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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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