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 cause d by electrons accelerated towards Earth by an upward-pointing electric fie ld(1-3). On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth(4-11 ). Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted(12), but how they could be maintained is still a matter for debate(13). The spatial and temporal beha viour of the electric fields-a knowledge of which is crucial to an understa nding of their nature-cannot be resolved uniquely by single satellite measu rements. Here we report on the first observations by a formation of identic ally instrumented satellites crossing a beam of upward-accelerated electron s. The structure of the electric potential accelerating the beam grew in ma gnitude and width for about 200 s, accompanied by a widening of the downwar d-current sheet, with the total current remaining constant. The 200-s times cale suggests that the evacuation of the electrons from the ionosphere cont ributes to the formation of the downward-pointing magnetic-field-aligned el ectric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.