FAST and IMAGE-FUV observations of a Substorm onset

S. B. Mende, C. W. Carlson, H. U. Frey, and T. J. Immel

Space Science Laboratory, University of California, Berkeley, CA 94720


On the 6th of February 2002, the NASA FAST satellite transited a 
substorm break up arc approximately one minute after substorm onset as 
identified by the NASA IMAGE far ultraviolet (FUV) instrument 
complement.  The bright onset feature had two particle components, a 
poleward super-thermal electron, and an equatorward quasi static 
"inverted V" electric field generated component.  The super-thermal 
electrons were closely aligned with the magnetic field with a 
non-peaked energy distribution consistent with an Alfven wave 
acceleration.  The onset arc is at the poleward boundary of the hot 
plasma sheet protons.  Separated from the Alfven wave accelerated 
aurora, on the equatorward side, an inverted V type aurora was seen 
with its quasi-static electric field signature.  The FAST particle 
measurements show an extended region of closed field lines separating 
the ground onset location (64.2 deg ILAT) from the closed/open field 
line boundary identified by polar rain electrons at 70.6 deg ILAT. 
No magnetic or particle activity related to substorm onset was 
detected in this high latitude region.  The onset is therefore located 
at some distance from the open-and-closed-field-line boundary in the 
tail, which is the location of the steady state convective 
reconnection region.  Prior work with FAST and IMAGE FUV data showed 
that during later phases of the substorm development, the Alfven wave 
accelerated electrons and the corresponding auroral surge were 
regularly seen much closer to the polar cap boundary.  Comparison of 
the onset event with those indicates that substorm field line 
configuration change leads to the surge propagating poleward through a 
region of closed field lines towards the open-and-closed-field-lines 
boundary.  These observations are consistent with both the near earth 
neutral line (NENL) and the tail current disruption substorm model. 

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Fall 2002 Meeting of the American Geophysical Union
San Francisco, CA, USA, 6-10 December 2002