Astrophysical plasma

An astrophysical plasma is a plasma (an ionised gas) found in astronomy whose physical properties are studied in the science of astrophysics. The vast majority of the volume 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):

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 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

λ=Mean free path. ρ= Lamor radius of electron. λ_D=Debye length. l_c=Characteristic length
Adapted From Cosmical Electrodynamics (2nd Ed. 1952) Alfvén and Fälthammar.^[2]

Alfvén and Fälthammar noted that the:

“table shows some features of interest. The first is that most densities are to be considered as very high. Except in the close vicinity of the earth there is no analogy to high-vacuum phenomena. The laboratory analogy of cosmic space is not the vacuum in a tank but a highly ionized gas of a very high density.

“Still more striking than the high densities are the very strong magnetic fields in the cosmos. In fact they are so strong that at present our laboratory resources do not suffice to produce field strengths large enough for model experiments.

“The powerful magnetic fields have two important consequences. The first is that the motion of charged particles is usually of a different type from that we are familiar with in the laboratory. The radius of curvature is very small and the particles more in the direction of the magnetic field or ‘drift’ perpendicular to it. [..]

“The other consequence is that strong electric fields are easily produced by any motion across the magnetic field. To give an example, in a reduced magnetic field of 10⁶ gauss, a velocity of 3.10⁵cm/sec causes an electric field of E = 10 e.s.u. – 3000 V/cm, and in a field of 11¹⁰ gauss the same velocity gives 30.10⁶V/cm. Thus also the electric fields in the cosmos, when reduced to laboratory scale, are often very strong.

“Finally, the time-scale transformation in [the] Table is of interest. Solar flares, coronal arcs, and also the initial phase of a magnetic storm should be regarded as very short-lived phenomena. In fact their equivalent duration is of the order of the ignition time of an electric discharge. This means that transient phenomena are very important in cosmical physics”.^[2]

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.

Anomalous characteristics

In his summary of low-density plasmas, Fälthammar notes that:^[3]

“Low-density plasmas fill the rest of the universe; i.e., they dominate planetary magnetospheres, the solar corona, and interplanetary, interstellar, and intergalactic space. They represent a very complicated state of matter which can entertain a host of waves and instabilities. Their electrodynamic properties are such that in general there does not even exist a local macroscopic relation between the electric field and electric current density. For example, finite electric current may coexist with a zero electric field, and vice versa. This means that the concept of conductivity as a local property becomes meaningless, and even remote parts of the current circuit must be taken into account.

“A simple example is that discussed in [7, sec. 5.1.31; namely, of electrons and ions trapped between two magnetic mirrors and having different angular distributions, and hence differently distributed mirror points. Even if the total charge is zero, the electrons and ions contribute differently to the local charge density, and hence they maintain an electric field without any net current. This example has been elaborated in greater detail in [42]^[4]. A case of experimentally observed magnetic-field-aligned electric fields in a magnetic mirror with no electric current has been reported in [38]^[5].” [See footnotes for quoted references, 40 and 38]

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. ^[6]

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 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.

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 ^[7] 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.”

Pseudo-Plasma Versus Real Plasma

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

Source: Evolution of the Solar System, TABLE 15.3.1.^[8]

Footnotes

External links

• “US / Russia Collaboration in Plasma Astrophysics”