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with the cusp region shifted duskward (dawnward) in the northern (south-
ern) hemisphere for positive values. The same effect is observable on the
Earth, at high- and low-altitudes, from both particles and auroral data.
The
right panels
of Fig. 3 sketch the dayside magnetosphere of the planet,
with the magnetic field lines marked as above (red = open on the dayside,
gray = open on the tail). The color coded surface represents the estimated
magnetosheath particle flux injected along the dayside open field lines, and
mapped over the corresponding area of the magnetopause (gray mesh). The
fluxes are calculated by taking into account the energy gain produced by
the superimposition of the field line dragging caused by the magnetosheath
flow, and by the field line tension (as described by Eqs. (1)-(5)). Moreover,
it was assumed that 50% of the ions are reflected back by the magnetopause
boundary, that is, a transmission factor of 0.5, which is a typical value in
the case of the Earth (e.g., Ref. 8). To have an idea of the fraction of
ions that can actually reach the surface, the fluxes shown in Fig. 3 must
be multiplied by a factor of about 0.2, which is equivalent to a loss-cone
of 35
◦
. In addition, finite gyro-radius effects, grad-
B
and
E
B
drifts are
expected to spread out, to some extent, the plasma precipitation patterns
depicted in Fig. 3, as the ions precipitate throughout the magnetosphere to
the planet surface (see e.g., Refs. 5 and 6). A big unknown in this context
is the poor knowledge of the intensity and geometry of the electric field,
which can strongly affect the displacement of the plasma away from the
injection regions. Figure 3 shows that the strongest fluxes are located near
the sub-solar region of the magnetopause, where the magnetosheath plasma
should be about 3.5 higher than the free solar wind (as expect on the base of
Spreiter
et al.
4
gasdynamic approximation). The energy distribution (not
shown) typically peaks on the equatorial side of the open field regions,
and then decreases gradually while moving toward the high latitudes,
as observed on the Earth's LLBL, cusp and mantle magnetospheric
regions.
3
×
4. Conclusions
We discussed some results from a modelization of the plasma entry on
the dayside magnetosphere of Mercury. This analysis was performed by
means of the TH93 model that we adapted
ad hoc
to reproduce the key
features of the planet. The results confirm our previous findings based on
a modified version of the Tsyganenko (T96) model,
3
but display also other
