Diagram of features of the 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. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. The Sun (Sol is the Star at the center of the Solar System.

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.

Solar wind

Main articles: Solar wind and interplanetary medium

Coronal mass ejections sending material out into the heliosphere. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as Planets Asteroids

The solar wind consists of particles, ionized atoms from the solar corona, and fields, in particular magnetic fields. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge A corona is a type of plasma " atmosphere " of the Sun or other celestial body extending millions of Kilometres into space most easily In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges As the Sun rotates once in approximately 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation 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. A geomagnetic storm or solar storm is a temporary disturbance of the Earth 's Magnetosphere caused by a disturbance in Space weather. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object.

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. The Solar and Heliospheric Observatory ( SOHO) is a Spacecraft that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December ^[3]

Structure

Heliospheric current sheet

The heliospheric current sheet out to the orbit of Jupiter

Main article: Heliospheric current sheet

The heliospheric current sheet is a ripple in the heliosphere created by the Sun's rotating magnetic field. The heliospheric current sheet (HCS is the surface within the Solar System where the polarity of the Sun 's Magnetic field changes from The heliospheric current sheet (HCS is the surface within the Solar System where the polarity of the Sun 's Magnetic field changes from 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 solar system, in logarithmic scale, showing the outer extent of the heliosphere, the Oort cloud and Alpha Centauri

The heliosphere's outer structure is determined by the interactions between the solar wind and the winds of interstellar space. The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, Alpha Centauri (α Centauri / α Cen also known as Rigil Kentaurus, Rigil Kent, or Toliman, is the brightest Star in the southern Constellation 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. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. This takes place in several stages:

• The solar wind is traveling at supersonic speeds within the solar system. For other uses see Supersonic. The term supersonic is used to define a speed that is over the Speed of sound ( Mach 1 At the termination shock, a standing shock wave, the solar wind falls below its speed of sound and becomes subsonic. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called
• 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 Galactic Center is the rotational center of the Milky Way Galaxy. The bow shock, outside the heliopause, is a turbulent region caused by the pressure of the advancing heliopause against the interstellar medium. A bow shock is a boundary between a Magnetosphere and an ambient medium

Termination shock

An example of the termination shock, relating it to a sink basin.

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. Physical compression is the result of the subjection of a material to Compressive stress, resulting in reduction of Volume. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges In our solar system the termination shock is believed to be 75 to 90 astronomical units^[5] from the Sun. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the The termination shock boundary fluctuates in its distance from the sun as a result of fluctuations in solar flare activity; i. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares 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. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called

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. In Plumbing, a sink or basin is a bowl-shaped fixture that is used for washing hands or small objects such as Food, dishes A hydraulic jump is a phenomenon in the science of Hydraulics which is frequently observed in open channel flow such as Rivers and Spillways. 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. A hydraulic jump is a phenomenon in the science of Hydraulics which is frequently observed in open channel flow such as Rivers and Spillways. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the A bow shock is a boundary between a Magnetosphere and an ambient medium

Evidence presented at a meeting of the American Geophysical Union in May 2005 by Dr. The American Geophysical Union (or AGU) is a nonprofit organization of Geophysicists consisting of over 50000 members from over 135 countries 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. Edward C "Ed" Stone is a Professor of Physics at Caltech. The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, In contrast, Voyager 2 began detecting returning particles when it was only 76 AU from the sun, in May 2006. Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. 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. The Interstellar Boundary Explorer ( IBEX) is a NASA

Heliosheath

A diagram depicting Voyager 1 at its relative position in the heliosheath. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the Since then, Voyager 2 has joined it in the 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. The Sun (Sol is the Star at the center of the Solar System. The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the In Astronomy, a coma (from the Latin word for "hair" is the nebulous envelope around the nucleus of a Comet. 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. The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977.

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. A spacecraft is a Vehicle or machine designed for Spaceflight. Pioneer 10 ( Pioneer-F) was the first Spacecraft to travel through the Asteroid belt, which it entered on July 15, 1972 Pioneer 11 was the second mission (after its sister probe Pioneer 10) to investigate Jupiter and the Outer solar system and the first to explore the planet The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. The Solar System consists of the Sun and those celestial objects bound to it by Gravity.

• It is believed that Voyager 1 crossed the termination shock and entered the heliosheath in the middle of December 2004, at a distance of 94 AU. The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the ^[8] An earlier report that this had occurred as early as August 2002 (at 85 AU) is now generally believed to have been premature. ^[9]
• However, Voyager 2 crossed the termination shock on August 30, 2007 at 84 AU,^[10] showing evidence of denting in the heliosphere, believed to be caused by an interstellar magnetic field. Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. Events 1363 - Beginning date of the Battle of Lake Poyang; the forces of two Chinese rebel leaders— Chen Youliang and Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. ^[11]

Since the Pioneer 10 and 11 probes have both stopped communicating, it will never be known where they pass into the heliosheath. Pioneer 10 ( Pioneer-F) was the first Spacecraft to travel through the Asteroid belt, which it entered on July 15, 1972 Pioneer 11 was the second mission (after its sister probe Pioneer 10) to investigate Jupiter and the Outer solar system and the first to explore the planet

Heliopause

The heliopause is the boundary between the heliosphere and the interstellar medium outside the solar system. The Solar System consists of the Sun and those celestial objects bound to it by Gravity. As the solar wind approaches the heliopause, it slows suddenly, forming a shock wave called the termination shock of the solar wind. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the

The heliopause is the theoretical 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. The Sun (Sol is the Star at the center of the Solar System. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star.

