Nonlinear impact of plasma sheet density on the storm-time ring current

Y. Ebihara, M.-C. Fok, R. A. Wolf, M. F. Thomsen, T. E. Moore


We investigated the nonlinear impact of the plasma sheet density on the total
energy of the storm-time ring current by means of a numerical simulation that
self-consistently solves the kinetic equation of ring current protons and the
closure of the electric current between the magnetosphere and ionosphere.
Results of the simulation indicate that when the convection electric field is
self-consistently coupled with the ring current, the total energy of the ring
current ions trapped by the Earth's magnetic field is roughly proportional to
~Nps^1/2, where Nps is the plasma sheet density. This nonlinear response
results from the strengthened shielding electric field with increasing Nps.
The total energy is almost proportional to Nps when using an empirical
convection electric field, which is independent of the condition of the
simulated ring current. An empirical relationship between Nps and the solar
wind density was used to estimate time-dependent Nps. The result shows that
the calculated Dst* tends to overshoot the observed one when the
non-self-consistent electric field is employed. A better agreement was obtained
with the self-consistent electric field. We suggest that the nonlinear response
of the ring current to Nps is one of the mechanisms that impedes the growth of
the storm-time ring current. Another mechanism is probably the saturation of 
the polar cap potential drop for high solar wind electric field. 

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Journal of Geophysical Research, 110, A02208, doi:10.1029/2004JA010435, 2005