Temporal evolution of the electric field accelerating electrons away from the 
auroral ionosphere 

G.T. Marklund, N. Ivchenko, T. Karlsson, A. Fazakerley, M. Dunlop, 
P.-A. Lindqvist, S. Buchert, C. Owen, M. Taylor, A. Vaivalds, P. Carter, 
M. Andre and A. Balogh


The bright night-time aurorae that are visible to the unaided eye are caused by
electrons accelerated towards Earth by an upwardpointing electric field. On
adjacent geomagnetic field lines the reverse process occurs: a downward-pointing
electric šeld accelerates electrons away from Earth. Such magnetic-field-aligned
electric fields in the collisionless plasma above the auroral ionosphere have
been predicted, but how they could be maintained is still a matter for debate.
The spatial and temporal behaviour of the electric fields - a knowledge of which
is crucial to an understanding of their nature - cannot be resolved uniquely by
single satellite measurements. Here we report on the first observations by a
formation of identically instrumented satellites crossing a beam of
upward-accelerated electrons. The structure of the electric potential
accelerating the beam grew in magnitude and width for about 200 s, accompanied
by a widening of the downward-current sheet, with the total current remaining
constant. The 200-s timescale suggests that the evacuation of the electrons from
the ionosphere contributes to the formation of the downward-pointing
magnetic-field-aligned electric fields. This evolution implies a growing load in
the downward leg of the current circuit, which may affect the visible discrete
aurorae.

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Nature, 414, 724-727, 2001