FG12. Substorm Expansion Onset: The First 10 Minutes

2008 GEM Substorm Expansion Onset: The First 10 Minutes[1]

Conveners: Vassilis Angelopoulos <vassilis @ucla.edu>, Shin Ohtani <Shin.Ohtani @ jhuapl.edu> and Kazuo Shiokawa <shiokawa@ stelab.nagoya-u.ac.jp>

This is the first year for our new focus group, "Substorm Expansion Onset: The First 10 Minutes". We had 3 breakout sessions for three topical areas, that is, substorm onset timing, breakup aurora, and mapping. The following is a brief summary of each session.

For the timing session the primary question we discussed was:

"What is the time sequence of onset-related phenomena observed in space and on the ground, and what are the implications for substorm onset processes?"

The substorm trigger in the magnetotail is generally addressed in terms of two different ideas, that is, outside-in and inside-out models. The outside-in model proposes that the near-Earth neutral line (NENL) is formed before substorm onset, and that the braking of the ejected fast earthward flow causes the reduction of tail current in the near-Earth region. In contrast, the inside-out model proposes that the tail current reduction is the very initial process of substorm trigger, and the model considers the formation of a NENL as its consequence. The distinction of the two models is also one of the primary objectives of the THEMIS mission. Dave Sibeck gave a summary review of these two substorm models explaining how this issue can be addressed with THEMIS satellite and ground observations as well as with other existing data sets. For this session we pre-selected two substorm events, one on January 29, 2008 (07-09 UT) and another on March 1, 2008 (01-05 UT). For both events the THEMIS satellites were in major conjunction in the magnetotail allowing us to address radial propagation of onset signatures with their footpoints well covered by networks of ground observations, which provide unambiguous onset timing; those two events may be browsed at http://www.igpp.ucla.edu/public/THEMIS/SCI/events/. Stephen Mende presented the January 29 event on behalf of Tony Lui. For this event THEMIS satellite observation indicates that the main onset took place in the near-Earth region, between X = -8 and -11 RE. Since there was no fast flow detected before the onset and also because ground auroral images show no disturbance with arcs poleward of the onset arc, they concluded that this onset can be explained in terms of the inside-out model. There was another substorm period later on the same day. Benoit Lavraud (on behalf of Christian Jacquey) discussed the propagation of substorm-related signatures for three events during this later period. The preliminary result suggests the near-Earth initiation of a substorm followed by the tailward propagation of an active region (NENL or current disruption). Vassilis Angelopoulos presented the March 1 on behalf of Andrei Runov. In this event, particle acceleration, FACs, and plasma bulk flow were detected at X = -15 ~ -17 RE before the ground onset, while the tail current continued to build up in the near-Earth region. Dipolarization/current disruption was observed later in association with the major onset. Thus they concluded that the sequence is consistent with the outside-in model. The initiation of reconnection prior to a substorm onset was also reported for the February 28, 2008 event by Jiang Liu. As for the propagation of fast plasma flow, Tung-Shin Hsu presented a superposed epoch analysis based on the THEMIS data set. The result shows that the fast flow is decelerated as it approaches Earth and accordingly, magnetic flux is piled up in the near-Earth region. In addition, onset-related wave signatures were discussed for THEMIS events by Joe Baker (Pi2 observed by SuperDARN) and Peter Chi (Pi2 and magnetoseismology).

In the "breakup aurora" session, we addressed the following three questions:

1) How does aurora evolve around substorm onsets?

2) Is the onset arc formation an outcome of the M-I coupling, or is it a manifestation of a tail process?

3) Is the breakup arc Alfvenic or inverted-V? For the former, how is the associated process related to the formation of the substorm wedge system?

Stephen Mende made an overall review of recent observations of high-time resolution ground all-sky imaging. The auroral emission starts to increase gradually 1-2 min before the sharp enhancement. The structuring of aurora arcs and formation of a new arc occur during this 2-min interval. He also presented that auroral breakup occurs in the central region of the pre-existing proton precipitation. Eric Donovan also presented based on recent ground observations the sequence of auroral breakup, including auroral fading prior to the onset. He also pointed out that arcs poleward of the breakup arc are not disturbed before onset. Mark Lessard discussed PiB waves in the context of Alfven aurora. Mike Henderson addressed substorm dynamics with Polar auroral image data. Bob Lysak reviewed and discussed the role of Alfven wave in auroral breakup focusing on its temporal and spatial scales. An Alfven wave is trapped between the ionosphere and the peak of the altitudinal peak of the Alfven velocity, which creates Alfven resonator with resonant periods of 1-10 s. This is the typical period range of waves commonly observed on the ground. He also showed that in the presence of perpendicular density gradient, the phases of Alfven waves are mixed creating small-scale structures, which may explain the scale of the onset arc. Yan Song addressed the role of Alfven waves in substorm processes with an emphasis of the break down of the frozen-in condition.

The target question of the "mapping" session is:

How does the presence and evolution of pre-onset and expansion-phase onset current systems affect the link between auroral and plasma sheet locations and processes?

Three possible approaches are discussed, that is, physical mapping, field-line modeling, and phenomenological mapping. Joachim Birn theoretically addressed whether the thin current sheet in the tail can be detected at the ionosphere. He suggested that in a thin current sheet, in which a current is carried by the ExB drift of electrons, the magnetic field lines are dragged dawnward along with electron motion. Thus the rapid thinning causes an Alfvenic pulse, which propagates toward the ionosphere and therefore may be detected at the ionosphere. Misha Sitnov presented the update of a dynamical field-line model, which he has been developing with Kolya Tsyganenko. Their current model follows the sequence of magnetospheric storms, and its extension to substorms is the target of their future efforts. Larry Lyons and Shasha Zou addressed the mapping of auroral breakup using measurements of various quantities as references. They reported that auroral breakup takes place at the center of convection shear (Harang discontinuity) and that the breakup arc is located just poleward of SAPS, from which they inferred that the substorm onset takes place near the inner edge of the electron plasma sheet. Based on these results they suggested that the physics of the R2 current system is important for substorm trigger.