A pseudo-plasma is one that approximates a real plasma, and consequently one whose properties and characteristics may not accurately describe a real plasma.
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 [1] 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|| along magnetic 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.[2]
Dr Timothy E. Eastman, senior scientist at NASA’s Goddard Space Flight Center, also notes that:
“One commonly held myth is that plasma processes are only significant for high ionization fraction; in fact, ionospheric plasmas at ~100 km altitude have ionization fractions less than about 1% and yet plasma processes are essential in treating ionospheric dynamics. Another commonly held myth is that plasma effects are 100% shielded out for scale lengths large compared to the Debye length; although direct electric field influences are typically shielded out within a few Debye lengths (an effect offset by overlapping Debye spheres), it is becoming increasingly recognized that long-range magnetic fields and currents have no known scale length limit [7, 10]”[3]
Footnotes
- ↑ Alfvén, Hannes, “Model of the plasma universe”, IEEE Transactions on Plasma Science (ISSN 0093-3813), vol. PS-14, Dec. 1986, p. 629-638
- ↑ Hannes Alfvén and Gustaf Arrhenius, Evolution of the Solar System Chapter 15
- ↑ Eastman, Timothy E , “A Survey of Plasmas and Their Applications”, Plasma Physics Applied, Research Signpost C. Grabbe, editor, page 6 (citing [7] Goedbloed, J. P. and S. Poedts, 2004. Principles of Magnetohydrodynamics: Fith Applications to Laboratory and Astrophysical Plasmas, Cambridge: Cambridge University Press.; [10] Peratt, Anthony, 1992. Physics of the Plasma Universe, New York Springer-Verlag. ).