Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh14a..04h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SH14A-04
Physics
2104 Cosmic Rays, 2114 Energetic Particles (7514), 2124 Heliopause And Solar Wind Termination, 2126 Heliosphere/Interstellar Medium Interactions, 2162 Solar Cycle Variations (7536)
Scientific paper
Recently, L. Wang, R. P. Lin, D. E. Larson and J. G. Luhmann reported the detection of 4-20 keV energetic neutral atoms (ENA), most likely H atoms, coming from the frontal lobe of the heliosphere (Nature, Vol. 454, p. 81-83, 3 July 2008). The detection was performed by the suprathermal electron (STE) sensor on the STEREO A and B spacecraft from June to October 2007. The report showed the ENA flux peaking at about 5 and 20 degrees in ecliptic longitude, respectively, on either side of the Apex, the direction of Sun's motion relative to the local interstellar medium. Each peak has a full-width at half-maximum of about 20 degrees, with the one at the lower ecliptic longitude having a peak flux about three times that of the peak at the higher ecliptic longitude. The same report also derived the spectral shape of the shock-accelerated pick-up ions in the heliosheath to be two power-law spectra with a knee at about 11 keV. Assuming a healiosheath thickness of 40 AU at Voyager 2's crossing of the termination shock, the extrapolation of this proton spectrum into higher energy meets the extrapolation of the ion spectrum measured by Voyager 1 at about 25 keV. These results, especially the double peaking in the ENA flux, appear puzzling to some theorists and modelers of the heliosphere. This talk shall review the analysis of the STE/STEREO data, and go on to discuss the interpretation of the data and its consequences concerning the thickness and shape of the heliosheath as well as the variation of the availability of shock-accelerated ions in the frontal portion of the termination shock. We also compare this most recent ENA observation and implication on the heliosheath with the earlier observation of heliospheric neutral H atoms by SOHO in the energy interval 55-88 keV and the information on the heliosheath deduced from it. If we accept the analysis by Wang et al., then the STE/STEREO observation not only confirms that ion populations in the remotest regions of our heliosphere can be sensed near 1 AU via ENA, but also prepares the way for the next stage of observation by IBEX in an even lower energy interval, 0.01-6 keV. Observation of the same regions of the outer heliosphere in ENA by STE/STEREO and IBEX over the energy interval 0.01-20 keV would provide the much-welcomed information to guide theorists and modelers working on the termination shock and the heliosheath.
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