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of Mercury's magnetosphere (Fig. 1 left panel) and subsequent sputtering
of the surface by energetic incident solar protons leading to ejection of a
significant number of volatiles such that they can be seen from the Earth
ground-based observatories.
5
Solar wind sputtering is in particular an ener-
getic process of ejection and is therefore, able to eject refractory species
into Mercury's exosphere. Therefore, such refractory species, like calcium,
should be found preferentially above the surface submitted to solar wind
sputtering.
5
,
17
As a matter of fact, the first observations of the calcium
in Mercury's exosphere reported brighter emissions at high latitudes than
at the equator.
17
However, it has been recently suggested after analyzing
4 years of observations, that the origin of the observed exospheric calcium
atoms is preferentially formed by micrometeoroid ejection as molecule and
subsequent photodissociation.
33
3.2.2.
Solar event encounter
The encounter of a solar event with Mercury could lead to a significant
enhancement of the incident dynamic pressure, such that the dayside mag-
netosphere is no longer able to e
ciently shield Mercury's surface from the
incident solar wind and energetic particles.
34
,
35
Potter
et al.
36
suggested
a relationship between the observation of a global and rapid increase of
the total content of Mercury's sodium exosphere and the occurrence of
Mercury-directed Coronal Mass Ejections observed by the SOHO space-
craft coronagraphs. Leblanc
et al.
37
used a sample energetic particle event
observed at Earth to estimate some of the effects of such an event on the sur-
face sputtering contribution to Mercury's sodium exosphere. These authors
concluded that fluxes several orders of magnitude larger would be required
to produce Potter
et al.
's
36
inferred sodium enhancements. Such events are
fully within the range of variation of known solar energetic particle events.
3.2.3.
Magnetospheric recycling
Delcourt
et al.
26
found that planetary Na
+
ions may convect to the night-
side, be accelerated and then hit the surface in a non-uniform way.
29
,
38
This
feature is confirmed by hybrid simulations of ion circulation at Mercury.
34
Leblanc
et al.
28
concluded that less than 15% of the photoion reimpact
Mercury's surface. These authors using typical solar wind conditions found
that enhancement in the Na emissions could be correlated to these bands
of re-implantation. They also suggested that any change in the solar wind
