Saw spotlight from airplane shining on the ground from something behind plane.

I was in the very back of an alaska airlines flight from santa ana to portland or. It was night time, somewhere around 9:30pmish. The plane just began its decent to land at PDX, and I was looking out the window. I was looking at the ground, and lights on the ground were pretty sparse, so I figured we were flying over somewhere rural. At some point I noticed a light moving with the trajectory of rhe plane on the ground. It was like a spot light shining on dark areas on the ground from something tailing the plane. In completly dark areas on the ground it apeared like a VERY large bright circle, but when it crossed areas of light like houses or cars, it would shift shapes around terrestrial light sources as if to evade or doge them. This spot light from an arial source followed with the plane for about 45 seconds, and disapeared before the plane entered airspace above the larger Portland area.