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areas of the magnetosphere. However, finite gyro-radius effects, grad-
B
and
E
B
drifts are expected to spread out the plasma patterns as the injected
ions precipitate through the magnetosphere (e.g., Refs. 5 and 6).
×
2. Open Field Regions
In Fig. 2, we report the fraction of the Mercury's surface that is crossed
by open magnetic field lines, mapping on the dayside magnetopause, as a
function of the solar wind pressure (
P
dyn
)andoftheIMF
B
z
component.
On the
left panels
, the results we had obtained by means of the T96* model
3
are shown:
upper panel
,IMF
B
=(0
,
0
,B
z
)and
P
dyn
=20nPa;
lower panel
,
IMF
B
=(0
,
0
,
10) nT. On the
right panels
, we show the results derived
with the present model (TH93*) using the IMF geometries reported on
the legends:
B
z
-dominated (black lines) and three configurations with a
fixed
B
x
/
B
y
−
ratio along the nominal Parker's spiral direction at Mercury
(gray lines).
In the T96* case, the open area is equally distributed in both northern
and southern hemispheres. In the TH93* case, the area is completely, or
Fig. 2. Fraction of Mercury's surface that is characterized by open magnetic field lines,
mapping on the dayside magnetopause, as calculated by means of both the T96* (
left
panels
) and the TH93* (
right panels
)models.
Upper panels
: open field area as a function
of the IMF
B
z
component, for fixed solar wind pressure (
P
dyn
= 20 nPa): the black
lines mark the IMF
B
=(0
,
0
,B
z
) configurations, while the gray lines show the results
obtained with the IMF aligned along the nominal Parker's spiral direction (TH93* only,
since T96* is independent from
B
x
).
Lower panel
:openfieldareaasafunctionofthe
solar wind pressure, derived with the IMF configurations indicated in the legends.
