Origins and variation of terrestrial energetic neutral atoms outflow

G. R. Wilson and T. E. Moore


Analysis of ENA data from the LENA instrument on the IMAGE spacecraft shows that
the terrestrial atmosphere is a copious emitter of energetic neutral atoms (<300
eV) under all conditions. When activity is low, the observed emissions are
concentrated close to the Earth and are presumed to be the high-energy tail of
the warm oxygen geocorona, with energies <2 eV. When activity increases, the
relative abundance of the higher-energy neutrals increases, and the emissions
can be seen farther from the Earth. Because of the close correlation between the
postperigee ENA flux (fluxes seen 1Ð2 hours after spacecraft perigee) and Ap and
the fact that the postperigee fluxes are seen when no magnetic storm is in
progress we conclude that many of the emitted ENA come from the auroral zone and
are produced by energized ionospheric ions rather than by precipitating
energetic ions. In more spectacular events, such as the Bastille Day storm event
(14Ð16 July 2000), oxygen neutral emissions produced by precipitation of keV
ring current oxygen ions can also make an important contribution to the total
neutral emission. We conclude that diurnal variation in ENA emissions is a
winter hemisphere feature that is absent in the summer hemisphere. As activity
increases, the altitude range of the auroral oval ENA emission region increases.

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Journal of Geophysical Research, 110, A02207, doi:10.1029/2003JA010356, 2005