Auroral poleward boundary intensifications and modes of energy transport in the 
plasma sheet

E. Zesta (1), L. Lyons (1), E. Donovan (2), H. U. Frey (3), T. Nagai (4)

(1) UCLA, Atmospheric Sciences
(2) University of Calgary, Physics Dept.
(3) University of California, Berkeley


Auroral poleward boundary intensifications (PBIs) have an auroral signature in 
ground meridional scanning photometer (MSP) data that appears as an increase in 
intensity at or near the magnetic separatrix. This increase is often seen to extend 
equatorward through the ionospheric mapping of the plasma sheet. PBIs are also 
associated with fast flows in the tail plasma sheet and are thus an important 
disturbance of the auroral zone and plasma sheet. We have recently used simultaneous 
auroral observations from the CANOPUS MSPs and either the Freja UV imager or the 
CANOPUS Gillam all-sky imager (ASI) to investigate the two-dimensional structure of 
auroral intensifications near the poleward boundary of the oval. We found that 
equatorward extending PBIs are either north-south (NS) aligned structures or 
east-west (EW) arcs that mostly propagate equatorward, but we have not been able to 
determine without doubt which type is the most prevalent. The different 
two-dimensional orientations for equatorward extending PBIs suggests that they may 
be the auroral footprint of two major modes of energy transfer in the plasma sheet: 
multiple, narrow, earthward fast-flow channels (associated with the NS structures) 
and sequences of azimuthally broad and primarily earthward propagating phase fronts 
initiating near the separatrix (associated with the EW arcs). We test this 
hypothesis by combining data from the CANOPUS MSPs, the all-sky imagers of the newly 
installed NORSTAR array in northern Canada which cover the poleward boundary of the 
auroral oval, auroral images from the IMAGE spacecraft, and magnetic field and 
plasma data from the Geotail spacecraft. We discuss, in particular, two events. 
First, an event from Jan 3, 2001 when multiple PBIs occurred during the long 
recovery phase of a substorm. We find that PBIs correlate well with plasma sheet 
fast flows that occur within the local time sector of PBIs. Most of the PBIs are EW 
arcs that initiate near the poleward boundary and then propagate equatorward. They 
often tilt and become approximately NS structures as they propa-gate equatorward. 
Second, an event from Jan 13, 2001 that occurs during very quiet (northward IMF) 
conditions and is not associated with a substorm. The PBIs are EW arcs and the 
ionospheric footprint of a triple flow burst in the plasma sheet. These two events 
suggest that, contrary to our previous suggestion, the same dynamics likely produce 
both EW and NS structures and that similar plasma sheet dynamics may occur both 
during the recovery phase of a substorm under southward IMF and during a quiet, 
non-substorm, northward IMF time period.

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Presented at the 34th COSPAR Scientific Assembly, Houston, Texas, U.S.A.  
10-19 October 2002