A Case Study of Near‐Earth Magnetotail Conditions at Substorm and Pseudosubstorm Onsets

Abstract

AbstractWhile 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.