While a substorm involves auroral poleward expansion after initial brightening, a pseudosubstorm (pseudobreakup) subsides without progressing to poleward expansion. To understand what makes this difference, we studied near‐Earth magnetotail conditions at a pseudosubstorm onset and the subsequent substorm onset, using multipoint Time History of Events and Macroscale Interactions during Substorms spacecraft data. In the present event, near‐Earth magnetic reconnection possibly occurred before initial brightening for both pseudosubstorm and substorm. In the near‐Earth magnetotail at X ∼− 10 RE, the ion β, ion pressure, and ion and total (ion plus magnetic) pressure gradient projection along two closely located spacecraft were smaller and magnetic field lines were less stretched around the pseudosubstorm initial brightening than around the substorm initial brightening. Dipolarization did not occur for the pseudosubstorm, whereas it began just before poleward expansion for the substorm. These observations suggest that conditions of the near‐Earth magnetotail possibly affect whether the initial action develops into a full‐fledged substorm.

Pseudosubstorms (pseudobreakups) and substorms are similar phenomena. In terms of auroral morphology, however, the former are not accompanied by poleward expansion, while the latter are. To understand what causes this difference, we studied temporal and spatial development of the near-Earth magnetotail at X = −7 to −11R E around pseudosubstorm and substorm onsets, based on superposed epoch analysis of Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. We find that the earthward flow begins to increase at X = −10 to −11R E just before onset and dipolarization for both pseudosubstorms and substorms, possibly due to near-Earth magnetic reconnection or the preceding relaxation of the thin current sheet in a tailward region, but the earthward flow is slower for pseudosubstorms than for substorms. Dipolarization, together with magnetic field fluctuation, is nearly the same at X = −8R E for both cases, but it is weaker at other distances for pseudosubstorms than for substorms. This result suggests that the current disruption related to dipolarization does not expand tailward and hence auroral poleward expansion does not occur for pseudosubstorms. Furthermore, the total pressure is larger at X = −8 to −11R E for several minutes before onset for substorms than for pseudosubstorms. The total pressure gradient increases more largely after onset for substorms than for pseudosubstorms. We suggest that these differences are important factors for determining whether ballooning instability causing current disruption grows in a wide area, that is, whether the initial action develops into a substorm or subsides as a pseudosubstorm.