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. The substorm is a very complex phenomenon and the triggering mechanisms behind it and their sequence of processes in the magnetosphere are still not fully understood. Geomagnetic pulsations and perturbations are considered as excellent indicators for onset timing, and Pi pulsation's arrival time on the ground may also be used to identify the location of substorm onset in the magnetotail. We studied the stepwise development in geomagnetic perturbations and Pi pulsations at substorm onsets and their association with the development of auroral onset arc which is a four-step phenomenon. We observed that the geomagnetic perturbations and pulsations which are the magnetic signature of the substorm on the ground show stepwise changes and excitation similar to the development of the auroral onset arc. Additionally, we suggest that further enhancement of auroral onset arc (FE), which did not get much attention previously, is another important and separate step besides initial brightening and poleward expansion. As the actual mechanism behind FE is still unknown, we anticipate that a detailed analysis of the FE step using ground- and space-based data may provide crucial information on the mechanism behind FE as well as the sequence of mechanisms behind the substorm. Pi pulsations and magnetic perturbations are excited in steps during substorm onset A relation exists between further enhancement of auroral onset arc and large-amplitude, high-frequency Pi pulsations We suggest that further enhancement of the auroral onset arc is an important step in auroral onset arc development