Some "missing" elements of constraint in substorm initiation modeling

An auroral breakup is the first indicator of substorm expansion onset. It i s thus crucial to use auroral observations prior to breakups to time variou s substorm phenomena in the magnetosphere. This paper addresses the relevan ce of differentiating the necessary and sufficient conditions that lead to the sudden initiation of an auroral substorm, and points out that any subst orm model must include an auroral component that is repeatedly observable f or every isolated substorm. It is important to realize that an auroral brea kup has a definite origin, i.e., it emanates from a preexisting auroral are . This are has shifted equatorward slowly during the growth phase of the su bstorm. The strong statistical dependence of latitude of substorm initiatio n, as well as the probability of substorm occurrence, on the southward comp onent of the interplanetary magnetic field (IMF) seems to be missing from t he current modeling of substorm initiation. It is contended that because a certain region of the magnetosphere, determined by IMF conditions, becomes unstable and susceptible to substorm expansions, it is not very meaningful to attempt to identify, regardless of the IMF time history, a particular lo cation in the magnetotail where the chain of substorm initiation processes begins. (C) 2001 Elsevier Science Ltd. All rights reserved.