Geoscience Reference
In-Depth Information
circulating over the North pole, and the H-ENA signal originated by protons
that are bouncing and drifting westward. The first produce an intense H-
ENA signal in the upper part of the three H-ENA pictures. The second
population of protons reaches low altitude at the mirroring points, where
exospheric densities are higher: even if, here, H
+
flux is generally lower than
H
+
flux in the dayside, they may produce an intense ENA signal that can be
seen on the right side of the pictures. This westward circulation is generated
by
B
-grad drift; this drift prevails over the
E
B
drift if the PD is low. For
this reason, the second ENA peak is more evident for low values of PD.
These considerations apply to a generic ENA sensor; as far as it concerns
ELENA, the proper FoV is shown by the black bold grid in the center
of Fig. 3, where each square represents an instrument sector (2
◦
×
×
2
◦
).
The color-scale, in this case, gives also simulated counts, integrated over
1 min. Thanks to s/c motion it is possible to investigate this region from
different vantage points, but it is not straightforward how to deconvolve
these “images” to obtain plasma properties. In principle, the problem is
similar to that of obtaining information on a 3D plasma distribution from
a single, typical 2D ENA image, since the lack of information is the same;
however, this problem will be faced in a future study.
As far as it regards a single ELENA 1D “image”, these simulations show
that it will be possible to monitor the westward-drifting H
+
population,
by looking at the instantaneous H-ENA signal in the last three sectors.
An increase or decrease of this signal could, in principle, be correlated to
external variations (such as potential drop, in this example); moreover,
the 2
◦
×
2
◦
instrument resolution results, at the surface-closest point, in
a 200 km altitude resolution, which allows a rough reconstruction of the
vertical profile of H
+
flux. Another information that could be obtained
from ELENA data in this configuration is how many protons circulate in
the nightside, by means of those sectors that look towards the planet.
Figure 4 is similar to Fig. 3, but the vantage point here is in the
dawn region (0.1,
1.6, 0
R
M
). In this configuration, ELENA looks directly
towards the westward drifting population. Hence, from this vantage point it
is possible to monitor instantaneously the H
+
population drifting westward
(central sectors), as well as the precipitating one (right sectors) since, in
most cases (
low
and
medium
PD), all sectors have noticeable count rates.
The same considerations made about Fig. 2, concerning PD influence on H-
ENA images, apply also here. In particular, if the PD is high (right panel),
protons pass over the poles at higher altitudes, where exosphere density is
lower, and CE process is less relevant.
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