Around the time the steady convection model was being developed, Akasofu (1964) was arranging ground-based magnetometer and all-sky camera observations of the complex time dependence of nightside auroral activity into the central phenomenological conception of tune-dependent magnetospheric physics—the auroral substorm. In this chapter, we assemble a description of a substorm from modern observations. We will see that observations of electric fields, auroral X rays, cosmic noise absorption, ionospheric density, and geomagnetic micropulsations have also been successfully ordered by the substorm paradigm. At the same time, it will become clear that each individual substorm has its own irreducible individuality, and that our summary description is really a list of effects that anyone thinking about substorms ought to consider. No real substorm will look exactly like the one described here. Spacecraft observations of auroral light, precipitation, currents, and fields from polar orbit have held out high promise for unified understanding of the development of the auroral substorm around the entire oval. Without truly global auroral observations, it would be difficult to establish decisive contact with observations of large-scale convection and the associated changes in magnetospheric configuration. Despite the high promise and the many other successes of spacecraft observations of the aurora, synthetic understanding of the time development of the auroral substorm at all local times, dayside and nightside, evening and dawn, has been slow in emerging, perhaps because a stringent combination of field of view, sensitivity, space and time resolution, and multispectral capability is required. One needs images of the whole oval with sufficient space resolution to identify important arc structures (50-100 km or better) in a temporal sequence that can articulate the evolution of activity on better than the 10-minute time scale on which polar cap convection develops. Only recently has it been possible to observe auroral activity at all local tunes around the auroral oval simultaneously and follow its time development from the beginning of the growth phase until well into the expansion phase. This amplification of the original paradigm is the subject of Sections 12.2 and 12.3.
Large-scale electron acceleration by parallel electric fields during magnetic reconnection