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on the maximum detectable energy; however, high-energy particles could
hit the MCP before the shuttering mask has been closed, and could be lost,
being mixed with photons.
The shuttering mask technique does not introduce “disturbing” detector
elements, which may affect particle trajectory or energy. This is particularly
important in this case, since the detection of ion-sputtered ENAs with
energies down to few tens of eV is crucial. Furthermore, this technique has
a really good photon rejection, important for measurement in the Hermean
environment.
5. ENA Imaging
In this section we present our results in terms of simulated ENA images, and
we discuss them in the frame of the MPO mission. The MPO has a polar
orbit, with a 400 km periherm and a 1,500 km apoherm; ELENA FoV is
always nadir-pointing, and most sectors look toward the planetary surface.
We will discuss about CE H-ENAs and ion-sputtering O-ENAs separately,
since instrumental simulations show that the related signals can be easily
distinguished.
Charge-exchange
: According to our proton circulation model, and
assuming the exospheric profiles introduced in Sec. 3, CE process occurs
mostly in the dayside and dawnside regions close to the planetary sur-
face (up to altitudes of hundreds of km). In a medium PD configura-
tion, the H-ENA production rate for unitary length reaches values up to
10 (cm
2
/
s
/
sr
/
m), close to the dayside planetary surface. This value, if inte-
grated over a long line-of-sight, could lead to high values of H-ENA flux. To
facilitate the detection of H-ENAs, the ELENA central axis is tilted, with
respect to the s/c nadir axis, by 8
◦
. In this way at least three sectors point
outward of the planet limb if the s/c is in a
15
◦
orbital arc centered at
the apoherm, while one sector points outward in a
±
90
◦
arc. The H-ENA
signal is lower in sectors looking towards the planet, because the integration
path is shorter. The s/c apoherm position will move, in longitude, during
the MPO mission, thus allowing different, optimal vantage points. Here we
examine two possible examples.
Figure 3 shows H-ENA simulated images, as seen from the nightside;
the vantage point is in (
±
1.5, 0, 0.4)
R
M
MSE. For simplicity, H-ENA fluxes
are integrated over all energies, even if ELENA can resolve particle energy.
In principle, from this vantage point it is possible to detect the H-ENA
signal generated by protons that are precipitating into the cusp regions or
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