Shock Waves in Space

Shock waves that we now call sonic booms first made their impression on mankind in the form of thunderclaps. Before long, man learned to create his own sonic boom by cracking a whip, in which the tip of the whip exceeds the speed of sound. Anyone who has changed targets in the pit of a firing range knows that they hear the snap of a bullet breaking the sound barrier over their head before they hear the distant crack of the rifle that fired it.

We are confronted with shock waves all the time. Ordinarily, when we think of sonic booms, we picture a supersonic airplane flying overhead. The sonic boom tracks along the ground at the same speed as the aircraft--the faster the plane, the more the boom lags behind. Larger meteors entering the atmosphere create shock waves which can be detected on the ground as low-frequency vibration known as infrasound.

Shock waves result when the matter through which the wave is passing is compressed and the molecules collide and vibrate. When the velocity of the disturbance is extreme, such as in the case of a meteor, electrons are knocked loose and the molecules are ionized.

In space, explosions are constantly occurring. It is somewhat paradoxical that the ultimate explosion, the "Big Bang" in which the universe is believed to have been created some 20 billion years ago, could not have produced a shock wave. Although "things" were sent flying in every direction, space was a total vacuum then and there was nothing to compress.

Now, there are a few particles in every cubic centimeter of space and shock waves are abundant. The ones which effect us most directly originate from solar flares on the sun. When a large solar flare erupts, electromagnetic radiation in the form of X-rays and radio waves travel to the earth at the speed of light and arrive here in about 8 minutes. The shock wave, however, is produced by an expanding, teardrop-shaped "piston" of ionized gases driving toward the earth at about 600 miles a second (the shape traces out magnetic field lines of the sun).

On arrival, the shock wave and the ionized solar gases interact with the earth's magnetic field. The hot gas is so tenuous that it scarcely affects conditions in our atmosphere, but it profoundly affects radio communications, often resulting in a total "blackout." It is also during times of large solar flares that we are likely to experience the most brilliant and spectacular auroras.