[1] We report first results from a spatiotemporal statistical analysis of ionospheric emissions as observed by the Ultraviolet Imager (UVI) onboard the POLAR spacecraft during 4 months of 1997 and 1998. Approximately 12,300 individual emission events near local midnight with durations exceeding the sampling time of the image sequences are investigated. The probability distributions of these events over the lifetime T, maximum area A, integrated area S, maximum power W, and integrated energy output are shown to obey distinct power law relationsover a wide range of scales. The observed behavior is consistent with the behavior of statistical-physical avalanche models near a stationary critical state. These results support the hypothesis of self-organized critical dynamics of the magnetosphere and suggest an important role for cross-scale coupling effects in the development of geomagnetic disturbances.

A nonlinear filtering method is introduced for the study of the solar wind — magnetosphere coupling and related to earlier linear techniques. The filters are derived from the magnetospheric state, a representation of the magnetospheric conditions in terms of a few global variables, here the auroral electrojet indices. The filters also couple to the input, a representation of the solar wind variables, here the rectified electric field. Filter‐based iterative prediction of the indices has been obtained for up to 20 hours. The prediction is stable with respect to perturbations in the initial magnetospheric state; these decrease exponentially at the rate of 30 min−1. The performance of the method is examined for a wide range of parameters and is superior to that of other linear and nonlinear techniques. In the magnetospheric state representation the coupling is modeled as a small number of nonlinear equations under a time‐dependent input.

The linear prediction filters computed by Bargatze et al. [1985] have resulted in a turning point in the study of solar wind‐magnetosphere coupling. The evolution of the filters with varying activity provides a clear demonstration that the coupling is nonlinear. The filters have thus brought about the end of one era of linear correlative studies and the beginning of a new era of nonlinear dynamical studies. Two separate, but complementary, approaches have emerged in these dynamical studies, analogue modeling and data‐based phase space reconstruction. The reconstruction research has evolved from the original autonomous method studies, which were not conclusive, to the more recent input‐output studies that are more appropriate for the solar wind‐driven magnetosphere and have produced more reliable results. At present it appears that the modeling and reconstruction approaches may be merged in future attempts to produce analogue models directly from the results of the input‐output data‐based methods. If this can be accomplished, it will constitute a major step forward toward the goal of a low‐dimensional analogue model of the magnetospheric dynamics derived directly from data and interpreted in terms of magnetospheric physics. These developments are reviewed in three sections: autonomous data analysis methods, analogue models, and input‐output data analysis methods. The introduction provides sufficient information to read each of these sections independently.

Abstract. We show that distinct changes in scaling parameters of the D st index time series occur as an intense magnetic storm approaches, revealing a gradual reduction in complexity. The remarkable acceleration of energy release -manifested in the increase in susceptibility -couples to the transition from anti-persistent (negative feedback) to persistent (positive feedback) behavior and indicates that the occurence of an intense magnetic storm is imminent. The main driver of the D st index, the V B South electric field component, does not reveal a similar transition to persistency prior to the storm. This indicates that while the magnetosphere is mostly driven by the solar wind the critical feature of persistency in the magnetosphere is the result of a combination of solar wind and internal magnetospheric activity rather than solar wind variations alone. Our results suggest that the development of an intense magnetic storm can be studied in terms of "intermittent criticality" that is of a more general character than the classical self-organized criticality phenomena, implying the predictability of the magnetosphere.