Postmidnight storm-time enhancement of tens-of-keV proton flux

Y. Ebihara and M.-C. Fok
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA


We investigated the global morphology of the storm-time distribution of ring
current protons and energetic neutral atoms (ENAs) observed by the High-Energy
Neutral Atom (HENA) imager on the IMAGE satellite. The postmidnight enhancements
of the proton and ENA fluxes were in particular focused on in this study, and
the following six possible mechanisms causing the postmidnight enhancements were
tested by using a self-consistent kinetic simulation of the ring current
protons: (1) shielding electric field, (2) gap between the region 2
field-aligned current and the auroral oval, (3) strong gradient of the
ionospheric conductivity near the terminator, (4) plasma sheet density, (5)
plasma sheet temperature, and (6) local-time dependence of the plasma sheet
density. When the ring current is self-consistently coupled with the ionosphere
through the region 2 field-aligned current, the simulated postmidnight
enhancements are found to agree well with the IMAGE/HENA observations, even
though effects other than shielding fields were not included. The overall
convection strength is found to have a substantial influence on the morphology
of the distribution of the ring current protons. The MLT of the flux peak is
also shown to depend slightly on the plasma sheet density and solar activity. A
local-time dependence of the plasma sheet density can produce a pronounced
postmidnight enhancement without introducing the self-consistent electric field.
Other possible mechanisms causing the postmidnight enhancements are also
discussed in detail.

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Journal of Geophysical Research, 109, A12209, doi:10.1029/2004JA010523, 2004