Multipoint in situ and ground‐based observations during auroral intensifications

Abstract

Analysis of in situ and ground‐based observations is performed to establish the precise timing of observed signatures during two successive auroral activations. The first, minor activation was interpreted as a pseudobreakup, while the second, major one was classified as a substorm. Timing of observations aboard four of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes situated in the plasma sheet in the tail‐aligned conjunction indicates initial activity at X GSM ≈ −15 R E . Magnetic field variations, tailward fast flows, and signatures in particle behavior observed by two THEMIS probes in the midtail plasma sheet suggest magnetic reconnection as the source of the first activation. The substorm onset, detected 6 min after the pseudobreakup, was found to be associated with the rapid decrease of the magnetic field strength, dipolarization, and increase of plasma density and pressure, i.e., signatures of the cross‐tail current reduction (disruption), observed in the near‐Earth plasma sheet at X GSM > −10 R E . Thus, in this case, reconnection in the midtail preceded the near‐Earth current reduction. A scenario based on a model of the near‐Earth breakup triggered by the fast earthward flow, generated by preceding reconnection, is proposed.