Self-Organized Criticality: What is it? How does it relate to substorm physics?

Dr. Alex Klimas
Lab. for Extraterrestrial Physics
NASA Goddard Space Flight Center

Our recent analyses of electrojet index and Polar UVI image data have added to the evidence for self-organized criticality (SOC) in the magnetospheric dynamics. These analyses will be reviewed. The results support our earlier suggestion that this SOC component of the dynamics is centered in the plasma sheet and that it is related to the flow bursts and associated localized reconnections that have been observed there. It is necessary now to develop an interpretation of SOC in a plasma physical context. A plasma sheet model that may evolve into SOC will be discussed. 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. In general terms, the model strongly couples fluid phenomena at MHD scales with kinetic phenomena at ion scales in an idealized manner. This strong coupling between such divergent scales introduces the possibility of multi-scale behavior in the intervening range of scales, and thus to the possibility of scale-free self-organized criticality. Numerical simulations of the plasma sheet model will be presented. The model exhibits quiet loading intervals interspersed with extremely dynamic unloading intervals during which magnetic flux is both annihilated and lost through plasmoid ejection. Based on the simulation results, a possible mechanism for the transition in the plasma sheet from sporadic localized reconnection to global organized reconnection at or near substorm onset will be discussed.