Comparative study of post-midnight ENA flux enhancements during the August 2000
and April 2002 storms

Y. Ebihara, M. Fok, R.A. Wolf, M.R. Hairston, P.C. Brandt, D.G. Mitchell, 
T.J. Immel, and T.E. Moore


The high energy neutral atom (HENA) imager on-board the IMAGE satellite captured
remarkable images showing that the peak of the dominant ENA flux sometimes comes
from the post-midnight region during magnetic storms(Brandt et al., GRL, 29(20),
1962, doi: 10.1029/2002GL015160, 2002). The peak of the 39-50 keV ENA flux
occurred around 0500 MLT during the 12 August 2000 storm, while it peaked near
midnight during the April 20, 2002 storm. This means that the post-midnight
enhancement of the ENA flux does not always appear during all the magnetic
storms. Fok et al. (Space Sci. Rev., 109, 77, 2003) performed the
self-consistent ring current simulation that couples with the ionosphere, and
found that the post-midnight enhancement appears when the ionospheric convection
pattern is highly deformed by charge deposited by the strong Region 2
field-aligned current. Their calculations showed a robust reversal of the
convection velocity equatorward of the auroral oval in the post-midnight sector
due to the shielding electric field. Indeed, data from DMSP F13 shows that the
ionospheric ion flow reversal occurred at -43 MLAT (0442 MLT) and the westward
drift velocity reached around 600 m/s when IMAGE captured the post-midnight
enhancement on August 12, 2000. The reversal occurred at -50 MLAT (0430 MLT) and
the westward drift velocity reached only around 250 m/s when the midnight
enhancement was observed on April 20, 2002. This may reflect the fact that the
shielding electric field driven on August 12, 2000 is much stronger than on
April 20, 2002 and the convection pattern is highly skewed during the August
2000 storm. We performed a self-consistent ring current simulation with
realistic polar cap potential distribution from the Weimer 2001 model and a
realistic auroral conductivity model derived from the IMAGE/FUV optical data.
The result fairly accounts for the tendency of the DMSP observations and the
HENA observations, likely meaning that the shielding electric field plays an
important role in the formation of the post-midnight enhancement of the ENA
flux.

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Presentation, Fall A.G.U. Meeting, San Francisco, CA, 13-17 December 2004