Theoretical modeling for ENA investigations at Earth  

Yusuke Ebihara and Mei-Ching Fok


Theoretical modeling of ENA essentially involves modeling of the 3-dimensional
distribution of ÒseedÓ ions. Solving kinetic equation of the ions trapped in
EarthÕs magnetosphere [1], we have calculated evolution of the phase space
density of the ions with energy between keV and hundreds of keV ranges. The ions
are transported by the convection electric field with its intensity depending on
solar wind and IMF conditions under the assumptions that the first second
invariants are conserved. The primary output of the simulation is the
4-dimenaional equatorial phase space density, which can be easily mapped to the
directional differential number flux at off-equator. We employed the Chamberlain
type model for the H distribution and MSIS-E90 for the O distribution. ENA
emission is usually strongest at the lowest altitude where the ambient neutral
density is the highest. The lowest altitude of the ENA calculation was set to be
1200 km because contribution from secondary products due to the stripping,
ionization and excitation collision was neglected. We are still under constraint
to make the assumption that there is no emission at altitudes less than 1200 km
and there is no secondary emission because of limited computer resources.
Therefore, we avoid focusing on the ENA emission from the near-limb region. In
spite of the assumption, some of features are found to agree well with the
IMAGE/HENA observation. A noticeable feature is the global morphology of the ENA
emission, especially the post-midnight enhancement of the ENA emission during
large storms [2]. When the magnetosphere is self-consistently coupled with the
ionosphere in terms of conservation of electric currents, our simulations show
that the peak of ENA emission is rotated toward the morning side due to the
enhanced shielding electric field. The degree of the rotation depends on the
intensity of the convection electric field, the ionospheric conductivity and the
plasma sheet density as well. ENA observations together with theoretical
modeling are shown to be powerful tools for investigating not only the global
morphology of the magnetospheric ions, but also the complicated physical
processes connecting from the solar wind to the ionosphere.  

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Presentation, 2nd Annual Asia Oceanic Geosciences Society Meeting, Singapore 
20-24 June 2005