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			 **   THE GEM MESSENGER   **
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						     Volume 11, Number 26
						     August 14, 2001

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Reports of 2001 GEM Snowmass Workshops
Workshop on Tail/Substorms: Substorm Triggering Working Group
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From: Larry Kepko (lkepko at bu.edu)
      Aaron Ridley (ridley at mho.engin.umich.edu)

The substorm triggering session this year held one session which
focused primarily on the observational aspects of triggering.
Previous research had suggested that the probability of the upstream
solar wind monitor observing the trigger decreased in proportion with
the radial distance of the spacecraft from the earth-sun line. New
results using data from IMP8 and ISEE were presented that showed no
change in the association of solar wind trigger and substorm onset
with radial distance from the earth-sun line. A second study examined
if solar wind triggers were associated with substorm onset by chance.
In other words, after a 20-minute period of southward IMF, what is
the probability of a northward turning? If this probability was high,
then it would be likely that the associated between trigger and onset
was chance. The study showed that this probability was low, and
remained low for a long period of time. It was also shown
substorms where an IMF trigger was observed constitute a distinct 
class of substorms and are associated with larger decreases in AL and 
a larger change in the lobe magnetic field than are substorms without 
an observed IMF trigger. Further observations dayside radar 
measurements showed large-scale convection reductions at the time of 
three substorm onsets. However, there was no consensus on how this 
was associated with substorm onset.

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