Geoscience Reference
In-Depth Information
Fig. 6. Sky maps of average count rates (1/s) for NPI eclipse observations in 2004 (left)
and 2005 (right), i.e., we see the average count rate that the NPI sectors have registered
when pointing in that direction. The coordinate system is ecliptic with longitudes on
x
-axes, and latitudes on
y
-axes, and the grid is 128
64 cells. Note the different color
scales. The maximum value on the colorbar is 6 in 2004, and 35 in 2005. Black denotes
no coverage.
×
this coordinate system, Mars will not have constant size and position during
an orbit and from orbit to orbit, but for the deep umbra data the change
is small (Mars can increase in apparent size by 50
◦
) and does not affect
the different signals discussed later on. Another signal is seen coming from
Mars' ram direction (the direction of the planets velocity vector) around
200
◦
longitude. We will get back to this signal in the 2005 data. Note
that there is also a signal from below the ecliptic near 0 longitude. We
also see that the coverage out of the ecliptic is sparse during 2004. The
cell size of the longitude-latitude grid was chosen smaller than the sector
FOV (128
×
64 cells). Since we have many observations, we can achieve this
superresolution. In 2005 we have more observations away from the ecliptic
plane and we directly see the very strong emissions from below the ecliptic
plane. These correspond well with the galactic plane.
The different characteristics of the individual sectors (sensitivity and
noise level as discussed in the previous section) in a way make the NPI
composed of 30 different sensors. Therefore, one has to be careful when
comparing observations from different sectors. That the strong signals seen
in the sky maps are not artifacts of the normalization and averaging of
different sectors has been checked by identifying the signals in the time serie
of individual sectors, and it is also possible to see the features discussed if
one looks at sky maps for individual sectors.
4. Analysis
The big question to try and answer is what signals are from ENAs and what
are from UV? What is neutrals and what is photons? Some approaches to
separate UV and ENA signals are as follows:
•
Compare the observed sky maps with UV sky maps.
