Geoscience Reference
In-Depth Information
Table 1.
Specifications of MIA.
Energy resolution
∆
E/E ∼
15% (FWHM, high
g
-factor mode) ∆
E/E ∼
3%
(FWHM, low
g
-factor mode)
10
◦
×
360
◦
(high
g
-factor mode) 5
◦
×
360
◦
(low
g
-factor mode)
Field of view
5
.
625
◦
×
5
.
625
◦
(min.: solar wind) 22
.
5
◦
×
22
.
5
◦
(max. Mercury
ion)
Angular resolution
Time resolution
2 s/3D distribution function
g
-factor
high
g
-factor mode
5
10
−
4
cm
2
sr keV/keV at 10
◦
×
22
.
5
◦
×
10
−
5
cm
2
sr keV/keV at 10
◦
×
22
.
5
◦
5
×
(low
g
-factor solar wind sector)
low
g
-factor mode
10
−
5
cm
2
sr keV/keV (min.) at 5
◦
×
5
.
625
◦
1
×
10
−
7
cm
2
sr keV/keV (min.) at 5
◦
×
5
.
625
◦
5
×
(low
g
-factor solar wind sector)
The estimated dynamic range of the low energy ion flux around Mercury
including both intense solar wind ions and weak magnetospheric ions is as
wide as 10
6
.
5
In order to measure both solar wind ions without saturation
and Mercury magnetospheric ions with enough counting statistics, MIA
has a function to change
g
-factor electrically. Sensitivity of the analyzer is
controlled by applying high voltage to the “top hat” part. The center of the
“top hat” part is insulated from the surrounding structures. By applying
high voltage between 0 V and +5 kV,
g
-factor can be reduced down to
1/50. In addition to the electrical
g
-factor control, attenuation grid (10%
transmission) is placed at limited sector of the exit part of the analyzer in
order to further reduce geometrical factor for solar wind ion measurement.
Table 1 summarizes the specifications of MIA.
4. Design of MIA
Figure 2 shows a 3D model of MIA. The MIA is divided into two parts,
the analyzer part with toroidal deflectors and an ion detector, and the
electronics with pre-amplifier, high-voltage power supply, circuits to control
and monitor analyzer, and other circuits including spacecraft I/F. Both
parts are integrated into one package. In order to reduce weight, some of
the mechanical parts are made of magnesium alloy.
The solar radiation around Mercury is about 10 times more intense than
around the Earth. Since solar UV is a significant source of the noise count,
the following several steps to reduce solar UV are indispensable. (1) Toroidal
