Precursor activation and substorm expansion associated with observations of a dipolarization front by Time History of Events and Macroscale Interactions during Substorms (THEMIS)

Abstract

We analyzed simultaneous Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes and ground‐based observations during a set of successive auroral activations between 0600 and 0900 UT on 27 February 2009. The first (minor) activation, interpreted as a pseudobreakup, may be related to transient reconnection on closed plasma sheet field lines. During the pseudobreakup, a plasmoid was observed in the midtail. Other minor auroral activations were associated with earthward flow bursts accompanied by magnetic field disturbance in the near‐Earth plasma sheet. The effects of these minor activations (auroral brightening and ground responses) were limited in spatial scale, and weak or local variations in the near‐Earth magnetotail were observed. The slow but continuous increase in total lobe pressure suggests continuous flux accumulation in the magnetotail. A new reconnection episode likely occurred in the magnetotail, as evidenced by fast earthward flows (600∼800 km/s) accompanied by Bz > 0 observed by P1 and P2 at ∼0738 UT. At substorm expansion, an abrupt reduction in the lobe field at P5 (which had been increasing continuously) was observed. The rapid lobe flux reduction may indicate magnetotail lobe magnetic reconnection (TLR) and evolution of the expansion phase. Substorm expansion onset, as evidenced by a global expansion of auroral intensification and strong ground Pi2 pulsations, was accompanied by a decrease in total pressure and significant dipolarization and electron injections in the near‐Earth magnetotail. We interpret these minor activations (including a pseudobreakup and other minor activations at ∼0657 and ∼0705 UT) as precursors of a substorm onset. In this scenario, closed field line reconnection results in ionospheric effects consistent with very localized auroral intensifications, including limited poleward expansion. TLR provides most of the flux and energy required for fully developed substorms.