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An astrophysical plasma is a plasma (an ionized gas) found in astronomy whose physical properties are studied in the science of astrophysics. The vast majority of the baryonic 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 electrons and positively charged ions. Although influenced by gravity, because the particles are charged, they are also strongly influenced by electromagnetic forces, that is, by magnetic and electric fields.
All known astrophysical plasmas are magnetic. They also contain equal numbers of electrons and ions so that they are electrically neutral overall. And because plasmas are highly conductive, any charge imbalances are readily neutralised. However, because plasma phenonenon are very complex, charge imbalances can occur, resulting in a characteristic known as quasineutrality. An example is the influence of our Sun's magnetic field on the electrons and ions in the interplanetary medium (or Solar wind) resulting in the heliospheric current sheet, the largest structure in the Solar system.
Characteristics
Space plasma pioneers Hannes Alfvén and Carl-Gunne Fälthammar divided cosmic plasmas into three different categories (note that other characteristics of low-particle-density interstellar and intergalactic plasmas, means that they are characterised as medium density):
| 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 Ionopshere above 70km | 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 (eg. between mirror points) | Oscillations (eg. between mirror points) |
| Debye Distance λD | λD << lc | λD << lc | λD << lc | λD >> lc |
| Magnetohydrodynamics suitability | Yes | Approximately | No | No |
Adapted From Cosmical Electrodynamics (2nd Ed. 1952) Alfvén and Fälthammar
Astrophysical plasma may be studied in a variety of ways since they emit electromagnetic radiation across a wide range of the electromagnetic spectrum. For example, cosmic plasmas in stars emits light as can be seen by gazing at the night sky. And because astrophysical plasmas are generally hot, (meaning that they are fully ionized), electrons in the plasmas are continually emitting X-rays through a process called bremsstrahlung, when electrons nearly collide with atomic nuclei. This raditation may be detected with X-ray observatories, performed in the upper atmosphere or space, such as by the Chandra X-ray Observatory satellite. Space plasmas also emit radio waves and gamma rays.
Research and investigation
Both plasma physicists and astrophysicists are interested in active galactic nuclei, 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 believe that most phenomena on the largest scales do not involve plasma effects.
In physical cosmology
In the big bang cosmology the entire universe was a plasma prior to recombination. Afterwards, much of the universe reionized after the first quasars formed and emitted radiation which reionized most of the universe, which largely remains in plasma form. It is believed by many scientists that very little baryonic matter is neutral. In particular, the intergalactic medium, the interstellar medium, the interplanetary medium and solar winds are all mainly diffuse plasmas, and stars 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 clusters and superclusters.
In the 1950s, Hannes Alfvén proposed an ambiplasma model of the universe involving interacting plasmas of matter and antimatter. This model has been falsified by observations, however a small minority of plasma researchers advocate plasma cosmology, a non-standard cosmology based in which plasma effects are relevant on the largest scales.
History
In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions 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. "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720)
In 1937, when interstellar space was thought to be a vacuum, plasma physicist Hannes Alfvén argued that if plasma pervaded the universe, then it could carry electric currents that could generate a galactic magnetic field. During the 1940s and 50s, Alfvén develeoped 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.
However, Hannes Alfvén and co-author Carl-Gunne Fälthammar, wrote in their book Cosmical Electrodynamics (1952, 2nd Ed.):
- "It should be noted that the fundamental equations of magnetohydrodynamics rest on the assumption that the conducting medium can be considered as a fluid. This is an important limitation, for if the medium is a plasma it is sometimes necessary to use a microscopic description in which the motion of the constituent particles is taken into account. Examples of plasma phenomena invalidating a hydromagnetic description are ambipolar diffusion, electron runaway, and generation of microwaves".
In 1974, Alfvén's theoretical work on field-aligned electric currents in the aurora, based on earlier work by Kristian Birkeland, was confirmed by satellite, and Birkeland currents were discovered. In his later years, Alfvén was known for highlighting the importance of treating astrophysical plasmas in a proper theoretical fashion Alfvén, Hannes, "Model of the plasma universe", IEEE Transactions on Plasma Science (ISSN 0093-3813), vol. PS-14, Dec. 1986, p. 629-638 He wrote:
- "The basic difference between the first and second approaches is to some extent illustrated by the terms ionized gas and plasma which, although in reality synonymous, convey different general notions. The first term gives an impression of a medium that is basically similar to a gas, especially the atmospheric gas we are most familiar with. In contrast to this, a plasma, particularly a fully ionized magnetized plasma, is a medium with basically different properties: Typically it is strongly inhomogeneous and consists of a network of filaments produced by line currents and surfaces of discontinuity. These are sometimes due to current sheaths and, sometimes, to electrostatic double layers."
| First approach (pseudo-plasma) | Second approach (real plasma) |
| Homogeneous models | Space plasmas often have a complicated inhomogeneous structure |
| Conductivity σE = ∞ | σE depends on current and often suddenly vanishes |
| Electric field E|| alongmagnetic field = 0 | E|| often <> ∞ |
| Magnetic field lines are "frozen-in" and "move" with the plasma | Frozen-in picture is often completely misleading |
| Electrostatic double layers are neglected | Electrostatic double layers are of decisive importance in low-density plasma |
| Instabilities are neglected | Many plasma configurations are unrealistic because they are unstable |
| Electromagnetic conditions are illustrated by magnetic field line pictures | It is equally important to draw the current lines and discuss the electric circuit |
| Filamentary structures and current sheets are neglected or treated inadequately | Currents produce filaments or flow in thin sheets |
| Maxwellian velocity distribution | Non-Maxwellian effects are often decisive Cosmic plasmas have a tendency to produce high-energy particles |
| Theories are mathematically elegant and very "well developed" | Theories are not very well developed and are partly phenomenological |
Footnotes
External links
"Astrophysical plasma"