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 .
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