Time domain techniques are used to identify a dynamical model for the evolution of the Dst index under the influence of the solar wind. Frequency domain analysis of this model is used to study spectral properties of the Dst index dynamics. These properties indicate that the dynamics of the Dst index is similar to that of a linear oscillator under the action of an external force. This dynamical system should possess two main properties. Firstly the external driving force should be a nonlinear function of the solar wind parameter VBs and secondly the eigen frequency of the oscillator should be very low in comparison to the temporal characteristics of external force variations. The main process of energy loading in such systems involves nonlinear coupling between different spectral components of the external force with a transfer of their energy to the main resonant frequency. ¿ 2001 American Geophysical Union