magnetosphere, (3) Mercury — solar wind inter-
action, (4) atmospheric abundances, structure, and generation/loss process,
and (5) solar wind between 0.3 and 0.47 AU. In order to enable the required
measurements, MIA should measure 3-dimensional (3D) distribution func-
tion of solar wind ions around Mercury (0.3-0.47 AU), and Mercury magne-
tospheric ions simultaneously. According to Mukai
et al.
,
5
the predicted pro-
ton fluxes ranges 10
6
-10
12
/
cm
2
/
s
/
str
/
keV. Accordingly, MIA has to have
very wide dynamic range of
>
10
6
. The MIA realizes this wide dynamic
range by using electrical sensitivity control and attenuation grid simulta-
neously. The energy range of MIA is 5 eV/q-30 keV/q. This energy range
will cover predicted energy distribution of solar wind ions around Mercury,
Mercury cold plasma sheet ions, and Mercury hot plasma sheet ions. The
maximum number of energy step is 64 that are necessary for distinguishing
solar wind alpha particles from solar wind protons. The angular resolu-
tion for measuring solar wind ions is 5.625
◦
, while the angular resolution
for measuring Mercury magnetospheric ions is 11.25
◦
or 22.5
◦
, depending
on the allocated capacity of the telemetry data. This angular resolution
is enough for fulfilling required measurements including non-Maxwellian
characteristics of the ion distribution function predicted in the Mercury
magnetosphere and solar wind — Mercury magnetosphere interaction.
According to our knowledge of the Earth's magnetosphere, full 3D mea-
surements of low-energy ions with high time resolution are indispensable
