Astrophysical plasma
Encyclopedia
An astrophysical plasma is a plasma
(an ionized gas) the physical properties of which are studied as part of astrophysics
. Much of the baryon
ic matter of the universe
is thought to consist of plasma, a state of matter
in which atoms and molecules are so hot, that they have ionized
by breaking up into their constituent parts, negatively charged electron
s and positively charged ion
s. Because the particles are charged
, they are strongly influenced by electromagnetic forces, that is, by magnetic and electric fields.
All known astrophysical plasmas are influenced by magnetic field
s. Since plasmas contain equal numbers of electrons and ions, they are electrically neutral overall and thus electric field
s play a lesser dynamical role. Because plasmas are highly conductive, any charge imbalances are readily neutralised.
across a wide range of the electromagnetic spectrum
. Because astrophysical plasmas are generally hot
, (meaning that they are fully ionized), electron
s in the plasmas are continually emitting X-ray
s through a process called bremsstrahlung
, when electrons nearly collide with atomic nuclei. This radiation may be detected with X-ray observatories
, performed in the upper atmosphere or space, such as by the Chandra X-ray Observatory
satellite. Astrophysical plasmas also emit radio waves and gamma rays.
into three different categories:
Classification of Magnetic Cosmic Plasmas
λ=Mean free path
. ρ= Larmor radius (gyroradius) of electron. λD=Debye length
. lc=Characteristic length
Adapted From Cosmical Electrodynamics (2nd Ed. 1952) Alfvén and Fälthammar
, because they are the astrophysical plasmas most directly related to the plasmas studied in the laboratory, and those studied in fusion power
experiments. They exhibit an array of complex magnetohydrodynamic
behaviors, such as turbulence
and instabilities
. Although these phenomena can occur on scales as large as the galactic core, most physicists therorize that most phenomena on the largest scales do not involve plasma effects.
cosmology
the entire universe was a plasma prior to recombination. Afterwards, much of the universe reionized
after the first quasar
s formed and emitted radiation which reionized most of the universe, which largely remains in plasma form. It is assumed by many scientists that very little baryonic matter is neutral. In particular, the intergalactic medium, the interstellar medium
, the interplanetary medium
and solar wind
s are all mainly diffuse plasmas, and star
s are made of dense plasma. The study of astrophysical plasmas is part of the mainstream of academic astrophysics and is taken in account for in the standard cosmological model; however, current models indicate that plasma processes have little role to play in forming the very largest structures, such as voids
, galaxy cluster
s and supercluster
s.
may have been the first to predict that space is filled with plasma
. He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electron
s and flying electric ion
s of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space."
In 1937, plasma physicist Hannes Alfvén
argued that if plasma pervaded the universe, then it could generate a galactic magnetic field. During the 1940s and 50s, Alfvén developed magnetohydrodynamics
(MHD) which enables plasmas to be modelled as waves in a fluid, for which Alfvén won the 1970 Nobel Prize for physics. MHD is a standard astronomical tool.
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
(an ionized gas) the physical properties of which are studied as part of astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
. Much of the baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
ic matter of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
is thought to consist of plasma, a state of matter
State of matter
States of matter are the distinct forms that different phases of matter take on. Solid, liquid and gas are the most common states of matter on Earth. However, much of the baryonic matter of the universe is in the form of hot plasma, both as rarefied interstellar medium and as dense...
in which atoms and molecules are so hot, that they have ionized
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
by breaking up into their constituent parts, negatively charged electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and positively charged ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s. Because the particles are charged
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
, they are strongly influenced by electromagnetic forces, that is, by magnetic and electric fields.
All known astrophysical plasmas are influenced by magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s. Since plasmas contain equal numbers of electrons and ions, they are electrically neutral overall and thus electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s play a lesser dynamical role. Because plasmas are highly conductive, any charge imbalances are readily neutralised.
Observational evidence
Astrophysical plasma may be studied in a variety of ways since they emit electromagnetic radiationElectromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
across a wide range of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. Because astrophysical plasmas are generally hot
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, (meaning that they are fully ionized), electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in the plasmas are continually emitting X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s through a process called bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
, when electrons nearly collide with atomic nuclei. This radiation may be detected with X-ray observatories
X-ray astronomy
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
, performed in the upper atmosphere or space, such as by the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
satellite. Astrophysical plasmas also emit radio waves and gamma rays.
