Geoscience Reference
In-Depth Information
3D angular distribution function,
v
is the direction from
P
to
Q
.Itisthen
sucient to integrate over all grid cells along the
line-of-sight
(
v
)toobtain
the ENA image at
Q
. In this simulation we include only H, H
2
,He,andO,
and we assume the exosphere composition derived by Wurz and Lammer.
24
They have used a Monte-Carlo model guided by some input parameters:
surface density (in m
−
2
,H:2
.
3
×
10
7
;H
2
:2
.
6
×
10
13
;He:6
×
10
9
;O:
4
.
4
10
10
), temperature (for thermal desorption, in K, H: 420, H
2
: 540, He:
575) and binding energy (for ion-sputtering, O: 3.4 eV). In particular, H
2
gives the most significant contribution to ENA generation. Since H
2
has not
been directly observed, its exospheric vertical profile has been intentionally
depleted (in accordance with authors' suggestions); the factor adopted here
(0.01) is intentionally very low, to avoid any possible overestimation of ENA
fluxes; the corrected surface density is hence 2
.
6
×
10
11
m
−
2
.
×
4. ELENA Sensor
The ELENA sensor, part of the SERENA experiment,
11
,
26
,
27
is a time-of-
flight (ToF) detector, with energy, mass and azimuth resolution. Particles
enter the ToF chamber through the holes of a couple of masks; one of those
masks shutters with respect to the other, driven by a piezo-electric engine
at frequencies up to 100 kHz.
The holes on the masks let the particles enter only during a defined time-
window (
tw
) (approximately one-tenth of the duty-cycle time). During the
subsequent flight toward the micro-channel plate (MCP), ions are removed
by high-voltage deflectors. The delay between the center of the opening
tw
and the Stop event on the MCP is the measured ToF. The pulse height
signal on the MCP gives also information about particle energy (
E
p
), mass
is calculated by knowing
E
p
and ToF. The position of the event gives the
particle direction; the viewing geometry is explained in Fig. 2. Presently, a
relative error on energy down to 35% could by hypothesized, and up to six
mass channels could be identified. The minimum detectable energy depends
on the MCP eciency, which strongly decreases at low energies, and it is of
the order of 20 eV (see, e.g., Fig. 3 in Ref. 28). In principle, there is no limit
Table 1.
Performances of ELENA instrument.
Aperture: 1 cm
2
30
◦
-46
◦
cross track,
1
◦
along tr.
FoV:
−
±
Angular resolution: 2
◦
×
2
◦
ToF resolution ∆
t/t
:
∼
8%
Energy range: 20 eV-5 keV
Energy resolution ∆
E/E
:
∼
35%
Nadir,-Z MPO axis+8
◦
tilt
Mass channels: up to six
Pointing:
∼
10
−
4
cm
2
sr
G. Factor (w/o MCP): 2
.
5
×
Piezo freq.: up to 100 kHz
