Substorm timings and timescales: A new aspect

The magnetospheric substorm is a fundamental element of magnetospheric disturbances. After more than 40 years of intensive studies, various aspects of substorm morphology have been qualitatively established. Observations from the International Solar-Terrestrial Physics (ISTP) mission during the last decade have provided more detailed and complete pictures of substorms than before and, consequently, have provided new insights into substorm mechanisms. From the global auroral imaging it is shown that substorm onsets are locally confined; however, the effects of substorms involve a very large space at different times. Observations relying on in situ techniques can be misleading and can introduce confusion if not properly interpreted. On the other hand, remote sensing techniques such as global auroral imaging not only provide a robust means for studying substorm phenomenology but also yield relatively consistent results. This article reviews and summarizes a number of substorm studies conducted based primarily on global auroral images from NASA's Polar satellite, with a main focus on 'quantitative' substorm morphology (i.e., onset timing, locations, energy input, and substorm timescales). These studies conclude that (1) auroral breakups are the most reliable substorm indicator, whereas other commonly used onset proxies may not always be associated with substorms and are subject to a propagation delay; (2) after breakup, the expanded auroral bulge can move either westward (60%) or eastward (40%); and (3) a typical substorm expansion phase lasts ∼ 10 minutes and increases with increasing distances from the onset. A key conclusion from some recent studies seems to suggest that magnetotail reconnection, if it ever exists, is a consequence of substorm expansion onset. These findings provide constraints for substorm models and theories.