My Tesla Coil is powered by household alternating current, which alternates at 60 cycles per second. For each cycle, the current flows one way, stops, flows the other direction and then stops. That means that there are 120 times per second where the current is not flowing. During those times, the spark also stops. When the spark starts again, it follows the ionized path left by the previous spark, but because the spark heats the air, that path is rising, so each time the spark restarts, the path is a little higher, showing as bands in the photo.