Intensities of energetic neutral atoms produced by high-energy tails of pickup
protons in the solar wind

Edmond C. Roelof
Johns Hopkins University/Applied Physics Laboratory, Laurel, MD 20723-6099 USA


Gloeckler (Proc.SolarWind X, 2003) has summarized the behavior of the ubiquitous
high-energy tails of pickup ions in the solar wind. The tails are characterized by
power-laws in velocity in the solar wind frame. When these ions have
charge-exchange collisions with the interstellar neutral atoms that flow through
the heliosphere, energetic neutral atoms (ENAs) will be generated. We have
examined the ENA H intensities from 10eV to 100 keV that would be viewed when
looking anti-sunward along a radial line of sight from 1 AU to the boundary of the
supersonic solar wind (taken to be 85 AU). We have considered ENAs from tail
distributions that are time-averaged, CME-dominated, and CIR-dominated. The tail
distributions have been normalized to proton spectra measured at 1 AU (ACE) and
3-5 AU (Ulysses) and extrapolated to the outer heliosphere based on theoretical
estimates of the spatial dependence of the pickup protons as well as protons
accelerated by CMEs and CIRs. The extrapolated spatial dependence of the tails is
corroborated by comparison with energetic proton measurements >25 keV throughout
the heliosphere by ACE/EPAM, Ulysses/Hi-SCALE, and Voyager/LECP. The ENA
intensities at 1AU predicted from the ubiquitous tails fall 1-2 orders of
magnitude below the 10-50 keV ENA H upper limits established with the HENA imager
on IMAGE during 2000-2002 (Roelof et al., Proc.Third IGPP Conf. Outer Heliosphere,
2004). The estimated protontail-generated ENA intensities are also orders of
magnitude lower than the 10eV-10 keV ENA H intensities calculated by Gruntman et
al. (J.Geophys.Res., 106, 1576715781, 2001). Those authors estimated the ENA
intenisities at 1AU that would be generated by solar wind thermal and pickup
protons that would be accelerated at a strong termination shock and would then
populate the subsonic flow beyond it in the heliosheath.

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Presented at the 35th COSPAR Scientific Assembly, Paris, France, July 18-25, 2004.