Plasma Injection During Substorms

We review the observational characteristics of the injection of plasma to the middle (⩽ 8 Re) and inner regions of the earth's magnetosphere during substorms, and we discuss how those characteristics should constrain and direct the development of physical models that can account for the particle transport associated with such injections. We emphasize that the transport and energization of particles to the middle regions appear to always be associated with highly impulsive and relatively localized processes driven by inductive electric fields. The important consequences of these properties of the injection mechanisms on the character of the observed particle distributions will be discussed. We note that the localized character of the injection processes indicates that the physics of injections within the middle regions involves mechanisms that, while coupled to them, are distinct from the mechanisms presumably associated with the initiation of the substorm expansion phase deeper in the magnetotail.