Geoscience Reference
In-Depth Information
Eventually, some of the westward-drifting protons precipitate onto the dawn
and night surface. Finally, according to this model, the H
+
circulation is
negligible on the night side.
3. ENA Simulations
In the present model, we consider only ion-sputtering and CE processes.
Ion-sputtering is the result of the impinging of an ion of mass
m
1
upon the
planetary surface; if the impact energy (
E
i
) is high enough, a new particle
(
m
2
), mostly neutral, may be released from the surface. Its ejection energy
(
E
e
) distribution function
f
S
(
E
e
)peaksatfeweV:
20
-
22
1
1
/
2
.
E
e
+
E
b
E
i
(
m
1
+
m
2
)
2
4
m
1
m
2
E
e
(
E
e
+
E
b
)
3
f
S
(
E
e
,E
i
)
∝
−
(2)
Here,
E
b
is the surface binding energy of the atomic species extracted. The
consequent neutral differential flux is
=
YR
E
max
E
min
d
Φ
dE
e
d
Φ
H
+
dE
i
f
S
(
E
e
,E
i
)
dE
i
,
(3)
where
Y
and
R
are the yield and the surface relative abundance of the
atomic species considered, and Φ
H
+
is the ion flux. In this simulation,
E
min
and
E
max
are the same of the H
+
accumulation grid, i.e., 100 eV and 10 keV,
respectively. In this study, we have concentrated our attention to Oxygen
only, observed in the exosphere of Mercury by Mariner-10.
23
For our spec-
ulations, since the surface composition is not known, we assume a uniform
O surface abundance of 50% and a rough value for
Y
of 0.1;
24
the binding
energy considered here is 3.5 eV.
The CE process occurs when an energetic ion collides with an exospheric
neutral atom; the result is an ENA, which retains both the energy and the
direction of the incoming ion.
25
To simulate an ENA image, we start from
the ENA production rate for unitary length in any point
P
of the grid:
d
Φ
ENA
(
P
)
ds
n
H
+
w
dw
dt
,
=
(4)
Φ
ENA
(
Q
)=
d
Φ
ENA
(
P
)
ds
1
∆
Vf
α
(
v
)
2
.
(5)
|
QP
|
Equation (5) gives the total flux escaping from a cell located in
P
and
detected in
Q
,where∆
V
is the volume of the grid cell in
P
;
f
α
(
v
)isthe
