Geoscience Reference
In-Depth Information
wind protons are admitted into Mercury's magnetosphere only when
B
z
is negative. In the “
mean
PD” conditions, the total amount of protons
precipitating onto the surface of Mercury is approximately equal to 4
×
10
25
s
−
1
. For comparison, the amount of protons that exchange their charge,
in 1 s, is 4
10
24
s
−
1
. The first number depends only on the H
+
model
adopted, and is similar to other results.
8
The second number depends also
on the exospheric profiles assumed. In summary, approximately 10% of the
injected protons precipitate onto the planet, 1% is neutralized due to CE
and the remaining part leaves the magnetosphere.
Some more specific issues, related to BC/MPO mission, can be outlined.
The MPO periherm is so close to the planet that H-ENAs are generated
partially above the s/c. Since ELENA FoV is nadir-pointing, CE H-ENA
imaging is better performed when the s/c is close to its apoherm, especially
in the dayside, dawn and nightside regions. On the other hand, the MPO
orbital configuration permits a very good spatial resolution, for both CE
and ion-sputtering ENA imaging. BepiColombo program also foresees an
ENA detector on board Mercury Magnetospheric Orbiter (BC/MMO). This
spacecraft will be orbiting at higher distances, and will allow a global ENA
imaging with lower spatial resolution. For this reason, it will be very useful
to have simultaneous ENA images from both instruments.
For some vantage points (in dawn and dayside regions), the instrument
is able to monitor, at the same time, proton circulation (via CE H-ENA
detection) and precipitation (via the detection of ENAs sputtered from the
surface), thus permitting the evaluation of the relative importance of such
loss processes. In the case of sputtered ENAs, the s/c motion will help to
obtain 2D images from 1D ELENA FoV, in the case of H-ENA, s/c motion
could permit to observe the same emission region from different vantage
point. The feasibility of such a deconvolution will be studied in a future
paper.
The intensity of the directional ENA signal originated from ion-
sputtering depends on both proton precipitation flux and surface properties
(composition and yield). However, our simulations show that it is possible
to discriminate between these two factors. In fact, by visually inspecting
Fig. 5, we can note that the proton flux on the surface is roughly constant
on a scale of 100 km (about two pixels); since ELENA spatial resolution
is lower (between 15 and 50 km), any small-scale spatial change in ion-
sputtering ENA signal is probably due to surface property variations. On
the contrary, temporal variability in the ENA signal should be ascribed only
to modifications in the proton circulation properties. It is worth noting that
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