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                     **   THE GEM MESSENGER   **
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                                                  Volume 22, Number 26
                                                    September 13, 2012
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2012 WORKSHOP REPORT: Substorm Expansion Onset:
The First 10 Minutes Focus Group
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From: V. Angelopoulos, A. Runov, S.-I. Ohtani and K. Shiokawa 
      <shiokawa at stelab.nagoya-u.ac.jp>, Conveners

This year this focus group covers the following four topics.  

* Relative timing between onset signatures in space and on the ground
* Substorm signatures propagation from tail toward the inner 
magnetosphere and to the ground
* Substorm-related processes in the tail-dipole transition region
* Substorm signatures beyond -30 RE, including Lunar orbit and distant 
tail 

We had three sessions on June 20, 19, 2012.  The third session 
(starting at 3:30) was joint with the Magnetic Mapping Focus group and 
discussed problems of late growth phase mapping.  Here are notes on 
the presentations and discussions made during the sessions. 

Session 1 (10:30-12:15) 							
	
* Feifei Jiang discussed what is the cause of the pre-existing arc?  
Feifei showed FAST statistics of 210 events of pre-existing arcs 
before subtorm onset in 1998 using E-field measurements.  Most 
equatorward electron acceleration structures observed within one hour 
before the substorm onset are considered as the pre-existing arcs. 
Most of the pre-existing arcs are located within 1 degree of the 
boundary of the region 1 and 2 field-aligned currents (FACs).  
Equatorward E-field increases significantly from lower to higher 
latitudes of the preexisting arc in the postmidnight.  In the pre-
midnight, such enhancement was not seen.  Relation between flow shear 
associated with the arc and large-scale convection flow should be 
clarified. This result is consistent with the kinetic ballooning model, 
since ballooning instability becomes most unstable near the boundary 
of R1/R2 currents (Cheng and Zaharia, 2004). 

* Eric Donovan reported results reported by Motoba et al. (GRL, 2012) 
which show clear conjugacy of onset auroral beads at conjugate 
stations in northern and southern hemispheres.  	The beads move 
eastward with a maximum speed of~5 km/s in the ionosphere.  This high 
speed is difficult to be explained by the ballooning instability (too 
fast).  There are some evidences that these beads occur after the flow 
bursts.   Drift-mirror instability can also make similar structure.
	
* Larry Lyons showed three unexpected suggestions: 1) current wedge 
response on ground magnetic field data delayed to the auroral 
brightening, and responds much stronger to auroral streamers (plasma 
sheet flow channels); 2) flow channels leading to pre-substorm onet 
PBIs and streamers can extend from well within the polar cap towards 
the polar cap boundary; 3) polar-cap boundary streamers after onset 
make additional poleward expansion and brightening of aurora when they 
touch the brightening aurora.  As the low-entropy plasma of flow burst 
touches the equatorward arc, instability in the inner magnetosphere 
seems to develop further.  Questions arises how the low-entropy plasma 
makes precipitation. ? Does the instability necessary?  Just a braking 
process may be enough to cause aurora brightening.  The issue 3) can 
be explained either by the idea that the flow braking causes auroral 
brightening, or the idea that the flow burst causes near-earth plasma 
instability.  

* Toshi Nishimura showed correspondence between aurroal signatures and 
midlatitude Pi2 pulsations to separate different Pi2 models (directly 
driven by BBF, ballooning instability, or cavity mode resonance).  
Quasi-periodic auroral streamers (brightenings) appear repeatingly 
after onset.  The on/off of these streamers corresponds to the Pi2 
pulsations at middle and low latitudes, consistent with the model that 
multiple BBF creates Pi2. However, the correspondence between 
repeating streamers and flow burst in the tail is not so clear.	

* Vassilis Angelopoulos showed that an earthward flow burst (FB) 
causes a pair of upward/downward currents.  Tailward FB causes an 
opposite pair of upward/downward currents.  The currents are inferred 
from ground magnetic field data. 

* Misha Sitnov showed accumulation of magnetic flux at the tailward 
end of the thin tail current sheet.  It was theoretically expected.  
Geotail observation by Machida et al. (2009) show such a signature of 
Bz increase before the onset.  A particle simulation shows that the 
accumulation (tearing instability, slippage) accelerates particles 
earthward before the reconnection (Sitnov and Swisdak, 2011), 
consistently with the idea of catapult slingshot scenario for 
substorms by Machida et al. (2009).  Time difference between non-
reconnection flow and reconnection start seems to be too short in the 
model (less than 1 min) to explain the Machida's observation (more 
than a few min). 

