Astrophysical plasma

An astrophysical plasma is a plasma (an ionized gas) the physical properties of which are studied as part of astrophysics. Much 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. 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 fields. Since plasmas contain equal numbers of electrons and ions, they are electrically neutral overall and thus electric fields 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 radiation across a wide range of the electromagnetic spectrum. 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 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.

Space plasma characteristics

Space plasma pioneers Hannes Alfvén and Carl-Gunne Fälthammar divided the plasmas in the solar system 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 suitability	Yes	Approximately	No	No

λ=Mean free path. ρ= Larmor radius (gyroradius) of electron. λ_D=Debye length. l_c=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 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 therorize 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 assumed 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.

History

In 1913, Norwegian explorer and physicist Kristian Birkeland 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 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."^[1]

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.

Footnotes

1. ^ "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720)

External links

• "US / Russia Collaboration in Plasma Astrophysics"
• Website about Astrophysical Plasma