Triggered Substorms

Kennel, Charles F

doi:10.1093/oso/9780195085297.003.0015

Chapter

28415 Triggered Substorms

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https://doi.org/10.1093/oso/9780195085297.003.0015

Pages

284–293

Published:

April 1996

Abstract

Do events in the solar wind “trigger” substorms, or do substorms occur independently? Is there a difference between triggered and untriggered substorms? What kinds of events trigger onsets? Expansions? Can we learn about their physics from the things that do and do not trigger them? Does the switch from expansion to recovery phase require external prompting? Even if it is not required, can the recovery be prompted anyway? If conditions in the solar wind are steady, can we still have substorms? In this chapter, we review studies that have addressed questions like these. “Pseudoexpansions” resemble the first several minutes of development of the dipolarization and westward surge, but later fail to sustain an expanding current wedge or auroral bulge (Section 15.2). They may be triggered by minor dayside reconnection events that occur before the threshold condition for the full substorm is reached. Interplanetary shocks can trigger a substorm within minutes of their arrival at earth, provided they are sufficiently strong and a growth phase is already in progress (Section 15.3). Other compressional solar wind discontinuities also can trigger substorms, but rarefactions cannot. Northward turnings of the interplanetary field can also trigger substorms after a growth phase has already started (Section 15.4). During intervals of steady southward interplanetary field, either quasisteady convection bays or quasiperiodically recurring substorms can occur (Section 15.5). An inkling of pseudoexpansion behavior has been around since before the auroral substorm existed (Elvey, 1957), and in his original paper Akasofu (1964) not only had taken them into account but suggested that they occur on other than the most equatorward arc. Later, Davis and Hallinan (1976) and Untiedt et al. (1978) pointed out that pseudoexpansions are brief activations of local small-scale auroral spirals to the ground observer. Now it appears that pseudoexpansions are attenuated versions of complete expansions in several important respects: brightening of an auroral arc, Pi 2 pulsation bursts, and enhancements of the auroral electrojet, (McPherron, 1991). In this section, we show the auroral pseudoexpansion has a counterpart in geostationary orbit, and present evidence that both may be responses to variations in the dayside reconnection rate.

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Contents

28415 Triggered Substorms

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