INDICATIONS OF LOW DIMENSIONALITY IN MAGNETOSPHERIC DYNAMICS

Using three separate but related approaches, we examine the question of whether the dynamic response of the magnetosphere to the solar wind input may be described by a low-order system of equations. First, we determine that the dimension of the subset (the "attractor") in the high-dimensional magnetospheric phase space associated with the westward auroral electrojet (AL) index for some of the data sets compiled by Bargatze et al. [1985] is 4.0 +/- 0.2, seemingly independent of activity level. Second, direct modeling of the magnetosphere considering the bulk properties of the tail plasma leads to a system of equations that is similar to those previously reported as a dripping faucet model; here we focus specifically on the prediction of a natural frequency in this model. Finally, we identify a peak with the predicted frequency in power spectra of AL computed for intervals with both low and high activity. Peaks at other frequencies also appear in the spectra, and such resonances would be expected for a chaotic nonlinear oscillator. Combining these approaches we conclude that at least some aspects of magnetospheric dynamics may be meaningfully modeled by low-dimensional sets of equations.