Observational and numerical modeling evidence demonstrates that substorms are a global, coherent set of processes within the magnetosphere and ionosphere. This supports the view that substorms are a configurational instability of the coupled system since the entire magnetosphere changes during the expansion phase onset. It is shown that the magnetosphere progresses through a specific sequence of energy-loading and stress-developing states until the entire system collapses. This energy loading-unloading sequence is the essential basis of the Faraday Loop non-linear linear dynamics model which has been quite successful in describing the fundamental behavior of substorms without invoking detailed treatments of the internal substorm instability mechanism. Present-day MHD models also are seen to produce substorm like global instabilities despite the fact that they do not treat realistically the extremely thin current sheets that play such an essential role in the near-tail dynamics prior to substorm onset. This paper discusses three-dimensional kinetic simulations that have recently shown a variety of initial plasma kinetic instability modes which all evolve quickly to a similar, globally unstable reconnection configuration. Continuing research concerning the substorm onset location and mechanisms addresses important questions of when and exactly how the substorm expansion develops. However, the loaded magnetosphere almost always progresses rapidly to the same basic reconnection configuration irrespective of the detailed localized initiation mechanism. This is likened to the catastrophic collapse of a sand dune that has reached a highly unstable configuration: Any small local perturbation can cause a complete and large-scale collapse irrespective of the perturbation details. It is concluded that the global magnetospheric substorm problem has now largely been solved and that future work should be directed toward understanding the detailed plasma physical processes that occur during substorms.
- GEOTAIL OBSERVATIONS
- PLASMA SHEET