MSASI
Mercury Sodium Atmospheric Spectral Imager
PI: Ichiro Yoshikawa (Univ. of Tokyo)
Co-PI: Oleg Korablev (IKI)
Co-I: Shoichi Okano (Tohoku University)
High-dispersion spectral imager for the global imaging of Sodium exosphere of Mercury
Target:
Observation of temporal variability (~min)
with high spatial resolution (40 km ~ scale height)
and high spectral resolution (R~90,000)
using Fabry-Perot Interferometer.
(MSASI can observe both Dawn-Dusk sides at perihelion.)
Mercury’s Sodium Exosphere
Why do we use Fabry-Perot interferometer?
Intensity: a few Mega Rayleigh
1A bandpass
Width of sodium emission line is several tens mA.
λ/Δλ>100,000 is necessary.
Fabry-Perot
interferometer Fringe
Fabry-Perot Etalon (BBM)
Light source: He-Ne
laser (Red)
Detector size
Radiation tolerance
CMOS detector (BBM)
Main objectives of MSASI
1. To image sodium exosphere (column density and spatial distribution) with a spatial
resolution of 1/64 RM and intensity range from 10M to 10K Rayleigh.
These are sufficient to find sodium rich spots on the surface,
if they do exist.
(e.g. radar bright spots and Caloris basin).
2. To measure local and temporal variations of the sodium exosphere (time scale:
less than a few hours)
- short-term:
Substorm
- middle:
North/South asymmetry
- long:
Seasonal variation and
places of interest (Caloris basin, polar regions, etc.)
3. To identify release mechanisms which are stimulated by incoming materials such as
solar wind ions and micro-meteoroids.
Close collaboration with MPO/Phebus
[Abundance for minor elements, High-resolution vertical distributions, etc.]
Expected Distribution and Brightness of Sodium
TAA=60 degree (High Solar Radiation Pressure)
Lower limit of MSAI
Predicted D2-line brightness based on Na
distribution models viewing from MMO orbit is
presented. Processes of photon stimulated desorption
(PSD), charged-particle sputtering (CPS), and micrometeoroid impact/vaporization (MIV) are assumed
to be operated in each panel.
Low SRP
Distribution of sodium atoms around the Mercury depending on
true anomaly angle (TAA). Sodium atoms are assumed to be
uniformly ejected upward from the surface with initial velocities
of 3 km/sec. The lifetime of the atom defined by the photoionization varies from 1.6 to 3.6 hours.
Current status of MSASI instrumentation:
●Critical parts have been verified.
●Internal Thermal problem is still open, but will be
solved.
●PDR for MSASI will be held on .November 6th .
Fabry-Perot etalon in
vibration test
Radiation tolerant sensor
EMC test@RISH
Spin-scan
& mirrorscan for
2-D image
Slit image at D2-line
Spin-scan covers the longitudinal direction.
Mirror-scan covers the latitudinal direction.
Longitudinal
At the best performance,
full disk image
with 1/64 RM resolution
in every minute.
Operation Plan
MSASI is operated
Mercury is in sight.
while
Nominal observation
(Window-1 + Window-2)
period occupies about 36%
per orbit.
• Normal mode 3.2Mbyte/orbit
• High priority mode
9.6Mbyte/orbit (e.g. True
Anomaly Angle ~ 60°.)
• Low priority mode
1.6Mbyte/orbit (e.g. True
Anomaly Angle ~ 180°.)