THE FREJA ULTRAVIOLET IMAGER

Auroral images acquired by satellite instrumentation have proven to be a crucial component of the scientific equiry into the physical processes of the Earth's magnetosphere. The Freja mission provided an opportunity to extend these measurements into the temporal and spatial regime commonly associated with ground-based optical imagers. Employing the basic procedure used successfully in the Viking program allowed image repetition rates of 6 s to be achieved with simultaneous exposure of all pixels within the field-of-view. Typical exposure times of 0.3 s required development of an improved image intensifier system and operational requirements resulted in image formats covering approximately one-third of a spin. Simultaneously exposing two broad-band UV cameras, the instrument generates in its normal operational mode 264.6 kbytes per image pair. Results from initial operations confirm the design approach and suggest such imagers can be routinely included on challenging low cost missions such as Freja.