Upper Limits on Intensities of Energetic  1-100 keV Neutral Hydrogen Atoms Generated beyond 
the Heliospheric Termination Shock:  Measurements from IMAGE/HENA/MENA

H. J. Fahr Universitaet Bonn/IAEF, 53121 Bonn, Germany
E. C. Roelof Johns Hopkins University/Applied Physics Laboratory, Laurel MD 20723
D. G. Mitchell, Johns Hopkins University/Applied Physics Laboratory, Laurel MD 20723
H. O. Funsten Los Alamos National Laboratory, Los Alamos NM 87545
M. A. Gruntman University of Southern California, Los Angeles, CA 90089
D. J. McComas Southwest Research Institute, San Antonio TX 78228


Energetic neutral hydrogen atoms (ENAH) are generated in the vicinity of the heliospheric 
termination shock(HTS)  by charge-exchange collisions of energetic protons with the cold 
(7000 degK) hydrogen atoms of the interstellar gas.  Recent calculations of accelerated 
pickup proton intensities upstream and downstream of the HST [Fahr and Lay, Astron. 
Astrophys., 356, 327-334, 2000] show that measurements of ENAH in the energy range that 
travel inward from the HTS at ~100 AU to the IMAGE spacecraft at 1 AU can provide 
significant diagnostics of the pre-shock and post-shock plasma conditions (and thereby of 
the strength of the HTS itself).  Pre-shock ENAH dominate the predicted spectrum in the 
energy range 0.5-2 keV, while the post-shock ENAH are dominant from 5 keV until the 
spectrum begins to fall off rapidly  at 100 keV due to the decrease in the 
charge-exchange cross-section.  The NASA IMAGE spacecraft now in Earth orbit carries two 
ENA cameras that cover this energy range :  MENA (1-30 keV) and HENA (10-200 keV).  When 
these imagers view the anti-Earthward portion of the sky, their counting rates are very 
low.  Thus their measured "sky backgrounds" can place rigorous upper limits as a function 
of energy on the intensity of ENAH reaching the Earth from the HTS in that direction of 
the sky.  For instance, suppose HENA obtained  1 count per 6 deg x 6 deg pixel in the 
energy range 20-30 keV during 2 hours while IMAGE was at apogee.   This would correspond 
to an upper limit on the ENAH intensity of 0.06 /cm2ssrkeV. Count rates lower than 
one/pixel in 2 hours been observed by HENA; many anti-Earthward pixels have zero counts.   
The ENAH intensities predicted by Fahr and Lay are in the range 0.001-1.0 /cm2ssrkeV in 
this energy interval (depending upon the strength of the HST and other parameters). 
Consequently, the MENA and HENA sky backgrounds can place meaningful constraints on 
theoretical models for the acceleration of protons up to 100 keV by the HST.  These 
protons should be the "seed" population for anomalous cosmic-ray  (ACR) protons observed 
at 10-100 MeV.  Accumulated "quiet-time" upper limits of ENAH intensities from MENA and 
HENA will be reported for different directions on the celestial sphere and their 
theoretical implications discussed.

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Presented at the Fall American Geophysical Union Meeting, 
San Francisco, CA., December 15-19, 2000