The Role of Self-Organized Criticality in the Substorm Phenomenon and its Relation to Localized Reconnection in the Plasma Sheet
Recent observations of the magnetotail plasma sheet have shown it to be a dynamic and turbulent region. Research has found strong turbulence in the plasma sheet at approximately 20 Earth's Radius tailward of Earth; the turbulence is observed at all activity levels. The existence of strong turbulence in the plasma sheet in the region associated with substorm onset might be thought difficult to reconcile with the coherence and repeatability of the substorm cycle. We review a variety of evidence that strongly suggests the magnetotail is driven, through magnetic flux transfer, into a state of "self-organized criticality" (SOC). It is an important property of physical systems that evolve into SOC that they self-organize into a unique, global dynamic state. This global state is inevitable, and repeatable. In this state, however, small-spatiotemporal-scale system fluctuations are unpredictable and can be only described statistically. This is the basis, we think, for the global coherence and repeatability of the substorm phenomenon in the turbulent plasma sheet. At, or near, substorm onset the plasma sheet can be described by a global SOC state containing significant small scale turbulence. In several recent studies, "sandpile" models were driven into SOC and then shown to reproduce various measures of substorm activity. We discuss the plasma physical foundation of these sandpile models. The evolution of simple continuum plasma sheet models into SOC-like states of many small reconnection events in the turbulent plasma sheet under the will be demonstrated. We view the substorm phenomenon as an avalanche assumption that the plasma sheet is in a SOC state.