Substorms: A global instability of the magnetosphere-ionosphere system

Observational and numerical modeling evidence demonstrates that substorms a re a global, coherent set of processes within the magnetosphere and ionosph ere. This supports the view that substorms are a configurational instabilit y of the coupled system since the entire magnetosphere changes during the e xpansion phase onset. It is shown that the magnetosphere progresses through a specific sequence of energy-loading and stress-developing states until t he entire system collapses. This energy loading-unloading sequence is the e ssential basis of the Faraday Loop non-linear linear dynamics model which h as been quite successful in describing the fundamental behavior of substorm s without invoking detailed treatments of the internal substorm instability mechanism. Present-day MHD models also are seen to produce substorm like g lobal instabilities despite the fact that they do not treat realistically t he extremely thin current sheets that play such an essential role in the ne ar-tail dynamics prior to substorm onset. This paper discusses three-dimens ional kinetic simulations that have recently shown a variety of initial pla sma kinetic instability modes which all evolve quickly to a similar, global ly unstable reconnection configuration. Continuing research concerning the substorm onset location and mechanisms addresses important questions of whe n and exactly how the substorm expansion develops. However, the loaded magn etosphere almost always progresses rapidly to the same basic reconnection c onfiguration irrespective of the detailed localized initiation mechanism. T his 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 la rgely been solved and that future work should be directed toward understand ing the detailed plasma physical processes that occur during substorms.