Yeah the rays come and go and so the best thing is to just keep clicking the shutter until the memory card is full :-)
I've bookmarked this page on my phone for when i am out. It updates every minute. If you wait until the Bz indicator goes (-)negative, then you should be in with a shot. The further into the neg. the better, and the higher the density and faster the speed all helps.
http://www.aurora-service.net/aurora-forecast/
Trev