Heliosphere

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The heliosphere is a bubble in space produced by the solar wind. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself.

Drawing of features of the heliosphere.

1 Overview
2 Detection by spacecraft
3 Solar wind
4 See also
5 Notes
6 References and further reading

[edit] Overview

The solar wind streams away from the Sun in all directions at speeds of several hundred km/s (about 1,000,000 mph) in the Earth's vicinity. At some distance from the Sun, well beyond the orbit of Pluto, this supersonic wind must slow down to meet the gases in the interstellar medium. This takes place in several stages:

The solar wind is traveling at supersonic speeds within the solar system. At the termination shock, a standing shock wave, the solar wind falls below its speed of sound (about 250,000 mph or 100 km/s) and becomes subsonic.
Once subsonic, the solar wind may be affected by the ambient flow of the interstellar medium. Pressures cause the wind to form a comet-like tail behind the Sun, called the heliosheath.
The outer surface of the heliosheath, where the heliosphere meets the interstellar medium, is called the heliopause. This is the edge of the entire heliosphere.
The heliopause causes turbulence in the interstellar medium as the sun orbits the Galactic Center. The bow shock, outside the heliopause, is a turbulent region caused by the pressure of the advancing heliopause against the interstellar medium.

[edit] Detection by spacecraft

Voyager 1 is in the heliosheath

The precise distance to, and shape of, the heliopause is still uncertain. Interplanetary spacecraft such as Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 are traveling outward through the solar system and will eventually pass through the heliopause.

In May 2005, it was announced that Voyager 1 had crossed the termination shock and entered the heliosheath in December 2004, at a distance of 85 AU. In contrast, Voyager II began detecting returning particles suggesting it was entering the termination shock when it was only 76 AU from the sun, in May 2006. This implies that the heliosphere may be irregularly shaped, bulging outwards in the sun's northern hemisphere and pushed inward in the south.[1].

[edit] Solar wind

The solar wind consists of particles, ionized atoms from the solar corona, and fields, in particular magnetic fields. As the Sun rotates once in about 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. Variations in the Sun's magnetic field are carried outward by the solar wind and can produce magnetic storms in the Earth's own magnetosphere.

In March 2005 it was reported that measurements by the Solar Wind Anisotropies (SWAN) instrument onboard the Solar and Heliospheric Observatory (SOHO) have shown that the heliosphere, the solar wind filled volume which prevents the solar system from becoming embedded in the local (ambient) interstellar medium, is not axisymmetrical, but is distorted, very likely under the effect of the local galactic magnetic field.^[2]