title: Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere.
abstract: The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field 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.
authors.affiliation: Division of Plasma Physics, Alfvén Laboratory, KTH, Royal Institute of Technology, SE 10044 Stockholm, Sweden. marklund@plasma.kth.se
authors.forename: G T
authors.initials: GT
authors.lastname: Marklund
authors.forename: N
authors.initials: N
authors.lastname: Ivchenko
authors.forename: T
authors.initials: T
authors.lastname: Karlsson
authors.forename: A
authors.initials: A
authors.lastname: Fazakerley
authors.forename: M
authors.initials: M
authors.lastname: Dunlop
authors.forename: P A
authors.initials: PA
authors.lastname: Lindqvist
authors.forename: S
authors.initials: S
authors.lastname: Buchert
authors.forename: C
authors.initials: C
authors.lastname: Owen
authors.forename: M
authors.initials: M
authors.lastname: Taylor
authors.forename: A
authors.initials: A
authors.lastname: Vaivalds
authors.forename: P
authors.initials: P
authors.lastname: Carter
authors.forename: M
authors.initials: M
authors.lastname: André
authors.forename: A
authors.initials: A
authors.lastname: Balogh