Geoscience Reference
In-Depth Information
•
Examine the time evolution of the signals. Stellar UV sources should be
stable in position and intensity over time.
•
Examine data from other ENA detectors to identify similar signals.
Assuming an hydrogen ENA signal in the 1 keV range, the current pos-
sibilities are the NPD on MEX and LENA
12
on IMAGE. This is a topic
for future studies.
Below we will address the first two items, but first of all: could we be seeing
something else than ENAs or UV, e.g., X-rays? Probably not, comparison
with published sky maps in other wavelengths show no similarities to the
features in Fig. 6.
Let us now compare the NPI skymaps in Fig. 6 with a UV skymap. In
Fig. 7, we show SOHO/SWAN UV sky maps.
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To the left in ecliptic and
to the right in galactic coordinates. At the top is the original data, and
at the bottom the same data smoothed by an NPI sector angular response
function Appendix A. The Milky Way is clearly visible both in the UV and
NPI skymaps. In ecliptic coordinates it is the U-shaped structure. We can
note that by comparing countrates in Fig. 6 and intensity in Fig. 7 we can
estimate the UV response of the NPI: 1 kR (uniformly over a sector) corre-
sponds approximately to 10 counts per second, assuming that the galactic
emissions are only UV. We can also compare the lower right plate in Fig. 6
with the NPI sky-map in galactic coordinates shown in Fig. 8. Most of the
strong UV signals in the galactic plane correspond well to the NPI mea-
surements. The UV source at longitude
150 is, however, an exception.
Turning now to the time evolution of the observed signals. We see in
Fig. 6 that there is a source in the ecliptic around longitude 200
◦
in 2004
and 2005. However, it has moved to a higher longitude by approximately 25
◦
from 2004 to 2005, if it is the same signal. However, this shift is inconsistent
with a UV source.
We further investigate the time evolution by examining how the ecliptic
plane signals evolve over time in Fig. 9. Both years seem to include signals
that are constant in longitude (suggesting UV) and change longitude at
the same rate as the Sun (maybe Mars related), but also other changes
are visible (possible ENAs), e.g., the emissions around longitude 200 look
different in 2004 and 2005, and do also vary over time. In Fig. 10 we show
how the galactic plane signals evolve over time. We see a clear change of
the signal at around
−
45
◦
longitude over time. This change is present even
if we only look at counts from the hemisphere away from Mars, excluding
a Mars-related source.
−
