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

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field1,2,3. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth4,5,6,7,8,9,10,11. 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.