Stepwise tailward retreat of magnetic reconnection: THEMIS observations of an auroral substorm

Abstract

Auroral stepwise poleward expansions were clarified by investigating a multiple‐onset substorm that occurred on 27 February 2009. Five successive auroral brightenings were identified in all‐sky images, occurring at approximately 10 min intervals. The first brightening was a faint precursor. The second brightening had a wide longitude; thus, it represented the Akasofu substorm onset. Other brightenings expanded poleward; thus, they were interpreted to be auroral breakups. These breakups occurred stepwise; that is, later breakups were initiated at higher latitudes. Corresponding reconnection signatures were studied using Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations between 8 and 24 RE down the magnetotail. The Akasofu substorm onset was not accompanied by a clear reconnection signature in the tail. In contrast, the three subsequent auroral breakups occurred simultaneously (within a few minutes) with three successive fast flows at 24 RE; thus, these were interpreted to be associated with impulsive reconnection episodes. These three fast flows consisted of a tailward flow and two subsequent earthward flows. The flow reversal at the second breakup indicated that a tailward retreat of the near‐Earth reconnection site occurred during the substorm expansion phase. In addition, the earthward flow at the third breakup was consistent with the classic tailward retreat near the end of the expansion phase; therefore, the tailward retreat is likely to have occurred in a stepwise manner. We interpreted the stepwise characteristics of the tailward retreat and poleward expansion to be potentially associated by a stepwise magnetic flux pileup.

Global simulation study for the time sequence of events leading to the substorm onset

“…Since the existence of detailed ionospheric structure during the onset is an observational fact, if the corresponding structure is shown in the magnetosphere, it should certainly help to make the cause of the onset clearer. In traditional substorm theories, detailed sequence of the onset is attributed to the dynamical wandering of reconnection point in the equatorial plane [Kadokura et al, 2002;Saito et al, 2010;Ieda et al, 2016]. In this paper, we try to give another explanation applying the near-Earth dynamo associated with the convection transient.…”

“…Recent simulation studies that reproduce plasma sheet dynamics leading to the substorm onset have shown the increasing ability of global simulation for the substorm study [Wiltberger et al, 2000;Tanaka, 2000;Tanaka et al, 2010;Ge et al, 2011;El-Alaoui et al, 2016]. It is found from more recent studies that increasing the resolution of FAC calculation in the global simulation, the simulation model exerts the ability to reproduce a realistic substorm not only in the magnetosphere but also in the ionosphere [Tanaka, 2015;Ebihara and Tanaka, 2015a, 2015b, 2016. In these studies, substorm variations in the ionosphere (the preonset N-S arc, the quiet arc, the initial brightening, the WTS, etc.)…”

“…In the traditional sequence, the detailed (two-step) structure of the onset is understood by adopting a complex wandering of the reconnection site in the equatorial plane [Kadokura et al, 2002;Saito et al, 2010;Ieda et al, 2016]. The prerequisite for this estimation is the triggering of the onset by the BBF from the NENL.…”

“…Otherwise, an alternative defect occurs somewhere. For example, the two-stage development of the onset is attributed to the wandering of reconnection position in the equatorial plane [Saito et al, 2010;Ieda et al, 2016]. In such consideration, no matter how the reconnection point is far from the Earth, the ionospheric disturbance is associated with variations in the equatorial plane through field-aligned projections.…”

Simultaneous observation of auroral substorm onset in Polar satellite global images and ground-based all-sky images

“…Practically, precursor brightenings are often observed prior to the AIB (e.g., Ieda et al. 2016). It is sometimes difficult to determine whether such a brightening is the AIB or a precursor, particularly when it does not decay significantly, leading to subjectivity in the duration of the AIB.…”