[GEM] THE GEM MESSENGER, Volume 25, Number 39

[Newsletter Editor]

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     THE GEM MESSENGER
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Volume 25, Number 39
September 14, 2015

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2015 Summer Workshop Report: Testing Proposed Links between Mesoscale Auroral and Polar Cap Dynamics and Substorms Focus Group
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From: Kyle Murphy, Toshi Nishimura (toshi at atmos.ucla.edu), Emma Spanswick, and Jian Yang

Motivated by the recent controversy over auroral streamers, flow bursts and possible connections to substorm triggering, we introduced a new focus group (FG), "Testing Proposed Links between Mesoscale Auroral and Polar Cap Dynamics and Substorms", intended to elucidate connections between auroral structures and their magnetospheric counterparts, and to bring closure to the question of substorm triggering. We had four sessions including one jointly with the Scientific Magnetic Mapping & Techniques FG. The first three talks reviewed current understanding and open questions, and subsequent talks in session 1 and 3 discussed a selected event and discussed how much the community can agree on the event sequence. Modelers also presented works on key issues on substorms. Session 2 discussed methods and techniques to quantify auroral signatures, and session 4 was for magnetosphere-ionosphere mapping of substorm signatures.

1. Event discussion

Larry Lyons gave a review of precursor streamer scenario, presented his view of the Kepko et al. [2009] event, and raised outstanding issues. If the Nishimura et al. [2010] scenario is very common, as he believes will turn out to be the case, then a number of quite interesting questions, including (1) How do flow channels do and do not lead to onset relate to entropy gradient changes from intruding low entropy flow channels? (2) How does substorm onset instability relate to entropy gradient changes caused by the intruding flow channels? (3) How are flow channels related to DNL/NENL and polar cap flows? (4) How is a current wedge involved in these?

Mike Henderson reviewed a historical aspect of substorm triggering by IMF orientation changes.  Then he commented on substorm pre-onset scenarios and near-Earth instability. He noted that the November 21, 2002 event in Henderson [2009] also shows streamer-like features prior to onset. The onset has characteristics of ballooning instability and precedes mid-tail X-line formation. He also raised questions why only some of the streamers trigger onset, how the magnetosphere is different compared to streamers that lead to other types of responses (e.g., torches and omega bands).

Larry Kepko discussed similarities and differences of existing scenarios and raised key questions (presented by Toshi Nishimura). He pointed out that there is no quantitative definition of streamers, and that different types of auroral activities have been used in the past statistics. Depending on how people identify each auroral signature, the occurrence probability of precursors may vary substantially, resulting in the lack of consensus about substorm precursors in the community. The community should establish quantitative definition of each auroral signature, and have a community-wide discussion of individual events and occurrence statistics. 

Shin Ohtani and Tetsuo Motoba examined Kepko et al. event with a focus on the initial location and propagation of auroral beads using the polar distribution of equivalent currents as a reference for global convection.  They found that an auroral structure propagating from poleward touched the auroral oval around the demarcation between the dawn and dusk convection cells and then, the auroral beads started to form without any noticeable delay.  The timing is consistent with the idea that the auroral breakup is caused by the penetration of a plasma flow into the near-Earth region.  However, they also found that the beads formation started noticeably dawnward of the convection demarcation and expanded dawnward toward the demarcation.  In addition, auroral beads themselves propagated dawnward.  They therefore suggested that the auroral activation is not a direct consequence of the flow penetration, but there is an additional process, for which the ion dynamics might be important.

Larry Lyons and Toshi Nishimura gave their view of Kepko et al. event. They agree that a streamer formed and contacted the growth phase arc prior to the onset. However, in contrast to Kepko et al.’s suggestion, they found that the streamer did not originate in the middle of the auroral oval but from a poleward boundary intensification (PBI). The PBI was preceded by a polar cap airglow, suggesting that the precursor of this substorm initiated in the dayside polar region, propagated across the polar cap and nightside open-closed boundary, and then reached the near-Earth plasma sheet prior to the substorm onset.

Kyle Murphy analyzed Kepko et al. event using his technique of tracing auroral structures. His method automatically identifies streamers and growth phase arc, and shows that streamers contacted the growth phase arc prior to the onset. In this event, his analysis supports Kepko et al.’s conclusion that the substorm onset is preceded by plasma sheet flows.

Yukinaga Miyashita suggested that for making discussions about the substorm triggering mechanism clear, it is essential to determine the timings of the three steps of the auroral development, i.e., initial brightening, enhancement of wave-like structure, and poleward expansion.  For the 25 February 2008 substorm event, he pointed out that the pre-onset auroral streamer reached the auroral onset arc away from the initial brightening site after the initial brightening already occurred.  He also showed that near-Earth reconnection possibly began 4 min before the initial brightening.

