Spreading Critical Dynamics in POLAR UVI Image Sequences as Evidence for Self-Organized Criticality in the Plasma Sheet

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. The scale-free UVI distributions due to Uritsky et al. were constructed under the assumption that the auroral bright spots are a direct image of the plasma sheet avalanches; i.e., the bright spots were treated as though they were the avalanches. If this assumption is valid, and if it is correct that the distributions imply self-organized criticality (SOC) in the plasma sheet, then the bright spots should exhibit other known properties of avalanching systems in SOC. We investigate the spreading dynamics of the auroral bright spots. We demonstrate that, on average, the bright spot evolution is consistent with the behavior known as spreading critical behavior in the general context of critical phenomena in many-body systems; it is a typical mean-field signature of SOC dynamics. We show that the so-called dynamical critical exponents controlling the spreading dynamics are consistent with the statistical exponents obtained earlier by Uritsky et al., thus providing further evidence of SOC dynamics in the plasma sheet.