The Heliosphere and the Boundary of the Solar System

The heliosphere is that part of space dominated by the Sun and is filled with charged particles streaming off the Sun – the 'Solar Wind'. For the first ten billion km of its journey from the Sun the solar wind travels at over 1,000,000 km/h. As it moves away from the Sun, it gets slower and weaker before dispersing into the interstellar medium. At some point the solar wind will slow to below the speed of sound. This happens at the termination shock. The shock arises because solar wind particles are emitted at about 400 km/s, while the speed of sound in the interstellar medium) is about 100 km/s and varies with density. The interstellar medium, although very low in density, nonetheless has a constant pressure associated with it; the pressure and speed of the solar wind decreases with the square of the distance from the star. When the speed of the wind falls below the speed of sound a shock wave is formed. The solar wind continues to slow until it reaches the heliopause where the interstellar medium and solar wind pressures balance. The termination shock was successfully detected by both Voyagers 1 and 2 in 2004 and 2007 respectively.

As the Sun rotates, its magnetic field rotates with it, sweeping out a spiral as the Solar Wind moves away from the Sun.

The 'stagnation region' within the heliosheath, starting around 113 AU, was detected by Voyager 1 in 2010. The solar wind velocity dropped to zero and the magnetic field intensity doubled, with the number of high energy electrons from the Milky Way increasing by a factor of one hundred. In May 2012 at 120 AU, Voyager 1 detected a sudden increase in cosmic rays, indicating the proximity of the heliopause.

The heliosheath is the region of the heliosphere beyond the termination shock in which the Solar wind is slowed, compressed and made turbulent by its interaction with the interstellar medium. Its distance from the Sun is approximately 80 to 100 astronomical units (AU) at its closest point. It was once hypothesised that as the Sun moves through the interstellar medium, it creates a bow wave, dues to the interstellar medium moving supersonically relative to the Solar Wind. By 2012, it was determined the Sun has no bow shock. The Solar Wind was not fast enough where it meets the interstellar medium. It was also supposed that the heliosphere would be swept back by the interstellar medium as it moved through space (below left) be it now appears to be bubble shaped (below right), dominated by particle pressure and magnetic fields.

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