Joachim Birn presented results from test particle tracing in an MHD simulation of near-tail reconnection and flow bursts, demonstrating the formation of field-aligned ion beams in the PSBL. The ion beams were generated by direct non-adiabatic acceleration in the vicinity of the neutral line, consistent with PIC simulation results, but adiabatically deformed into crescent shaped velocity distributions from propagation toward higher magnetic field. The energy dispersion of the beams permits a remote identification of the acceleration site.

Phil Pritchett showed results from a 3D PIC simulation of a portion of the magnetotail indicating that the ballooning/interchange instability should produce structuring of auroral streamers similar to THEMIS ASI observations. He emphasized that 3D PIC simulations have evolved to the point where they can be used to investigate other issues relevant to the Substorm focus group such as the influence of dayside flow channels in initializing localized tail reconnection.

Misha Sitnov discussed PIC simulations of dipolarization fronts and their ionospheric implications. Pritchett, Coroniti and Nishimura (JGR, 2015) first noticed that while equatorward portion of the streamer showed multiple arcs, the poleward portion of the streamer might stay essentially as a single arc. This observation is consistent with his 3D PIC simulations of fronts. Sitnov showed that, in contrast to flapping and buoyancy-driven perturbations of the dipolarization front causing its modulation in the dawn-dusk direction, the region well behind the front, including the new X-line forming in its wake, remains largely unstructured in the dawn-dusk direction, except relatively long wavelength flapping motions.

Vassilis Angelopoulos presented plans of the upcoming Heliophysics/Geospace System Observatory (HGSO). Although satellite missions are generally driven by their own mission goals, he proposes to coordinate satellites as well as ground observatories for studying cross-scale and cross-regional coupling processes in a broader scale than previously possible. HGSO will conduct simultaneous observations in the dayside and nightside reconnection regions by MMS and THEMIS, and coordinate with other satellite missions and ionospheric measurements for investigating (1) global effects of dayside transients, (2) cusp-dayside connections, (3) nightside reconnection and tail-inner magnetosphere coupling, (4) global processes, and (5) cross-scale coupling.

2. Tools and Methods Session

Bob McPherron presented a detailed overview on point processes and how this analysis could be applied to substorm research. Using data from ground-based magnetometers B. McPherron compared and contrasted different substorm lists including the SuperMAG, IMAGE, mid-latitude positive bay and Nishimura substorm lists. B. McPherron demonstrated that the SuperMAG, IMAGE and mid-latitude positive bay list were all in excellent agreement while the Nishimura list was poorly correlated with the others. 

Emma Spanswick and Eric Donovan presented an overview of the circle gram substorm aurora analysis technique. The circle gram determines whether onset occurs spontaneously with in a discrete region or is triggered by aurora outside of the region. The analysis technique is ideal for characterizing substorm triggering by auroral streamers. 

Nadine Kalmoni presented a new analysis technique for characterizing the auroral bead observed at substorm onset. The technique is able to determine both the auroral wave length and growth rates observed at substorm onset. These wavelengths and growth rates can then be compared to theoretical values to determine the most likely instability leading to development of auroral beads. 

Kyle Murphy presented a new analysis technique for auroral tracking. The quantitative algorithm track aurora and is able to determine whether streamers are a necessary condition for substorm onset. 

3. Mapping Session

Emma Spanswick presented observations from riometers and red line imagers in the mapping session. E. Spanswick demonstrated that the riometer signature of substorm onset is highly correlated with in-situ observations of the substorm injection. With an array of ground-based riometers the development of the substorm injection can be track in latitude, longitude and time. E. Spanswick also showed new 2D red line auroral imagers, new work is being done to determine if these imagers can routinely track the polar cap boundary across MLT and in time. 

Shin Ohtani gave two talks in the mapping session. In the first S. Ohtani discussed the overlap region of R1 and R2 currents where substorm onset can occur and which can be driven unstable by the interchange instability. In the second talk S. Ohtani discussed the mapping of R1 and R2 current relative to electron and ion plasma sheets.    

Jian Liu presented detailed observations of the substorm current wedge from the THEMIS constellation. 

Chao Yue presented a new mapping technique for mapping the growth phase auroral arc from the ionosphere to equatorial plane. C. Yue’s mapping suggests that substorm onset occurs in the inner magnetosphere in a region that is characteristically unstable to a ballooning instability. 

4. Future work

In a wrap up session the focus groups leaders discussed outstanding questions and ways to move forward with the focus group. Outstanding questions included:

1)	What is a streamer?
2)	What are the conditions for onset?
3)	What is the timing between streamers and auroral onset? And how close do streamer need to get to trigger onset?
4)	Is the association of streamers and onset coincidental?
5)	What information can we derive from optical/ground data that directly connects to satellite measurements?


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