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The second ENA sensor, the neutral particle detector (NPD), provides
measurements of the ENA flux, resolving velocity and mass (H and O) with
a coarser angular resolution. ENAs incident on a surface at a grazing angle
of 15 are reflected and cause secondary electrons used for a start signal. The
reflected ENAs hit a second surface and again produce secondary electrons
utilized for a stop signal. Time-of-flight electronics give the ENA velocity.
The pulse-height distribution of the stop signals roughly determine the ENA
mass. The NPD sensor is a new development.
ASPERA-3 provides the first ever ENA measurements in the solar wind
energy range at another planet. In this work we describe, and discuss, NPI
measurements in the shadow of Mars during the 2004 and early 2005 eclipse
seasons.
2. Background
The NPI measure the integral ENA flux (in the energy range 0.1-60 keV)
with no mass or energy resolution, but with good angular resolution. The
incoming ENAs are detected by 32 sectors in a plane for a 360 field of
view (FOV), with a 5 FOV perpendicular to the plane. Here we number
the sectors from 0 to 31. The direction between sector 0 and 31 is NPI's
x -axis, along the Mars Express z -axis that points toward the center of the
planet during nadir observations. Sectors 15 and 16 are physically blocked
and the count rates should be on the noise level, but we see higher count
rates in the data from these sectors. This is an instrumental effect that is
not yet understood, and these sectors should be disregarded if they appear
in any plot (they are not used in any of the analysis of this paper).
The time resolution is 1 s, so the raw data is 30 integer counts every
second. ASPERA-3's main unit is mounted on a scanning platform which
means that the NPI can rotate around its x -axis when the scanner is oper-
ated. Here we will only discuss observations when the scanner was not
operated. Then NPI is fix relative the spacecraft with its FOV plane per-
pendicular to MEX's y -axis (and the axis of the solar panels). Sector 8 is
looking in the general direction of MEX x -axis (and the high-gain antenna).
An illustration of the NPI geometry is shown in Fig. 1.
During different parts of Mars Express polar orbit, and during different
Martian seasons, the NPI samples different ENA populations. The NPI day-
side observations have been reported by Gunell et al. , 4 Brinkfeldt et al. , 5
and Kallio et al. 6 have investigated the decrease and increase in ENA flux at
entry and exit of the Martian shadow. Predictions by computer simulations
of the ENA generation from solar wind protons 7 and photoionized planetary
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