The Active Magnetosphere: Substorms and Storms

This chapter gives a review of recent progress in substorms and storms, focusing on substorm onset and auroral processes. Major advances have been made in these fields since the arrival of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. We primarily focus on advances in the THEMIS era (since 2007), particularly on substorm preonset and onset processes, substorm effects onto the inner magnetosphere, and storm-time substorms. Major advances have been made on the substorm precursors by auroral imaging. Sensitive imaging of growth phase streamers and a consideration of corresponding magnetosphere-ionosphere coupling processes are found to be critical for recognizing the precursor processes. Advances in understanding of substorm onset instability processes have been obtained from bead observations. Modeling of substorm processes has reproduced key features of substorms, although notable discrepancies exist. Despite these progresses, controversies remain and they often arise from the state of the field where there is no widely accepted quantitative definition of events. A robust definition of key substorm processes is needed. Substorm injections have major impacts on inner magnetospheric processes and their contribution to the ring current may be larger than previously considered. Similarities and differences between nonstorm-time and storm-time substorms are also briefly discussed.