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We now consider the interplanetary medium (heliosphere) within the Solar System as distinct from the local interstellar medium. The Sun moves with a velocity of about 20 km s-1 relative to the local interstellar medium. The solar wind produces a bow shock ahead of the Sun. This discontinuity (heliopause) defines the extent of the heliosphere in the upstream direction. It is anticipated that the Pioneer 11 or Voyager deep space probes will eventually confirm the existence of this boundary. The neutral interstellar gas is largely unaffected by the heliopause, so that the local neutral medium presumably streams relatively freely through the Solar System. In contrast, ions and charged dust grains are probably deflected by the advancing heliosphere. Cosmic rays are relativistic particles and therefore penetrate the heliosphere.

The most local gas we can associate with the local interstellar medium has the properties of the warm neutral medium. The solar radiation is backscattered by the local medium. Studies which exploit the resonance lines of hydrogen and helium show that the gas has a temperature of 8000 K, a hydrogen density of 0.25 cm-3 and a helium density of 0.02 cm-3. Absorption line measurements toward nearby stars indicate that this warm gas is only a few parsecs in extent with hotter 106K gas occupying most of the volume within 100 pc of the Sun.