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mainly, concentrated in the southern (northern) hemisphere for
B
x
posi-
tive (negative). The results obtained with the two models are similar in the
case of fixed solar wind pressure (
P
dyn
= 20 nPa, upper panels). For
B
x
/
B
y
fixed along the Parker's spiral direction (TH93* only), the open field area
increases progressively as a function of the total IMF strength. By allowing
the interconnection with the IMF
B
x
component, the TH93* model causes
a large fraction of the planetary surface to be exposed to the direct precipi-
tation of magnetosheath plasma, even when
B
z
∼
0, as previously discussed
by other authors (e.g., Ref. 7).
3. Plasma Penetration into Mercury's Dayside
Magnetosphere
Figure 3 illustrates the results obtained by applying the stress balance
condition to the Mercury's dayside magnetosphere, approximated by the
TH93* model. Calculations are made with a reference solar wind speed
with
V
SW
= 430 km/s and density
N
SW
=76cm
−
3
. In the test runs here
presented, three different interplanetary magnetic field configurations were
used, namely: IMF
B
=(30
,
10
,
0) nT (
top
), IMF
B
=(30
,
10
,
10) nT (
mid-
dle
), and IMF
B
=(30
,
10
,
10) nT (
bottom
), with a
B
x
/
B
y
ratio close to
the one expected at Mercury's orbit (about 20
◦
with respect to the radial
direction). The
left panels
show the foot-points of the magnetic field lines
on the surface of Mercury: closed (blue dots), open on the dayside (cusp
regions, red dots) and open tailward (polar caps, gray dots). Contrary to
the T96* model, the TH93* produces patterns in agreement with the Kallio
and Janhunen
5
numerical simulations, thanks to the hemispherical asym-
metry introduced by the interconnection with the IMF
B
x
component. The
interconnection with the IMF radial component causes the magnetosphere
to be “open” also for zero, or positive,
B
z
component (Fig. 3,
top
and
mid-
dle panels
). When
B
x
is positive, the open hemisphere is the southern one,
where we can see the footprint of the local cusp. In this case, the northern
cusp becomes visible only when a significant IMF
B
z
component is applied,
but its extension is in any case smaller than the southern one. This is due to
the fact that most of the open field lines in the northern hemisphere cross
the magnetopause on the nightside side, where the magnetosheath flux is
strongly antisunward and few particles can to precipitate along the con-
vected field lines. When the sign of
B
x
is negative, the situation reverses,
and the northern hemisphere is the most exposed to the plasma injection.
The sign of the
B
y
component is responsible of the dawn-dusk asymmetry,
−
