Giant bright blue beam shot across the sky.

I went out to walk my dog and looked up and there was a giant beam of deep blue light going from north to south.  The beam lasted for about 3 minutes and then stopped...it looked almost like a giant laser and someone put a piece of paper in front of it.  Towards the end of the beam, the light was a brighter, almost white blue.  The beam was big enough to make the sky divided into two and the beam had a clear shimmering around the edge.  My dog immediately took off when I saw it and ran back up to my apartment.  I've never seen or imagined of seeing something like this.