Substorms: Externally Driven Transition to Unstable State a few Minutes Before “Onset”

Abstract. Auroral breakup at substorm onset often is observed along a new arc that forms a few minutes prior to the time traditionally identified as substorm onset. The breakup arc increases in intensity approximately monotonically prior to onset and then becomes non-linear at onset, suggesting that the magnetosphere undergoes a transition to an unstable configuration a few minutes prior to onset and that the resulting instability is responsible for the substorm expansion phase. A reduction in the strength of dayside convection is also often observed to initiate few minutes prior onset, and we suggest that this reduction causes the magnetosphere to make the transition from stability to instability. If this inference is correct, then the critical outstanding question concerning substorms is how does a convection reduction cause the transition to instability and how does this transition lead to formation of the breakup arc prior to onset and to the substorm current wedge. 1