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Mars Express/ASPERA-3/NPI and IMAGE/LENA observations of energeticneutral atoms in Earth and Mars orbit



 
 
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Old January 16th 08, 08:29 PM posted to soc.culture.zimbabwe, sci.physics, sci.astro.ccd-imaging
ayaz[_2_]
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Default Mars Express/ASPERA-3/NPI and IMAGE/LENA observations of energeticneutral atoms in Earth and Mars orbit


hi,

this article was printed in advances in space research:

Mars Express/ASPERA-3/NPI and IMAGE/LENA observations of energetic
neutral atoms in Earth and Mars orbit

M. Holmströma, , M.R. Collierb, , , S. Barabasha, , K. Brinkfeldta, ,
T.E. Mooreb, and D. Simpsonb,
aSwedish Institute of Space Physics, Box 812, S-98 128, Kiruna, Sweden
bNASA/GSFC Code 673, Greenbelt, MD 20771, USA

The low energy neutral atom imagers on Mars Express and IMAGE have
revealed that the neutral atom populations in interplanetary space
come from a variety of sources and challenge our current understanding
of heliospheric physics. For example, both in cruise phase and at
Mars, the neutral particle instrument NPD on Mars Express observed
"unexplained neutral beams" unrelated to Mars which appear to be
either of heliospheric or solar wind origin. Likewise, the NPI
instrument on Mars Express has revealed streams of neutral atoms with
different properties than those observed by NPD. Independently, IMAGE/
LENA has reported neutral atom observations that may be interpreted as
a "secondary stream" having different characteristics and flowing from
a higher ecliptic longitude than the nominal upstream direction. Both
sets of observations do not appear to fit in easily with the neutral
atom environment from 1.0 to 1.57 AU as it is currently understood. In
this paper, we examine some highly suggestive similarities in the
IMAGE/LENA and Mars Express/ASPERA-3/NPI data to try to determine
potential origins for the observed signal.

------------------------------------------------

Conclusions
The source of the signals reported here observed by IMAGE/LENA and
Mars Express/NPI is unclear. The signals may be heliospheric in origin
with the observation direction greatly influenced by the motion of
Earth and Mars, respectively, in their orbits, as illustrated by Fig.
1. Whatever the origin, the IMAGE/LENA and Mars Express/NPI data show
some striking similarities that do suggest the two instruments are
observing the same signal.

First, the Mars Express signal is close to the Mars ram direction, as
shown in Fig. 4, while the LENA data are consistent with being close
to the Earth ram direction. This suggests the instruments are
observing particles that are not moving with the planets but are being
"rammed" by the planets' motion. Signals associated with the planets
themselves would not be expected to show this asymmetry.

Second, the two instruments observed the highest count rates from the
ram direction when Mars and Earth had about the same ecliptic
longitude, suggesting some sort of spatial structure exists at this
longitude from at least 1 AU to 1.57 AU, similar to the downstream
helium focusing cone. However, the observed signal does not appear to
be associated with the interstellar neutral helium focusing cone, as
the expected signal for the focusing cone is much smaller than the
observed "secondary stream" signal for LENA (Wurz et al., 2004) and
the expected energy of the helium in the focusing cone falls below the
Mars Express/NPI low energy cut-off.

Finally, the shape of the count rate profile in ecliptic longitude
observed by LENA is similar to that observed by NPI (see Fig. 9). This
might be expected if both spacecraft were traversing a spatial profile
organized by longitude.

Thus, like many other data sets related to heliospheric neutral atoms,
these two neutral atom data sets do not exhibit the expected symmetry
around the 74°/254° axis. Like most of the other data sets, these
appear shifted toward higher ecliptic longitudes, suggesting some sort
of heliospheric asymmetry. Indeed, Lallement et al. (2005), using SOHO/
SWAN data have concluded that the heliosphere is asymmetric with the
heliopause closest to the Sun at higher ecliptic longitudes than the
nominal upstream direction, perhaps as a result of a tilted
interstellar magnetic field (e.g. Ratkiewicz et al., 1998). The
Voyager 1 LECP data also support the notion of a heliosphere
substantially tilted toward higher ecliptic longitudes than the
nominal upstream direction (Decker et al., 2005). Frisch (in press)
has shown that small interstellar grains captured in interstellar
magnetic fields show a maximum polarization direction offset in
ecliptic longitude about 35° higher than the upwind direction, about
the same magnitude of offset observed in the neutral atom observations
reported here. She interprets these data as evidence for an
interstellar magnetic field forming an angle of about 75° with the
nominal upstream direction. In short, the evidence does seem to
suggest that the symmetry axis of the heliosphere is not along the 74°/
254° axis but rather at higher ecliptic longitudes, closer to the
Galactic center direction or the apex of the Sun's way.

The field of low energy neutral atom imaging is very young and still
maturing, relative to their charged particle cousins. The initial two
low energy neutral atom imagers, the Swedish Astrid/PIPPI (e.g. Brandt
et al., 2002) and IMAGE/LENA (Moore et al., 2000) were both launched
within about the past decade, so the field, along with an
understanding of the experimental techniques, is still maturing and,
as discussed in the paper, the data presented here do not appear to
suffer instrumental effects such as UV sources or direct solar wind
charged particle leakage.

Luckily, there are exciting current and future missions which will
have neutral atom imaging capability, potentially contributing to
answering or even resolving some of the questions that the LENA and
Mars Express observations have raised: The BepiColombo mission to the
Planet Mercury will include low energy neutral atom instrumentation
and Solar Orbiter may have a dedicated neutral solar wind instrument
(Hsieh and Orsini, private communication). The Mercury Planetary
Orbiter is anticipated to have a neutral atom imager. The Mercury
Magnetospheric Orbiter will carry a low energy neutral atom sensor
called ENA. The Chandraayan-1 mission to the Moon will carry a copy of
the ENA sensor. Finally, the Venus Express mission carries ASPERA-4
which is a copy of the Mars Express instrument with both the NPI and
NPD sensors on it. Thus, we will eventually have data sets from
similar instruments outside of 1 AU, at 1 AU, and inside of 1 AU for
comparison.



 




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