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the
x
component should play a role in the magnetosphere solar wind
coupling,
15
,
16
because of the different orientation of the Parker's spiral at
Mercury with respect to the Earth's case. In principle, one could expect to
have more proton penetration with a
B
x
-interconnected magnetosphere.
17
Unfortunately, the present magnetic field model does not allow us to explore
such configurations. In this paper, we consider only the particular config-
uration
B
IMF
=(0
,
0
,
−
20) nT, which allows reconnection in the dayside
magnetosphere and thus the entrance of solar wind protons. Different con-
figurations of
B
IMF
have been discussed in a previous study.
7
The electric potential in any point of the space is obtained assuming that
E
B
is zero, and hence following the
B
field line as far as it reaches any
region where
V
is known. As a zero-order hypothesis, here we have assumed
that the potential at the surface of Mercury is similar to Volland
13
potential
at the ionosphere of the Earth. The absence of an ionosphere at Mercury
may make it dicult to comprehend whether and how the field-aligned
currents, if present, can close the electric circuit between the planet and
the cross-tail potential drop (PD). The argument is still widely debated,
even if some evidences of the presence of field-aligned current at Mercury
exist.
3
Using the given solar wind parameters, and taking into account the
different size of Mercury's magnetospheric cavity with respect to the Earth's
one, we can estimate a cross-polar cap PD between 10 and 100 kV.
14
To
explore the behavior of the model under different external conditions, here
we use PD = 10 kV as “reference/mean” condition,
18
,
19
and the values of
100 and 1 kV to evaluate the importance of
E
in the reconstruction of the
proton circulation.
The proton distribution in the magnetosphere of Mercury has been
obtained by using a Monte-Carlo single-particle model.
7
·
The trajectories
of about 2
10
6
test-particles have been computed using classical equation
of motion
14
(including electro-magnetic forces and gravity), and accumu-
lated on a 5-dimensional (5D) grid (space, energy
E
, and pitch angle PA).
Each trajectory begins at the magnetopause of Mercury, with random initial
position and velocity; a weight
w
is associated to the test-particle, taking
into account the proton flux where this particle “was born” (Φ
0
). The dis-
tribution of protons at the MP has been taken from Massetti;
11
in this
simulation, unlike a previous paper,
7
we have started test particle trajecto-
ries from both north and south magnetopause. The test-particle trajectory
ends at the surface of the planet or when it reaches an IMF field line not
connected with Mercury's surface. Photoionization is not considered; CE
collisions between H
+
×
and exospheric neutrals are taken into account by
