Heliosphere

The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) 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.

For the first ten billion kilometres of its radius, the solar wind travels at over a million kilometres per hour.^[1][2] As it begins to collide with the interstellar medium, it slows to subsonic speeds before finally ceasing altogether. The point where the solar wind becomes subsonic is the termination shock; the point where the interstellar medium and solar wind pressures balance is called the heliopause; the point where the interstellar medium, travelling in the opposite direction, becomes subsonic as it collides with the heliosphere is the bow shock.

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Solar wind

Main articles: Solar wind and interplanetary medium

  The solar wind consists of particles, ionized atoms from the solar corona, and fields, in particular magnetic fields. As the Sun rotates once in approximately 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.^[3]

Structure

Heliospheric current sheet

Main article: Heliospheric current sheet

The heliospheric current sheet is a ripple in the heliosphere created by the Sun's rotating magnetic field. Extending throughout the heliosphere, it is considered the largest structure in the Solar System and is said to resemble a "ballerina's skirt"^[4]

Outer structure

  The heliosphere's outer structure is determined by the interactions between the solar wind and the winds of interstellar space. 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 Neptune, 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 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.

Termination shock

  The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (with respect to the star) due to interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. In our solar system the termination shock is believed to be 75 to 90 astronomical units^[5] from the Sun. The termination shock boundary fluctuates in its distance from the sun as a result of fluctuations in solar flare activity, i.e. changes in the ejections of gas and dust from the sun.

The shock arises because solar wind particles are emitted from stars at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s. (The exact speed depends on the density, which fluctuates considerably.) The interstellar medium, although very low in density, nonetheless has a constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the star. As one moves far enough away from the star, the pressure from the interstellar medium becomes sufficient to slow the solar wind down to below its speed of sound; this causes a shock wave.

Other termination shocks can be seen in terrestrial systems; perhaps the easiest may be seen by simply running a water tap into a sink creating a Hydraulic jump. Upon hitting the floor of the sink, the flowing water spreads out at a speed that is higher than the local wave speed, forming a disk of shallow, rapidly diverging flow (analogous to the tenuous, supersonic solar wind). Around the periphery of the disk, a shock front or wall of water forms; outside the shock front, the water moves slower than the local wave speed (analogous to the subsonic interstellar medium).

Further information: Hydraulic jump

Going outward from the sun, the termination shock is followed by the Heliopause where solar wind particles are stopped by the interstellar medium, then the bow shock past which particles from the interstellar medium are no longer excited.

Evidence presented at a meeting of the American Geophysical Union in May 2005 by Dr. Ed Stone suggests that the Voyager 1 spacecraft passed termination shock in December 2004, when it was about 94 AU from the sun, by virtue of the change in magnetic readings taken from the craft. In contrast, Voyager 2 began detecting returning particles 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.^[6]

The Interstellar Boundary Explorer (IBEX) mission will attempt to gather more data on the solar system's termination shock.

Heliosheath

The heliosheath is the region of the heliosphere beyond the termination shock. Here the 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; however, the heliosheath is shaped like the coma of a comet, and trails several times that distance in the direction opposite to the Sun's path through space. At its windward side, its thickness is estimated to be between 10 and 100 AU.^[7] The current mission of the Voyager 1 and Voyager 2 space probes includes studying the heliosheath.

In May 2005, NASA announced that Voyager 1 had crossed the termination shock and entered the heliosheath in December 2004, at a distance of 94 AU. An earlier report that this had occurred in August 2002 (at 85 AU) is now generally believed to have been premature.

Heliopause

The heliopause is the boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars.

Hypothesis

According to one hypothesis,^[8] there exists a region of hot hydrogen known as the hydrogen wall between the bow shock and the heliopause. The wall is composed of interstellar material interacting with the edge of the heliosphere.

Another hypothesis suggests that the heliopause could be smaller on the side of the solar system facing the Sun's orbital motion through the galaxy. It may also vary depending on the current velocity of the solar wind and the local density of the interstellar medium. It is known to lie far outside the orbit of Neptune. The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause.Voyager 1 reached the termination shock on May 23-24, 2006,^[9] and Voyager 2 reached it on August 30, 2007 according to NASA^[10]. It is anticipated that both missions may ultimately reach the heliopause itself. In the mean time, the Interstellar Boundary Explorer (IBEX) mission will attempt to image the heliopause from Earth orbit within two years of its 2008 launch.

When particles emitted by the sun bump into the interstellar ones, they slow down while releasing energy. Many particles accumulate in and around the heliopause, highly energised by their negative acceleration, creating a shock wave.

An alternative definition is that the heliopause is the magnetopause between the solar system's magnetosphere and the galaxy's plasma currents.

Detection by spacecraft

  The precise distance to, and shape of, the heliopause is still uncertain. Interplanetary/interstellar 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.^[6]

Bow shock

Main article: Bow shock

It is hypothesised that the Sun also has a bow shock as it travels through the interstellar medium, as shown in the figure. The bow shock gains its name from its resemblance to the wake left by the bow of a ship, and forms for much the same reason, albeit of plasma, rather than water. Bow shocks will occur if the interstellar medium is moving supersonically towards the Sun, since the sun's solar wind is moving supersonically away from the Sun. As the interstellar wind slams into the heliosphere, it slows, creating a region of turbulence. According to Robert Nemiroff and Jerry Bonnell of NASA, the solar bow shock may lie at around 230 AU^[5] from the Sun.
This phenomenon has actually been observed by the GALEX (NASA Galaxy Evolution Explorer) orbiting telescope. The well known red giant star Mira which can be observed in the constellation of Cetus has been seen to have both a comet like tail of debris ejected from the star and a distinct bow shock that precedes it as it travels through space at over 130 kilometers per second. A link to the GALEX web site can be found in the external links section of this article.

See also

• Solar wind
• Heliospheric current sheet
• Space weather
• Coronal mass ejection
• Solar flare
• Fermi glow
• Interstellar medium
• Stellar system
• Voyager Program

Notes

References and further reading

• The Heliosphere (Cosmicopia)
• The Heliosphere, MIT Space Plasma Group
• Voyager Interstellar Mission Objectives
• "Space probes reveal Solar System's bullet shape", COSMOS magazine, May 11, 2007. Retrieved on 2007-05-12. 
• UI's Don Gurnett Says Voyager 1 Is Approaching Edge Of Solar System December 8, 2003 Univ. of Iowa Press release
• "Heliopause Seems to Be 23 Billion Kilometres", Universe Today, December 9, 2003. Retrieved on 2007-08-08.