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			 **   THE GEM MESSENGER   **
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						     Volume 12, Number 26
						     August 5, 2002
				  
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GEM 2002 Tail/Substorm Observations Working Group Report
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From: Mark Moldwin <mmoldwin at igpp.ucla.edu>

The Tail/Substorm Observations working group held one session at GEM 
2002 in Telluride CO. In previous years the working group has 
attempted to identify the relative timing between substorm signatures 
in the near-Earth tail and in the mid-tail. Currently the two 
dominate models of substorms place the initiation of the substorm 
sequence in one or the other of these two locations. Unfortunately, 
the ability to time onset signatures (auroral brightenings, Pi 2 
pulsations, energetic particle injections etc.) all have 
uncertainties of about 2 minutes (due to either the cadence of the 
instrument, the difficulty of identifying the beginning of a long 
period wave, and the apparent localized nature of many plasma sheet 
phenomena associated with substorms). This is also the approximate 
fast mode travel time between the mid-tail and the near-Earth 
magnetosphere. Hence current measurements are not sufficient to 
distinguish between the two models.
This year we concentrated on results from energetic neutral atom 
imaging (ENA) observations that have the promise of addressing this 
issue since they can provide global perspective of the energetic 
particle population. Therefore, these images in combination with 
other in situ measurements can be used to provide new insights into 
the spatial and temporal structure of substorms and the 
substorm-storm relationship.
Geoff Reeves (LANL) presented work from Jorg-Micha Jahn (SwRI) that 
demonstrated that it was possible to quantify azimuthal propagation 
of energetic particles on the nightside of the Earth but it is more 
difficult to discern quantitatively radial motion. Geoff also 
presented simultaneous observations from MENA and the Los Alamos 
geosynchronous orbit satellites. The datasets were inter-compared to 
give a global context for in situ observations of the plasmasphere 
and plasmasheet and to directly compare the ion fluxes inferred from 
ENA image inversion with direct local measurements of those fluxes.
Mike Henderson (LANL) also utilized simultaneous observations of ENA 
and in situ particle data but also incorporated auroral imaging data 
in an effort to understand the difference between isolated substorms 
and those that occur associated with storms. Mike concludes that the 
substorm signatures are identical for both types of substorms however 
there is continuous substorm activity driven by solar wind-driven 
convection during storm-time substorms.
Pontus C:son Brandt (JHU/APL) presented ring current observations 
(between L<6 out to L=13) from HENA for periods of southward IMF when 
the geomagnetic field is stretched. During dipolarization the ENA 
flux from the L<8 region decreased rapidly. Pontus postulates that 
the flux decrease during dipolarization is caused by the 
dipolarization induced E-field, which brings ions to lower L-shells 
faster than they can be replenished by the overall convection, 
creating a void of particles in the plasma sheet. This scenario 
implies that the acceleration region must be localized to, from L=8 
out to, at least L=13.
Stephen Mende (UCB/SSL) presented observations from the FUV and HENA 
instruments on IMAGE.  Proton aurora expands dawnward following a 
substorm onset, which is accompanied by the expansion of the region 
of intensified ENA fluxes.  Mende interprets it in terms of the 
dawnward skewing of a plasma convection pattern in the inner 
magnetosphere.  Such a convection pattern was also presented in the 
tutorial talk given by Dick Wolf earlier at the workshop, and it can 
be regarded as a combined effect of the closure of region-2 currents 
in the ionosphere and the corotation electric field.
As the Tail-Substorm campaign winds down next year, the Observations 
Working group is exploring the idea of having a session on 
multi-perspective studies of substorms. Currently data from Polar, 
IMAGE, Cluster and a large number of operational satellites are 
making simultaneous multi-point and global imaging observations of 
substorm dynamics. If you are interested in participating in such a 
session please contact one of the co-chairs.

Shin Ohtani (ohtani at fluxgate.jhuapl.edu), Mark Moldwin (mmoldwin at igpp.ucla.edu)						    
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