When space is concerned, a more reasonable comparison of an earthly shock to that found in space is seen in the ocurrance of lightning. Electrical energy heats, pressurizes and ionizes the air within the channel, causing molecules to dissociate to nitrogen and oxygen atoms. These high energy particles form a stream of plasma, a good electrical conductor which is the visible lightning chain. The sound wave we hear when it thunders is generated when a shock wave radius increases and then drops in intensity by heating and compressing greater volumes of air. On a smaller scale, sending an electrical charge through a wire will cause the density of the wire to increase at the shock, expand along the wire and eventually break. Solar flares from the sun work in much the same way. Ionized gas locked into magnetic field lines accelerates towards the earth. Compression and heating across the shock wave takes place by magenetic field interactions rather than on the molecular/atomic collisional encounters, hence, collisionless shock. The small percentage of the ionized gas that reaches the Earth's atomosphere affects radio communications with its high electrically charged energy, and creates the natural phenomenon of the aurora borealis.
The most simple definition of a shock is a reduction of upstream to downstream flow of supersonic material to subsonic. What does this mean?? Basically, a fluid such as plasma, a product of the solar wind is traveling towards the Earth at a very high velocity- higher than the speed of sound. This fast-moving plasma then encounters the Earth's magnetosphere which has a magnetic field that diverts the plasma flow around it, thus preventing it from streaming right into the earth's atmosphere. When the plasma encounters the magnetosphere, interaction between the charged ions of the plasma and the magnetic field of the magnetosphere cause the plasma streams to decrease in velocity. This is the transition of the plasma streams from supersonic upstream to subsonic downstream flow. At this point, the ions contained in the plasma have two options: to flow through the magnetopause toward the Earth, or to be reflected back upstream by the shock, toward the foreshock. Driven at a high velocity by the solar wind, the plasma bunches up and forms ripples, a shock, as it encounters the magnetopshere. Characteristics of the shock include a multiplicity of waves, as well as turbulence and instability. The shock diverts the majority of the plasma ions around the magnetosphere, allowing only a few to pass though, creating such phenomena as the Aurora Borealis.
Last modified 08/13/96 by J. Linville