The Relation of N‐S Auroral Streamers to Auroral Expansion

Abstract

We investigate the relation of fast flows at the inner edge of the plasma sheet to the onset of auroral expansion. Recent work suggests that nearly all expansions are an instability triggered by an auroral streamer from far out in the magnetotail. We investigate an 8-hr interval of activity on 14 March 2008 using ground magnetometer and all-sky camera data to determine the onset times of six substorm expansions. We compare these times with Time History of Events and Macroscale Interactions during Substorms observations of plasma flow and magnetic field. We show that every expansion followed the arrival of a fast flow and dipolarization event at the inner edge of the plasma sheet. To relate the aurora to flows, we develop procedures for removing fixed lights, the moving Moon and its reflection, and contamination due to scattered moonlight. We scan movies of enhanced images for auroral streamers. Three onsets were tentatively associated with streamers. For two, the apparent source was very close to the growth phase arc mapping close to Earth. For one, an onset occurred in the recovery phase of an earlier substorm after a double oval had formed. For this one, the end of an N-S streamer stopped about 2°north of the breakup arc. For the remaining three expansions, no streamers were associated with the onsets. Most substorms exhibit N-S streamers in the recovery phase. These usually cannot be associated with fast flows. Either fast flows in the growth phase do not produce streamers or they make streamers that require significant image enhancement.Plain Language Summary Auroral streamers are narrow bands of auroral luminosity that originate near the poleward edge of the auroral oval. They generally propagate from north to south often with a large azimuthal component. Researchers think that these streamers impact the outer magnetosphere triggering an instability that appears as a localized expansion of aurora towards the poles. This paper describes the processing of ground auroral images to identify the existence of streamers. We compare their occurrence with ground magnetometer data signaling the onset of auroral expansion and with spacecraft data showing high-speed flows transporting magnetic flux to the inner edge of the plasma sheet. We find that all six expansions were associated with the arrival of fast flows transporting magnetic flux towards Earth. Only three of these could be associated with a streamer. Two were primarily E-W from a source close to Earth. The third was a classic N-S streamer that occurred in the recovery phase of an earlier substorm. At this time, aurora was present in two bands at the edges of the auroral oval. There was no obvious connection of the equatorward end of this streamer to the activated arc. We find that the fast flows causing substorm expansion either do not produce visible aurora or are so weak that special processing of the camera images is required to see the streamers.

Characteristics of Reconnection Sites and Fast Flow Channels in an MHD Simulation

“…It is important to emphasize that both in simulation and reality 04:00 UT is the start of the growth phase of a major substorm as we have described in McPherron et al (2020). Despite this, X‐lines fill the tail (at least 7), and 6–7 have fast earthward flows.…”

“…In our previous paper (McPherron et al, 2020), we carried out a detailed analysis of observations of the interval 0-9 UT on 14 March 2008. The question addressed was "does an MHD simulation predict 10.1029/2019JA027701 magnetotail flow channels which project to the ionosphere as auroral streamers?"…”

“…In our previous paper (McPherron et al, 2020), we carried out a detailed analysis of observations of the interval 0–9 UT on 14 March 2008. The question addressed was “does an MHD simulation predict magnetotail flow channels which project to the ionosphere as auroral streamers?” It has been argued that virtually all substorms are caused by the arrival of an auroral streamer at the inner edge of the magnetotail (Nishimura et al, 2010).…”

“…The simulation described here was driven by real solar wind conditions during the interval 02-08 UT 14 March 2008. We have extensively analyzed the observations for this interval in a companion paper (McPherron et al, 2020) where the solar wind data are summarized in Figure 3. The observation indicates there were six aurora expansions with accompanying negative bays.…”

“…In this paper, we discuss results of a similar high-resolution MHD simulation of substorm activity occurring during the interval 0-8 UT on 14 March 2008. The experimental observations during this event are described in a companion paper (McPherron et al, 2020). The original motivation for a study of this activity was to determine whether we could predict two streamer events seen in auroral data with the numerical simulation.…”