Auroral Poleward Boundary Intensifications and Modes of Energy
Transport in the Plasma Sheet

Zesta, E, Lyons, L, Donovan, E, Frey, H U, Nagai, T

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 important to plasma
sheet dynamics. 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
aligned structures or east-west 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
north-south structures) and sequences of azimuthally broad and
primarily earthward propagating phase fronts initiating near the
separatrix (associated with the east-west 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]
IMAGE [Image] spacecraft, and magnetic field and plasma data from the
Geotail spacecraft. We have identified a number of events from early
2001 when Geotail was in the midtail plasma sheet and seek to answer
the following questions: a) Are PBI structures observed at one location
associated with structures which simultaneously cover many hours of
local time on the nightside, b) Are all north-south PBI structures
associated with narrow channels of fast flows in the plasma sheet, and
c) Are the east-west PBI structures also associated with fast flows or
with some other dynamic structure in the plasma sheet? Finally, we use
events with overlapping images from both the ground ASIs and the
spacecraft imager to relate the two-dimensional structure seen with
high spatial resolution on the ground to the much larger spatial scale
structure observable from the spacecraft.