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content might be up to 2.5 times larger at aphelion than at perihelion. A
model which would not consider a variation of the surface content in Mer-
cury's surface would have predicted a global denser exosphere at perihelion
than at aphelion (up to a factor 2.3) because all processes ejecting sodium
atoms into Mercury's exosphere are in an average 2.3 more ecient at per-
ihelion than at aphelion. The only way that may compensate this effect
(and actually to invert it in order to be consistent with Killen
et al.
10
)isto
decrease such an eciency. A decrease of the available reservoir for ejection
from aphelion to perihelion is the only mechanism up to now that has been
suggested to explain this loss of eciency.
9
3.2.
Magnetosphere-exosphere relations
3.2.1.
Solar wind penetration
Observations of enhanced and localized peaks of sodium and potassium
emission in Mercury's exosphere (Fig. 1 right panel for the sodium emis-
sion) have been reported regularly soon after the first observation of sodium
in Mercury's exosphere.
4
Such peaks of emissions occur usually at high lati-
tudes and more frequently than one Mercury diurnal or sidereal cycle. These
latter properties strongly suggest a relation with solar wind penetration
Fig. 1. Left panel: simulated magnetic field lines of Mercury's magnetosphere for nom-
inal solar wind conditions and flux of proton solar wind impacting the surface. Maxima
of this flux correspond to gray colors whereas minima correspond to white colors. From
Ref. 34. Right panel: observation of high-latitude dayside peaks in Mercury's sodium
exosphere (from Ref. 5), E, N, W, and S are for East, North, West, and South of the
observers. The Sun is on the right part of the figure.
