Phase transition-like behavior of the magnetosphere during substorms

The behavior of substorms as sudden transitions of the magnetosphere is stu died using the Bargatze et al. [1985] data set of the solar wind induced el ectric field vB(s) and the auroral electrojet index AL. The data set is div ided into three subsets representing different levels of activity, and they are studied using the singular spectrum analysis. The points representing the evolution of the magnetosphere in the subspace of the eigenvectors corr esponding to the three largest eigenvalues can be approximated by two-dimen sional manifolds with a relative deviation of 10-20%. For the first two sub sets corresponding to small and medium activity levels the manifolds have a pleated structure typical of the cusp catastrophe. The dynamics of the mag netosphere near these pleated structures resembles the hysteresis phenomeno n typical of first-order phase transitions. The reconstructed manifold is s imilar to the "temperature-pressure-density" diagrams of equilibrium phase transitions, The singular spectra of vB(s), AL, and combined data have the power law dependence typical of second-order phase transitions and self-org anized criticality. The magnetosphere thus exhibits the signatures of both self-organization and self-organized criticality. It is concluded that the magnetospheric substorm is neither a pure catastrophe of the low-dimensiona l system nor a random set of avalanches of different scales described by th e simple sandpile models. The substorms behave like nonequilibrium phase tr ansitions, with features of both first- and second-order phase transitions.