After that, there will be a ball of fireball light "rising" for a long time after an interval of several hundred milliseconds to several seconds (from purple to green and finally frozen to white), followed by shock waves and radiation dust. This secondary light radiation is called "double flash syndrome".
This phenomenon occurs because the energy in the center of the explosion expands "rapidly" (indeed for a short time) at the moment of explosion, and then we see the first strong "flash". But the air around the explosion center was quickly squeezed and spread like water waves. After the density gradually increases, this layer of air is rubbed and "ionized" into particle gas.
At this time, the explosion light is blocked and refracted by this opaque air. We will temporarily lose sight of the light in the center of the explosion because it is blocked by this layer of ionized gas.
Then with the increase of distance, the air density began to decrease, and the light in the explosion center was released again, which showed us the energy light of nuclear explosion. After the above process, we will see the unique "double flicker sign" of nuclear explosion at the observation distance.
The longer the interval between two flashes, the greater the equivalent of a nuclear explosion. There is a well-known event in the double flash phenomenon: "Vela event", which is one of the "evidences" of suspecting that Israel possesses nuclear weapons.
The "Villa" optical exploration satellite launched by the United States originally used the principle of "double flashing sign" to monitor whether the Soviet Union violated the "partial nuclear test ban treaty" during the Cold War. However,1On September 22nd, 979, the satellite 69 1 1 of Villa System observed a double flash in the sea area between Prince Edward Island and Croze Islands at the junction of the South Atlantic and the Indian Ocean.