The transition to continuous unloading: Di erentiating storms and substorms in magnetospheric dynamics

We present a new paradigm for the driving of storms and substorms. Using a model that tracks the major energy components of the magnetosphere-ionosphere system, we demonstrate that both storms and substorms can be obtained from the same dynamical system for di erent levels of forcing. To say that storms are made up of substorms would be incorrect. Instead we see that periodic substorms result for weak forcing, while a steady storm ensues for strong forcing, with a continuous transition between the two. There is thus an inverse bifurcation in the state of the system with respect to driving between substorms and storms. While the forcing may initially be weak and produce substorms preceding a storm, they are, in fact, topologically di erent states of the magnetosphere (an attractor of dimension one for substorms and an attractor of dimension zero for storms). The properties of both states are discussed in detail. We are also able to characterize two types of unloading of tail ux: storage-unloading and continuous unloading. The rapid storage-unloading process is shown to be equivalent to the substorm expansion onset. The hypothesis is advanced that the state of continuous unloading represents a magnetic storm.