The Interplanetary Medium Family
Like the interstellar medium, the interplanetary medium is composed of gas and dust (P 15), as well as energetic particles from the solar wind (P 16) and cosmic rays (S 30). The solar wind creates the heliosphere, a bubble within the more general interstellar wind, and while most of the material in the heliosphere emanates from the Sun itself, a small part consists of neutral gas that makes its way into the Solar System rather than flowing outward from the Sun. Thus, whereas both the solar wind and most interplanetary dust originate within the Solar System, interplanetary gas is a rarefied neutral gas flowing through the Solar System at a speed of about 25 km/sec. More precisely, the Solar System itself is moving through what is sometimes called the “local interstellar cloud” at that speed, so space scientists measure the wind as would a motorcyclist speeding down the highway. The particles become charged as they approach the Sun, and eventually may become Anomalous Cosmic Rays (P 17).