1. Division of Plasma Physics, Alfvén Laboratory, KTH, Royal Institute of Technology, SE 10044 Stockholm, Sweden
2. University College, London, Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
3. Imperial College, Blackett Laboratory, Space and Atmospheric Physics Group, London SW7 2BW, UK
4. Swedish Institute of Space Physics, Ångströmlaboratoriet, Box 534, SE 751 21 Uppsala, Sweden

Correspondence to: G. T. Marklund¹ Correspondence and requests for materials should be addressed to G.T.M. (e-mail: Email: marklund@plasma.kth.se).

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^{1, 2, 3}. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth^{4, 5, 6, 7, 8, 9, 10, 11}. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted¹², but how they could be maintained is still a matter for debate¹³. 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.

1. Division of Plasma Physics, Alfvén Laboratory, KTH, Royal Institute of Technology, SE 10044 Stockholm, Sweden
2. University College, London, Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
3. Imperial College, Blackett Laboratory, Space and Atmospheric Physics Group, London SW7 2BW, UK
4. Swedish Institute of Space Physics, Ångströmlaboratoriet, Box 534, SE 751 21 Uppsala, Sweden

Correspondence to: G. T. Marklund¹ Correspondence and requests for materials should be addressed to G.T.M. (e-mail: Email: marklund@plasma.kth.se).