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Ann. Geophys., 26, 1507-1516, 2008
www.ann-geophys.net/26/1507/2008/
© European Geosciences Union 2008
Simulation of field-aligned H+ and He+ dynamics during late-stage plasmasphere refilling
J. Krall1, J. D. Huba1, and J. A. Fedder2
1Plasma Physics Div., Naval Research Lab., Code 6790, 4555 Overlook Ave., SW, Washington, D.C., 20375-5346, USA
2Icarus Research, Inc., P.O. Box 30780, Bethesda, MD 20824-0780, USA
Abstract. The refilling of the plasmasphere for 3≤L≤4
following a
model storm is simulated over long times (days) using the NRL ionosphere
code SAMI2 (Sami2 is Another Model of the Ionosphere).
Refilling is dependent on the supply of topside H+ and He+
ions with the result that H+ refilling rates decrease and He+ refilling
rates generally increase with increasing F10.7 index.
Both early- and late-stage refilling are affected by net ion flows
from the warmer to the colder geomagnetic hemisphere. When these
flows are strong, the ability of the "winter helium bulge" to increase
He+ refilling rates is suppressed.
When neutral winds are not included, refilling rates fall, typically by a
factor of two.
In most cases, late-stage He+ refilling is proportional to H+
refilling, with typical He+/H+ density ratios of 2%
for solar minimum and 10% for solar maximum. For high values
of F10.7, He+ refilling exhibits a strong diurnal variation
so that the He+/H+ density ratio varies by as much as a
factor of two during late-stage refilling.
Finally if the plasmasphere is left undisturbed,
the H+ density
can refill for as long as five weeks at L=3 and ten weeks at L=4,
with saturation densities nearly an order of magnitude
greater than typical observed densities. This confirms
that the plasmasphere at these L values rarely obtains saturation.
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