Search for ENA from the heliospheric boundary using IMAGE/HENA

E. Roelof, P. C:son Brandt, R. Demajistre, and D. Mitchell

Johns Hopkins University/Applied Physics Laboratory (edmond.roelof@jhuapl.edu)


The HENA instrument on the IMAGE spacecraft measures energetic neutral atoms 
(ENAs) using time-of-flight over the energy range 10-200 keV/nuc. Whenever there
are neither magnetospheric ENAs nor energetic ions in sufficient numbers to 
penetrate the electrostatic deflection system, HENA will respond to ENAs from the 
heliosphere.  These may either be ENAs produced within the solar wind by 
shock-accelerated ions that charge exchange with interstellar H-atoms [Roelof, 
1992], or ENA H-atoms produced by protons accelerated in the vicinity of the 
heliospheric termination shock and transported beyond it into the subsonic 
heliosheath [Gruntman et al., 2001]. During ~ 2 hr of every appropriate apogee 
pass of IMAGE, we select a region of the anti-Earthward sky approximately 60 deg 
x 60 deg and accumulate counts in 6 deg x 6 deg instrument pixels and in ~ 5 
energy channels. The energy spectrum and angular distribution accumulated during 
each apogee pass (usually containing >30 counts) are both examined for possible 
contamination. Acceptable counts are mapped into absolute intensities 
(cm^2 sr s keV)^-1 over the celestial sphere using the calibrated response 
function of the instrument. This allows counts from separate apogee passes to be 
properly concatenated. Any resulting intensity then gives an upper bound on ENAs 
from either within the heliosphere or from the termination shock. The portion of 
the celestial sphere covered so far is predominantly in the northern hemisphere. 
Preliminary results give an upper bound on the spectrum in the range 16-60 keV 
that is below the ENA intensity recently predicted for pickup protons accelerated 
at a strong heliospheric termination shock [Fahr and Lay, 2000]. However, this 
upper bound is well above the intensity we calculate for ENAs that would be 
produced by the recently reported ubiquitous high energy tails on 
long-term-averaged energetic ion spectra measured on the ACE spacecraft 
[Mewaldt et al., 2001].

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Presented at the 34th COSPAR Scientific Assembly, Houston, Texas, U.S.A.  
10-19 October 2002