Geoscience Reference
In-Depth Information
OBSERVATIONS IN THE SHADOW OF MARS BY THE
NEUTRAL PARTICLE IMAGER
M. HOLMSTR OM
∗,†,‡
,K.BRINKFELDT
∗
, S. BARABASH
∗
and R. LUNDIN
∗
∗
Swedish Institute of Space Physics, P.O. Box 812, SE-98128 Kiruna, Sweden
†
Currently at NASA Goddard Space Flight Center
Code 612.2, Greenbelt, MD 20771, USA
‡
matsh@irf.se
We present observations of energetic neutral atoms (ENAs) in the shadow of
Mars by the neutral particle imager (NPI), part of the ASPERA-3 experiment
on-board Mars Express. The observations are well into the umbra, where the
count rates are low, and contamination by UV light minimal. We present statis-
tics over all available observations, from the 2004 and the early 2005 eclipse
seasons. We investigate skymaps of the observations, and their time depen-
dence, and try infer the different origins of the observed fluxes: instrumental
effects, ENAs from Mars and its interaction with the solar wind, and ENAs of
possibly heliospheric origin. We also study the time evolution of the observed
signals. It is found that most of the observed emissions are consistent with
UV, but some are not, suggesting ENA fluxes — Mars related and non-Mars
related. The NPI measure the integral ENA flux (0.1-60 keV) with no mass
or energy resolution, but with high angular resolution (5
11
◦
). The sensor is
×
also sensitive to UV light.
1. Introduction
The Neutral Particle Imager (NPI) is part of the ASPERA-3 experiment
1
on-board the European Space Agency's Mars Express mission (MEX) that
was launched on June 2, 2003, and entered orbit around the planet early
on Christmas morning 2003.
The ASPERA-3 instrument is comprised of four sensors; two ENA sen-
sors, an electron spectrometer and an ion mass spectrometer.
The NPI provides measurements of the integral energetic neutral atom
(ENA) flux with no mass or energy resolution but high-angular resolution.
The sensor utilizes reflection and ion sputtering from a graphite surface to
detect ENAs and suppress the UV background. The NPI is a replica of the
ENA sensor used in the ASPERA-C experiment on the Mars-96 mission
2
and was successfully flown on the Swedish microsatellite Astrid,
3
launched
in 1995. The UV suppression is not perfect, and we will later discuss how
to separate ENAs from UV in the data.
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