CURRENT UNDERSTANDING OF MAGNETIC STORMS - STORM-SUBSTORM RELATIONSHIPS

This paper attempts to summarize the current understanding of the stor m/substorm relationship by clearing up a considerable amount of contro versy and by addressing the question of how solar wind energy is depos ited into and is dissipated in the constituent elements that are criti cal to magnetospheric and ionospheric processes during magnetic storms . (1) Four mechanisms are identified and discussed as the primary caus es of enhanced electric fields in the interplanetary medium responsibl e for geomagnetic storms. It is pointed out that in reality, these fou r mechanisms, which are not mutually exclusive, but interdependent, in teract differently from event to event. Interplanetary coronal mass ej ections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagneti c storms. The other two mechanisms, i.e., HILDCAA (high-intensity, lon g-duration continuous auroral electrojet activity) and the so-called R ussell-McPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in cre ating magnetic storms has yet to be quantitatively understood. (2) A s erious controversy exists as to whether the successive occurrence of i ntense substorms plays a direct role hn the energization of ring curre nt particles or whether the enhanced electric field associated with so uthward IMF enhances the effect of substorm expansions. While most of the Dst variance during magnetic storms can be solely reproduced by ch anges in the large-scale electric field in the solar wind and the resi duals are uncorrelated with substorms, recent satellite observations o f the ring current constituents during the main phase of magnetic stor ms show the importance of ionospheric ions. This implies that ionosphe ric ions, which are associated with the frequent occurrence of intense substorms, are accelerated upward along magnetic field lines, contrib uting to the energy density of the storm-time ring current. An apparen tly new controversy regarding the relative importance of the two proce sses is thus created. It is important to identify the role of substorm occurrence in the large-scale enhancement of magnetospheric convectio n driven by solar wind electric fields. (3) Numerical schemes for pred icting geomagnetic activity indices on the basis of solar/solar wind/i nterplanetary magnetic field parameters continue to be upgraded, ensur ing reliable techniques for forecasting magnetic storms under real-tim e conditions. There is a need to evaluate the prediction capability of geomagnetic indices on the basis of physical processes that occur dur ing storm time substorms. (4) It is crucial to differentiate between s torms and nonstorm time substorms in terms of energy transfer/conversi on processes, i.e., mechanical energy from the solar wind, electromagn etic energy in the magnetotail, and again, mechanical energy of partic les in the plasma sheet, ring current, and aurora. To help answer the question of the role of substorms in energizing ring current particles , it is crucial to find efficient magnetospheric processes that heat i ons up to some minimal energies so that they can have an effect on the strength of the storm time ring current. (5) The question of whether the Dst index is an accurate and effective measure of the storm time r ing current is also controversial. In particular, it is demonstrated t hat the dipolarization effect associated with substorm expansion acts to reduce the Dst magnitude, even though the ring current may still be growing.