Space plasma characteristics
Space plasma pioneers Hannes Alfvén and Carl-Gunne Fälthammar divided the plasmas in the solar systemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
into three different categories:
Classification of Magnetic Cosmic Plasmas
| Characteristic | Space plasma density categories (Note that density does not refer to only particle density) | Ideal comparison | ||
| High density | Medium Density | Low Density | ||
| Criterion | λ << ρ | λ << ρ << lc | lc << λ | lc << λD |
| Examples | Stellar interior Solar photosphere | Solar chromosphere/corona Interstellar/intergalactic space Ionosphere above 70 km | Magnetosphere during magnetic disturbance. Interplanetary space | Single charges in a high vacuum |
| Diffusion | Isotropic | Anisotropic | Anisotropic and small | No diffusion |
| Conductivity | Isotropic | Anisotropic | Not defined | Not defined |
| Electric field parallel to B in completely ionized gas | Small | Small | Any value | Any value |
| Particle motion in plane perpendicular to B | Almost straight path between collisions | Circle between collisions | Circle | Circle |
| Path of guiding centre parallel to B | Straight path between collisions | Straight path between collisions | Oscillations (e.g. between mirror points) | Oscillations (e.g. between mirror points) |
| Debye Distance λD | λD << lc | λD << lc | λD << lc | λD >> lc |
| Magnetohydrodynamics Magnetohydrodynamics Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes... suitability | Yes | Approximately | No | No |
λ=Mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
. ρ= Larmor radius (gyroradius) of electron. λD=Debye length
Debye length
In plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...
. lc=Characteristic length
Adapted From Cosmical Electrodynamics (2nd Ed. 1952) Alfvén and Fälthammar
Research and investigation
Both plasma physicists and astrophysicists are interested in active galactic nucleiActive galactic nucleus
An active galactic nucleus is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and...
, because they are the astrophysical plasmas most directly related to the plasmas studied in the laboratory, and those studied in fusion power
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...
experiments. They exhibit an array of complex magnetohydrodynamic
Magnetohydrodynamics
Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
behaviors, such as turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
and instabilities
Instability
In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds...
. Although these phenomena can occur on scales as large as the galactic core, most physicists therorize that most phenomena on the largest scales do not involve plasma effects.
In physical cosmology
In the big bangBig Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
the entire universe was a plasma prior to recombination. Afterwards, much of the universe reionized
Reionization
In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages," and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of...
after the first quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s formed and emitted radiation which reionized most of the universe, which largely remains in plasma form. It is assumed by many scientists that very little baryonic matter is neutral. In particular, the intergalactic medium, the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
, the interplanetary medium
Interplanetary medium
The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move.-Composition and physical characteristics:...
and solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
s are all mainly diffuse plasmas, and star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s are made of dense plasma. The study of astrophysical plasmas is part of the mainstream of academic astrophysics and is taken in account for in the standard cosmological model; however, current models indicate that plasma processes have little role to play in forming the very largest structures, such as voids
Void (astronomy)
In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe, that contain very few, or no, galaxies. They were first discovered in 1978 during a pioneering study by Stephen Gregory and Laird A. Thompson at the Kitt Peak National Observatory...
, galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
s and supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
s.
History
In 1913, Norwegian explorer and physicist Kristian BirkelandKristian Birkeland
Kristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
may have been the first to predict that space is filled with plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and flying electric ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space."
In 1937, plasma physicist Hannes Alfvén
Hannes Alfvén
Hannes Olof Gösta Alfvén was a Swedish electrical engineer, plasma physicist and winner of the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics . He described the class of MHD waves now known as Alfvén waves...
argued that if plasma pervaded the universe, then it could generate a galactic magnetic field. During the 1940s and 50s, Alfvén developed magnetohydrodynamics
Magnetohydrodynamics
Magnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
(MHD) which enables plasmas to be modelled as waves in a fluid, for which Alfvén won the 1970 Nobel Prize for physics. MHD is a standard astronomical tool.