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5. Discussion
The NPI has observed ENA and UV fluxes in the shadow of Mars in 2004
and 2005. The position of most of the strongest emissions in the NPI sky
maps correspond fairly well to UV sources, but there is at least one signal
that shifts position between the years, indicating that there are ENAs in
this signal and that the ENAs seem not to be related to the Mars-solar
wind interaction (they are well away from the Mars direction). In addition,
there are ENAs seen in the eclipse that emanates from the Mars-solar wind
interaction judging from the direction they come from (near Mars' limb).
There does not seem to be any obvious correlation between the NPI
observations and the neutral streams discussed by Collier et al. , 10 since
those fluxes have incoming directions of 260 -290 ecliptic longitude, and
we have not identified any possible neutral atom fluxes from these directions
in the NPI data. However, all previous observations of interplanetary ENAs
are indirect (except the ISN observations 9 ). Thus, it is dicult to rule
out that there is a connection between the NPI observations and previous
observations.
The next step in this investigation is to identify features that change over
time and examine the response of individual sectors to these signals, and
how they vary over time. Also, one should investigate if any such signal can
be identified in MEX NPD or IMAGE LENA data (this would confirm that
a signal is interplanetary since IMAGE is in Earth orbit), detectors that
are not UV sensitive. Since these detectors also have velocity resolution, we
could get more information on the ENA populations seen. A future compari-
son with measurements by the ASPERA-4 NPI sensor onboard ESA's Venus
Express mission could also confirm or rule out a possible interplanetary
ENA signal. Venus Express is scheduled to arrive at the planet in April 2006.
Acknowledgment
We thank Teemu Makinen at the Finnish Meteorological Institute (FMI),
and the SWAN team, for providing UV all-sky data.
Appendix A. NPI Angular and Energy Response
Here, we present the NPI angular response, as used in the smoothing of
the UV skymap in Fig. 7. The angular response was measured during on-
ground calibrations with an MCP bias voltage of
2500 V and is shown in
Table 3.
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