Based on an analysis of auroral electrojet (AE) index data, we demonstrate that the temporal evolution of magnetospheric perturbations exhibits non-trivial power-law relations consistent with the behavior of a general class of statistical physical models in the vicinity of a stationary critical point. We show that the ensemble average dynamics of the activity bursts in the AE index is essentially scale-free and can be characterized in terms of spreading critical exponents &eegr; and Δ. The lifetime T and the size S of the bursts show strong algebraic correlation that is approximated by the dependence S~T1+&eegr;+Δ. For times shorter than 3.5 hours, we find scaling features in the AE index that are independent of the solar wind input, thus indicating the internal magnetospheric origin of the revealed effects. ¿ 2001 American Geophysical Union