Self-Organized Criticality in Earth's Magnetotail Dynamics 

A recent analysis due to Uritsky et al.  [JGR, in press] of auroral emission 
bright spots in POLAR UVI images has revealed a remarkable set of 
scale-free power-law avalanche distributions for several of the spots' 
physical characteristics.  The most straightforward and, 
at present, sole explanation for these distributions is
that localized reconnection in the plasma sheet leads to a self-organized
critical avalanching system that is scale-free over a broad range of scales.
This analysis will be reviewed and a more recent study of the spreading dynamics
of the auroral bright spots will be discussed.  It will be shown that the
spreading dynamics provides further evidence of self-organized criticality (SOC)
in the plasma sheet.  We have constructed and are studying a plasma sheet model
that may evolve into SOC.  The model contains a two-dimensional resistive MHD
component coupled to a current driven instability that generates anomalous
resistivity when and wherever the current density exceeds a critical threshold.
The model strongly couples fluid phenomena at MHD scales with kinetic phenomena
at ion scales in an idealized manner.  The dynamics of a modeled current sheet
supporting a magnetic field reversal will be discussed.  It will be shown that
magnetic field is annihilated and reconnected in an avalanching process whose
statistics are surprisingly similar to those exhibited by the Uritsky et al.
UVI distributions.