Where and when does reconnection occur in the tail?

Abstract

We comment on the question of when and where reconnection begins in the tail and how it is related to the onset of auroral expansion. This question was addressed in a workshop dedicated to Unsolved Problems in Magnetospheric Physics held in Scarborough, UK, in September 2015. The answer is that it typically occurs first in the midtail a little beyond 20 R_e somewhat before midnight about 55 min after a southward turning of the solar wind magnetic field. It appears to be a consequence of plasma sheet thinning down to the scale of an ion gyroradius. The onset of the activation and expansion of auroral activity and accompanying magnetic signatures typically occur within 1 or 2 min of the appearance of signatures of midtail reconnection. The unanswered question is which comes first and whether reconnection is the cause of the auroral expansion. We point out that older observations clearly established that plasmoids (flux ropes) are released from near 20 R_e within a few minutes of the usual signatures of expansion onset. This is only possible if reconnection occurs close to the Earth. More detailed observations with modern spacecraft have led to the development of new explanations for the cause of the substorm expansion that appear to neglect older observations. We conclude that it is essential to carefully define the various terms used in the study of substorms and to develop quantitative methods that enable statistical studies of the various processes associated with auroral expansion onset.