Hypotheses

According to one hypothesis,^[12] there exists a region of hot hydrogen known as the hydrogen wall between the bow shock and the heliopause. A hypothesis (from Greek) consists either of a suggested explanation for a phenomenon (an event that is observable or of a reasoned proposal suggesting a possible 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. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause. The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. Voyager 1 reached the termination shock on May 23-24, 2005,^[13] and Voyager 2 reached it on August 30, 2007 according to NASA. The Voyager 1 spacecraft is a 722-kilogram robotic Space probe of the outer Solar system and beyond launched September 5, Events 1430 - Siege of Compiègne: Joan of Arc is captured by the Burgundians while leading an army to relieve Compiègne Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. Events 1363 - Beginning date of the Battle of Lake Poyang; the forces of two Chinese rebel leaders— Chen Youliang and Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program ^[14] 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. The Interstellar Boundary Explorer ( IBEX) is a NASA

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. A magnetopause flows along the boundary between a Magnetic field, (see Magnetosphere) and surrounding plasma. A magnetosphere' is a highly magnetized region around and possessed by an Astronomical object.

Image and artist's impression of the bow shock left by the star R Hydrae. R Hydrae is a Mira-type variable star in the Constellation Hydra.

Bowshock

Main article: Bow shock

It is hypothesized that the Sun also has a bowshock produced in its travels within the interstellar medium, as shown in the figure. A bow shock is a boundary between a Magnetosphere and an ambient medium The Sun (Sol is the Star at the center of the Solar System. A bow shock is a boundary between a Magnetosphere and an ambient medium The shock is named from its resemblance to the wake left by a ship's bow and is formed for similar reasons, though of plasma not water. Bowshocks will occur if the interstellar medium is moving supersonically 'toward' the sun, since its solar wind moves 'away' from Sol supersonically. When the interstellar wind hits the heliosphere it slows and creates a region of turbulence. NASA's Robert Nemiroff and Jerry Bonnell believe the solar bowshock may lie at around 230 AU^[5] from the Sun. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program

This phenomenon has been observed by NASA's orbital GALEX telescope. The Galaxy Evolution Explorer is an Orbiting Ultraviolet Space telescope that was launched on April 28, 2003. The red giant star Mira in the constellation Cetus has been shown to have both a cometlike debris tail of ejecta from the star, and a distinct bowshock preceding it in movement through space at over 130 kilometers per second. Mira, ˈmaɪrə also known as Omicron Ceti (or ο Ceti / ο Cet is a Red giant Star estimated 200-400 Light years away in Cetus ( Κῆτος Kētos, referring to the sea monster Cetus) is a Constellation of the northern winter sky in the region known as the Water

See also

• 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. Events 330 - Byzantium is renamed ''Nova Roma'' during a dedication ceremony but is more popularly referred to as Constantinople Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Retrieved on 2007-05-12. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1191 - Richard I of England marries Berengaria of Navarre.  
• UI's Don Gurnett Says Voyager 1 Is Approaching Edge Of Solar System December 8, 2003 Univ. Events 1609 - Biblioteca Ambrosiana opens its reading room the second public library of Europe. Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. of Iowa Press release
• "Heliopause Seems to Be 23 Billion Kilometres", Universe Today, December 9, 2003. Events 536 - Byzantine General Belisarius enters Rome while the Ostrogothic garrison peacefully leaves the city Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. Retrieved on 2007-08-08. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1220 - Sweden is defeated by Estonian tribes in the Battle of Lihula.  

External links

• The Solar and Heliospheric Research Group at the University of Michigan
• Observing objectives of NASA's Interstellar Probe. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program
• CNN: NASA: Voyager I enters solar system's final frontier – May 25, 2005
• New Scientist: Voyager 1 reaches the edge of the solar system – May 25, 2005
• Surprises from the Edge of the Solar System – Voyager 1 Newest Findings as of September 2006
• The heliospheric hydrogen wall and astrospheres
• Heliosphere, has a diagram. Events 1085 - Alfonso VI of Castile takes Toledo Spain back from the Moors. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Events 1085 - Alfonso VI of Castile takes Toledo Spain back from the Moors. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar.
• Global Structure of the Heliosphere
• Publications in Refereed Journals
• NASA GALEX (Galaxy evolution Explorer) homepage at Cal Tech
• Heliosphere Astronomy Cast episode #65, includes full transcript. Astronomy Cast is an educational Nonprofit Podcast discussing various topics in the field of Astronomy.