Rope trick effect

Rope trick is the term given by physicist John Malik to the curious lines and spikes which emanate from the fireball of certain nuclear explosions just after detonation.

The adjacent photograph shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface.

The surface of the fireball, with a temperature over 20,000 kelvins, emits huge amounts of visible light radiation, more than 100 times the intensity at the sun's surface. Anything solid in the area absorbs the light and rapidly heats. The "rope tricks" which protrude from the bottom of the fireball are caused by the heating, rapid vaporization and then expansion of mooring cables (or specialized rope trick test cables) which extend from the shot cab, the housing at the top of the tower that contains the explosive device, to the ground. Malik observed that when the rope was painted black, spike formation was enhanced, and if it were painted with reflective paint or wrapped in aluminium foil, no spikes were observed – thus confirming the hypothesis that it is heating and vaporization of the rope, induced by exposure to high-intensity visible light radiation, which causes the effect. Because of the lack of mooring ropes, no "rope trick" effects were observed in surface-detonation tests, free-flying weapons tests, or underground tests.

The cause of a surface mottling is more complex. In the initial microseconds after the explosion, a fireball is formed around the bomb by the massive numbers of thermal x-rays released by the explosion process. These x-rays cannot travel very far in the lower atmosphere before reacting with molecules in the air, so the result is a fireball that rapidly forms within about 10 metres (33 ft) and does not expand. This is known as a "radiatively driven" fireball.

Inside the radiative fireball the bomb itself is rapidly expanding due to the heat generated by the nuclear reactions. This moves outward at supersonic speeds, creating a hydrodynamic shock wave at its outer edge. After a brief period this shock front reaches and then passes the initial radiative fireball. The shock wave contains so much energy that the compression heating it causes in the air causes it to glow. At the point in the explosion captured in the above photo, the shock front has passed the original radiative fireball, and is now about twice its size.

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Wikipedia