THE EVOLUTION FROM WEAK TO STRONG GEOMAGNETIC-ACTIVITY - AN INTERPRETATION IN TERMS OF DETERMINISTIC CHAOS

Linear prediction filtering techniques have shown that the magnetospheric response to energy transfer from the solar wind contains both driven and unloading components. Filter elements ordered with respect to geomagnetic activity have shown a peak at 20 minutes due to the driven component and a second peak at 1 hour which has been interpreted in terms of the magnetotail unloading component. The peak at 1 hour was found to increase in strength with increasing activity up to a critical activity level beyond which the peak vanished. We study these features of geomagnetic activity in terms of a nonlinear dynamical model of the magnetospheric system. We base our model on the leaky faucet analogy of plasmoid formation (Hones, 1979) and the dripping faucet experiment and model of Shaw (1984). We have constructed a mechanical analogue to the magnetosphere which models both the driven and the loading‐unloading energy release mechanisms. This model is elementary, but is dynamically complete in that it is able to represent the entire cycle of substorm growth, expansion, and recovery. The model is able to explain many of the features of the linear filter results. In particular, the model suggests that the disappearance of the 1 hour peak is due to a chaotic transition beyond which the loading‐unloading mechanism continues, but aperiodically. This work suggests that, using the tools of chaos theory, we may gain considerable insight into the stormtime behavior of the magnetosphere. Copyright 1990 by the American Geophysical Union.