Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh52a..03r&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #SH52A-03
Other
2114 Energetic Particles (7514), 2124 Heliopause And Solar Wind Termination, 2126 Heliosphere/Interstellar Medium Interactions, 2151 Neutral Particles (7837)
Scientific paper
Energetic neutral atoms observable at 1AU are produced in the inner (IHS) and outer (OHS) heliosheath, from the termination shock (TS) to the heliopause, and from the heliopause to a possible bow shock, respectively. The ENA directional intensity is determined by line-of-sight integrals of the product of the charge exchange cross-section, the relatively cold ~10000 K interstellar neutral hydrogen density, and finally the Sunward- directed unidirectional intensity of the singly-charged ion species (mainly hydrogen). The last-mentioned can be strongly affected by advection with the heliosheath plasma flow. The density of interstellar neutral hydrogen atoms is ~0.1/cc and its local enhancement in the upwind OHS (the weak hydrogen wall) has been calculated. Therefore the key functions are the energetic proton intensity and the plasma flow velocity. Some recent global modeling efforts suggest that the thickness of the IHS (and maybe even the OHS) along the solar radial through Voyager 1 may be only several tens of AU. Estimates of the present location of the TS place Voyager 1 (at 100 AU) about 10 AU into the IHS. It is therefore plausible to characterize the IHS by using the estimates of plasma flow and energetic particle intensities observed so far by Voyager 1 at least out to the heliopause. Estimates of the radial plasma flow velocities using the LECP low-energy ion anisotropies seldom exceed 80 km/s and can approach zero during episodes of Sunward motion of the termination shock. This (80 km/s) is the velocity of a 33 eV proton, so ENA intensities measured at energies below a few hundred eV will be strongly affected by the measured radial flow. These lowest energy ENAs are generated by gas- dynamical heating of solar wind protons at the TS. On the other hand, the ENA intensity, produced by more energetic heliospheric pickup protons (~1 keV) heated by their crossing of the TS (and by further stochastic acceleration in the IHS), should be relatively insensitive to such low plasma flow velocities. Gloeckler and Fisk have argued that the suprathermal tail of the heated pickup proton spectrum extends into the LECP measurement range >20 keV, where indeed we have continued to find the predicted spectral index (-1.5) up till the present time. Consequently we have sufficient information to estimate the ENA intensity from the direction of Voyager 1, at least that portion generated within the IHS, over the overlapping energy ranges (tens of eV to 6 keV) covered by the IBEX-LO and IBEX-HI ENA telescopes on the NASA IBEX spacecraft to be launched in June, 2008.
Gruntman Michael A.
Roelof Edmond C.
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