Ring Current Energy Input and Decay

Kozyra, J. U. and M. W. Liemohn 

Comprehensive models, multi-satellite observations and global ENA images
have fueled a new understanding of ring current energy input and decay
processes. During intervals of strong storm-time convection, plasma sheet
particles move deep into the inner magnetosphere on open drift paths,
making one pass before being lost at the dayside magnetopause. As the
convection field weakens, these ions are trapped at low L values to form
the storm-time symmetric ring current. Major issues in ring current
build-up revolve around the relative roles of convection and induction
electric fields on ring current energy input, modifications in the
injection due to shielding and penetration electric fields, the role of
plasma sheet density and temperature variations in modulating the energy
input, the contribution of direct ionospheric injection, and the impact
of the saturation of the polar cap potential in limiting the energy input
during extreme events. There are two major ring current loss processes:
"flow-out" loss and charge-exchange collisions. During "flow-out" loss
plasma sheet dynamics are coupled to ring current evolution through the
existence of a significant percentage of ions moving along open drift
paths from the magnetotail into the inner magnetosphere. Major issues
related to ring current decay processes include: the relative importance
of charge-exchange and flow-out losses during the recovery phase of a
variety of storms, processes that cause two-phase decay, the relationship
to solar wind drivers to the decay, the details of ring current trapping
in response to a decreasing solar wind southward IMF Bz, and the impact
of high O+ content on the time-scales for loss. Other losses (Coulomb
collisions and wave-particle interactions) contribute only a small (~ few
percent) amount of the decay but are extremely important to thermal
plasma dynamics. Potential new loss processes in the literature are
associated with the interchange instability, magnetopause compressions,
and scattering of ions in stretched magnetic fields. Each of these topics
will be briefly discussed.

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To be presented at the Magnetospheric Imaging Workshop, 
Yosemite National Park, California, U.S.A., Feb. 5-8, 2002.