Session 2 (13:30-15:00)  	

* Joe Baker showed suppression of westward ionospheric convection for 
a few minutes at subauroral latitudes during auroral substorm onset in 
the onset meridian.  Currently there is no good explanation on this 
phenomenon.

* Jiang Liu showed measurements of current sheet at the dipolarization 
front (DF) using a THEMIS statistics at X=-6 to -13 Re.  Current at DF 
is more field aligned at higher latitudes, and more perpendicular to 
the field at lower latitudes.   Jx>0 in morning and Jx<0 in evening, 
which is consistent with the region-1 FAC sense.  

* Joo Hwang showed tailward-moving dipolarization front (DF) followed 
by an earthward-moving DF observed by Cluster at X=-14Re.  Tailward 
flow causes stretching of plasma sheet and may initiate X-type lobe 
reconnection that causes subsequent earthward flow.  Flow velocity is 
earthward during tailward-moving DF.  What does it mean?  DF may be 
just an enhancement of Bz.  

* Joachim Birn showed two issues; 1) a few-min timing delay from 
reconnection to the substorm current wedge formation, and 2) energetic 
electron/proton motion in the simulated BBF.  The particle has two 
source regions, one from tail frank side (early, higher energy), and 
the other from the reconnection region (later, lower energy). They 
have anisotropic pancake distribution at low latitudes, and cigar 
(field-aligned) distribution at high latitudes.  

* Xuzhi Zhou studied ion beams in the PSBL using THEMIS observation at 
two satellites (P4 and P5) for 18 events.  PSBL ion flow bursts are 
followed by adjacent CPS flow bursts and dipolarization fronts for 
16/18 events. 

* Stefan Kiehas showed several examples of ARTEMIS observation during 
substorm-like phenomena.  P1 and P2 are separated about 7 Re in X or 
in Y.   The scale size of the substorm signature (TCR/flux 
ropes/plasmoid) in the near-Earth tail at X~-60 Re does not extend 
over the entire tail.  

Session 3 (15:30-17:00)  joint session with the mapping FG

* Shin Ohtani showed that the DMSP FAC/particle data (large data set) 
show b3a (equatorward boundary of monoenergetic electron precipitation) 
occurs at the R1/R2 current boundary.  B3b (poleward boundary of 
monoenergetic electron precipitation) occurs at poleward boundary of 
the R1 current.  (Ohtani et al., 2010)	Caution was made that the 
growth-phase arc will be only a very small fraction of the used 
dataset.
  
* Toshi Nishimura showed using CHAMP FAC and THEMIS ASI that the pre-
onset arc was located (event 1) at the peak of the R1 currents, (event 
2) at the middle of the R2 currents, (event 3) at the middle of the R2 
current, and (event 4) at the poleward edge of R2 current.  The onset 
arc is in the R2 current in the onset meridian, while it is at the 
boundary of R1/R2 current at dusk/dawn side of the onset meridian.  
These observations seem to be inconsistent with the kinetic ballooning 
model, since ballooning instability becomes most unstable near the 
boundary of R1/R2 currents (Cheng and Zaharia, 2004). But they assume 
symmetric magnetosphere in the model. 

* Jun Liang showed using THEMIS E and A difference that the tailward 
boundary of upgoing quasi-parallel electron beam (QPEBs) in the CPS 
can be used to map the equatorward boundary of auroral arc region to 
the magnetosphere.   The pre-breakup arc region is found as situated 
in the near-tail region, i.e., a transition region from quasi-dipolar 
to stretched current sheet topology, inferred by estimating the 
magnetic field curvature.  

* Larry Lyons showed mapping implications of the very thin auroral 
oval in the late growth phase.  The sequence of "PBI -> eqauatorward-
moving streamer -> auroral brightening onset" was investigated for 
thick oval cases (typical, 97%) and thin oval cases (rare, 3%).   
THEMIS data show difference in Ptot increase.  Thin case: less PBIs, 
streamers, flow channels.  Thick case: stronger, monotonic increase of 
Ptot at growth phase, and more thinning of tail.   

* Jian Yiang showed development of a substorm-time magnetic field 
model based on the equilibrium version of RCM (SUMMER).  He also 
showed that even one has a very good empirical model, the equatorial 
crossing X-distance is very different.  

All these presentation and relevance to the various substorm models 
are summarized as a substorm onset matrix which is uploaded on the GEM 
Wiki page.
(see 
http://aten.igpp.ucla.edu/gemwiki/index.php/FG12._Substorm_Expansion_O
nset:_The_First_10_Minutes) 

At the end, we agreed to have a similar joint session with mapping FG 
next year.  Caution was made to distinguish morphological mapping and 
field-line mapping.  A possibility was also suggested to have a joint 
session with inner magnetosphere-Tail FG.


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