The geometry of the radiation point of an auroral corona is examined. The radiation point of two rays is the antidirection of the point within the Earth at which the rays meet or appear to meet. It is therefore incorrect to identify the radiant point of a corona with local auroral zenith. Their difference in direction is commonly 0.5� of zenith distance. The importance of rays as magnetic disturbance indicators in the height range 100-1000 km is stressed, particularly in view of possible deformations of the magnetosphere whose full effects may not be estimated from groundbased observations of the geomagnetic field.