Transition Between Substorm Auroral Onset and Expansion Phase Activity

Abstract This study investigates the evolution of substorm onset beads into poleward expansion, surge, and streamer formation during the substorm expansion phase. Using optical observations, we infer the transition from near‐Earth instability to the formation of a near‐Earth neutral line (NENL). We found that a thin, faint arc appeared immediately poleward of the onset arc shortly after substorm onset but prior to significant poleward expansion. Beads within the longitudinal extent of this poleward arc expanded poleward more rapidly than those outside this region. The western edge of the poleward‐expanding beads formed the surge, and streamers emanated from the poleward‐expanding arc. Poleward expansion occurred stepwise, with each step associated with a re‐intensification of the poleward arc. Analysis of an event with simultaneous observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite and THEMIS all‐sky imager showed a near‐simultaneous occurrence of stepwise poleward expansion and dipolarization fronts. The lack of a significant time delay suggests that an X‐line initiates in the near‐Earth plasma sheet at approximately 11.8 R E after onset. This stepwise poleward expansion suggests a corresponding stepwise tailward retreat of the X‐line toward NENL locations observed further tailward in earlier studies.