Importance of Energetic Neutral Atom Emission from the Exosphere:  IMAGE/HENA

Edmond C. Roelof, Donald G. Mitchell, Pontus C. Brandt, and Robert Demajistre

A major feature in energetic neutral atom (ENA) images taken by IMAGE/HENA is the very 
bright emission from low altitudes.  This feature is most prominent when the spacecraft 
is at mid to low latitudes.  The source of this ENA emission is the charge exchange of 
energetic singly-charged ions with exospheric atoms at altitudes of ~300-400 km.  It is 
here that the densities of atomic oxygen and helium (and perhaps di-atomic nitrogen) 
overwhelm the density of the geocoronal hydrogen that is the converter of ring current 
ions into ENAs at all higher altitudes.  This is because the scale height of H at 
exospheric altitudes is >1000 km, while that of O is ~60 km.  Energetic ions with 
nearly-mirroring pitch angles will therefore produce copious quantities of ENAs that will 
escape the exosphere to be imaged from above.  That is why the exospheric ENA emission is 
seen most strongly when IMAGE is not at polar latitudes; large pitch angles cannot be 
viewed from high latitudes on ring current field lines with 4 < L < 8.  An immediate 
consequence of this imaging geometry is that the exospheric ENA emission tends to be seen 
only at magnetic longitudes ~ 180 deg. from that of the spacecraft.  The basic formulas 
for calculating exospheric ENA emission [Roelof, Adv. Space Res. 20(3), 361-366, 1997] 
are now included in computer simulations for comparison with IMAGE/HENA images of 10-100 
keV ENA hydrogen.  The very bright exospheric ENA emission cannot be spatially resolved 
by the 6 degx6 deg HENA pixels, but it can easily dominate the higher altitude emission 
from the geocorona.  A variety of examples will be presented and discussed from the 
growing library of HENA images, illustrating exospheric ENA emission over a wide range of 
geomagnetic conditions.

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