The inner solar system is permeated by swiftly flowing charged particles emanating from the Sun. The outward rush of particles and the solar magnetic field carried with them carve an enormous cavity in the interstellar medium that reaches far beyond the orbit of the most distant planets of the solar system. This immense region, a magnetic bubble of solar dominance within the vastness of space, is called the heliosphere.

The general structure of the solar wind and the heliosphere was first outlined three decades age by Eugene N. Parker of the University of Chicago. It has now been confirmed that the solar wind, as it moves radially outward from the Sun, brings with it the imprint of the solar atmosphere: Certain parts of the sun emit a high speed conducting (plasma) wind in vast streams that flow hundreds of kilometers per second (about a million miles per hour).

As it travels, the solar wind expands over an increasingly large volume. Eventually the solar wind plasma is spread so thinly that it can no longer push outward against the small inward pressure of the local interstellar medium. The wind does not slow down gradually at this point, however, because its velocity is greater than that at which disturbances can propogate within it. Instead, the solar wind undergoes a sudden, violent change in speed.

This behavior follows from the fundamentals of supersonic fluid flow. For the motion to diminish incrementally, the downstream material must signal the upstream fluid to slow. These signals must be carried by sound waves moving through the medium. But such waves cannot propagate against flows moving faster than sound. As a result, the upstream fluid crashes into the fluid ahead, setting up a powerful disturbance called a shock wave.

As the solar wind passes through the shock, its outward velocity should slow to about one quarter of its original value. Some of the wind's kinetic energy is converted to heat, raising the temperature of the interstellar gas to more than a million degrees Celsius. Some kinetic energy goes into compressing the magnetic field.