Substorms: A global instability of the magnetosphere‐ionosphere system

Abstract

Abstract. 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 nonlinear 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.

Quantitative magnetotail characteristics of different magnetospheric states

“…Although the increase in the tail lobe field and flaring angle is well expected from previous experience, this is the first quantitative model allowing one to compute the amount of tail lobe magnetic field (and magnetic flux) increase prepared just before the onset of explosive large-scale tail instability (e.g. Baker et al, 1999) at different external conditions. Maezawa (1975) studied experimentally the tail radius increase during the substorm growth phase.…”

“…By the name "isolated" we mean that the lobe field B L has recovered well after the previous substorm. Although multiple substorm intensifications are well known (multiple-onset substorms), we hope such an approach allowed us to select the situations when the magnetotail was close to the onset of global instability (Baker et al, 1999); this is somewhat similar to the description of the "main onset" by Hsu and McPherron (1998). Substorms inside the storm periods (registered >2 h after the storm main phase commencement and/or corresponding to SYM<−25 nT) were excluded.…”

The magnetosphere as a complex system

“…First, the magnetotail plasma sheet appears to be a dynamic and turbulent region (Borovsky et al, 1997;Ohtani et al, 1998). Second, the complex behavior of the plasma sheet, in particular in the regions associated with substorm onset, is at first sight difficult to reconcile with the predictability of the geomagnetic indices (Vassiliadis et al, 1995;Valdivia et al, 1996) and the coherence and repeatability of the substorm cycle that has led to the identification of its distinct phases (Klimas et al, 1996;Baker et al, 1999). Therefore, it is natural to ask about the significance of the complexity observed in the magnetosphere, and the relationship with the two paradigms mentioned above.…”

“…The existence of a strongly turbulent plasma sheet in the regions where substorm onset is thought to occur is, at first sight, difficult to reconcile with the global coherence and repeatability of the substorm cycle that has led to the identification of its distinct phases (Klimas et al, 1996;Baker et al, 1999), unless the self organized state is invoked. Furthermore, modern input-output methods have been quite successful in modeling and/or predicting the geomagnetic activity of the magnetosphere (Vassiliadis et al, 1995;Valdivia et al, 1996Klimas et al, 1997).…”

Forced current sheet structure, formation and evolution: application to magnetic reconnection in the magnetosphere

“…on the quasi-static evolution stage, stability of the system is presumably dependent on the current density in the nearEarth portion of the magnetotail CS, where a transition takes place from the dipole-like to the tail-like field line structure (transition region, TR). A typical feature of TR is an intense CS thinning during substorm disturbances (Sergeev et al, 1993;Baker et al, 1999). Before the substorm activation (onset) the process may be treated as slow, quasistatic (Kropotkin and Lui, 1995;Schindler 1998), but during the activation it should become much faster.…”