Association of High‐Latitude Geomagnetic Perturbations and Pi1 and Pi2 Pulsations With the Three Steps of Auroral Onset Arc Development at Substorm Onset

Using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data, we studied the stepwise development in high‐latitude geomagnetic perturbations and Pi1 and Pi2 pulsations during substorm onsets and their association with stepwise auroral onset arc development by analyzing four substorm events. We found that the geomagnetic perturbations and pulsations which are magnetic signatures of the substorm on the ground show stepwise changes and excitation similar to the development of the auroral onset arc which is the visual manifestation of the substorm. We observed minor to small changes in magnetic perturbations and excitation of Pi2 pulsations before initial brightening (IB), and the subsequent excitation of Pi1 and the second Pi2 at or around the further enhancement of onset arc (FE). Then, a steep fall in the magnetic northward component, and the largest‐amplitude and highest‐frequency Pi1 and Pi2 pulsations appeared at or after poleward expansion (PE). The appearance of FE in all four events and its association with magnetic perturbations and pulsations suggest that FE is an important step in addition to IB and PE. The detailed analysis of the FE step using ground‐ and space‐based data may provide information on the substorm triggering mechanism, the sequence of mechanisms behind the substorm, as well as the mechanisms responsible for the excitation of Pi1 and Pi2 pulsations.