RADIAL EXPANSION OF A SYMMETRICAL CME BETWEEN TWO CORONAL STREAMERS

The CME event on May 6,1980,occurring between a pair of coronal streamers,is discussed in this paper.An empirical kinematical model is proposed for describing the motion of outer loop of the event,n which the observed profile of loop is fitted in terms of the Cassini function in a moving coordinate system.The development of the loop can be distinguished into two different phases.During the first phase,when the loop lay between the helmet structures of streamer pair,the development of event is dominated by expansion of angular width with the asymmetrical axis shifting to northern latitudes;the speed of radial expansion and outward motion were rather low with remarkable acceleration.During the second phase,when the outer loop moved beyond the height of helmet structure of coronal streamer,the angular expansion ceased;the motion approached to an expansion and lift mainly in the radial direction with uniform speed and invariable angular width.As the loop moved outward,both the top and center heights increased with time, however,the bottom remained nearly fixed.Based on the above observational characteristics,a possible mechanism is put forward for the formation of the event.