Flashing light keeps pace with commercial aircraft going dark and light again like a sparkler or roman candle.Around 8:30 P.M., I was flying a commercial propjet from Dallas to Wichita, Kansas when I noticed a flashing of light to the port side. The light was far enough behind the wing that I doubt the cabin crew saw it. We were flying at 18,000 feet. The flashing light paced our aircraft in steady northward direction. I can only describe the flashing as what the tail of a sparkler looks like or a roman candle. I noticed that the flashing would illuminate trees, so it was that close to the ground. It appeared that the light was traveling through trees occasionally. The light would flare-up and then it would go dark for 1-5 seconds and then appear again. The flashing light would only occurred in dark spots on the ground as if it was intentionally bypassing areas of man-made illumination. I turned asked a fellow passenger behing me if he saw the light and he replied in the affirmative. When the flashing light reached the south side of Wichita it then turned to the west and I lost sight of it. It appeared that the light deliberately avoided flying into Wichita. I did not see an object, only the flashing light.