................................................................. Temporal evolution of the electric ®eld accelerating electrons away from the auroral ionosphere G. T. Marklund*, N. Ivchenko*, T. Karlsson*, A. Fazakerley², M. Dunlop³, P.-A. Lindqvist*, S. Buchert§, C. Owen², M. Taylor², A. Vaivalds§, P. Carter², M. Andre  § & A. Balogh³ * Division of Plasma Physics, Alfve Ân 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 § Swedish Institute of Space Physics, A Ê ngstro Èmlaboratoriet, Box 534, SE 751 21 Uppsala, Sweden .............................................................................................................................................. The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward- pointing electric ®eld 1±3 . On adjacent geomagnetic ®eld lines the reverse process occurs: a downward-pointing electric ®eld accel- erates electrons away from Earth 4±11 . Such magnetic-®eld-aligned electric ®elds in the collisionless plasma above the auroral iono- sphere have been predicted 12 , but how they could be maintained is still a matter for debate 13 . The spatial and temporal behaviour of the electric ®eldsÐa knowledge of which is crucial to an under- standing of their natureÐcannot be resolved uniquely by single satellite measurements. Here we report on the ®rst observations letters to nature 724 NATURE | VOL 414 | 13 DECEMBER 2001 | www.nature.com Aurora Downward- accelerated electrons Upward- accelerated electrons Ionosphere EARTH Parallel current Parallel current Perpendicular current + + + + + + + + + – – – – – Electric field Electric field Figure 1 Acceleration structures (black) within the auroral current circuit (red). This diagram shows a negatively charged potential structure (indicated by equipotential contours) representative of the aurora (left) and a positively charged potential structure representative of the auroral return current (right). The two branches of the equipotential contours close at typical altitudes of 5,000±8,000 km in the upward-current region above the aurora and at 1,500±3,000 km in the auroral return-current region, respectively, above which they extend to very high altitudes along the geomagnetic ®eld, forming the characteristic U-shape. The ®eld-aligned currents are carried by the downward and upward accelerated electrons, respectively (blue). Together with the ionospheric closure current and the magnetospheric generator these form the complete auroral current circuit. The ®gure shows a north±south section through the structures that usually extend several hundreds of kilometres in the east±west direction. b a 1,000 500 0 –500 –1,000 R = 4.3R E B M Lat = 69.8 MLT = 3.6 80 70 60 6 0 12 18 Rumba dt = 105 s Samba dt = 0 s Salsa dt = –90 s Tango dt = –182 s –1,000 –500 0 East (km) North (km) 500 1,000 Figure 2 Location and con®guration of the Cluster satellite formation passing over the Northern Hemisphere auroral oval. It passed between 4:20 and 4:40 UT on 14 January 2001, in the recovery phase of a weak substorm, peaking at 04:00 UT. a, The nightside auroral oval as observed by the ultraviolet imager on the IMAGE satellite at 04:30 UT. The post-midnight auroral oval forms a relatively homogeneous diffuse band between 65 and 70 degrees magnetic latitude. The ionospheric projection of the Cluster orbit (between 4:20 and 04:45 UT) is shown by the white line intersecting the auroral oval near 3:00 magnetic local time. b, Cluster satellite con®guration on 14 January 2001. Relative positions and time differences (dt) between the Cluster 1, 2, and 4 spacecraft with respect to the reference Cluster spacecraft 3 (Samba) in a plane transverse to the magnetic ®eld. The four Cluster spacecraft are aligned nearly as pearls on a string, with the separation transverse to the velocity vector being small, and follow in the sequence 1-3-2-4. The direction of motion of Samba projected onto this plane is shown by the dotted line. B denotes the direction of the geomagnetic ®eld, R is the geocentric distance of the Samba spacecraft expressed in units of the Earth radius (R E ), M Lat denotes magnetic latitude, and MTL denotes magnetic local time.