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A Birkeland current generally refers to any electric current in a space plasma, but more specifically when charged particles in the current follow magnetic field lines (hence, Birkeland currents are also known as field-aligned currents). They are caused by the movement of a plasma perpendicular to a magnetic field. Birkeland currents often show filamentary, or twisted "rope-like" magnetic structure.
Originally Birkeland currents referred to electric currents that contribute to the aurora, caused by the interaction of the plasma in the Solar Wind with the Earth's magnetosphere. The current flows earthwards down the morning side of the Earth's ionosphere, around the polar regions, and spacewards up the evening side of the ionosphere. These Birkeland currents are now sometimes called auroral electrojets. The currents were predicted in 1903 by Norwegian explorer and physicist Kristian Birkeland, who undertook expeditions into the Arctic Circle to study the aurora.
Professor Emeritus of the Alfvén Laboratory in Sweden, Carl-Gunne Fälthammar wrote (1986): "A reason why Birkeland currents are particularly interesting is that, in the plasma forced to carry them, they cause a number of plasma physical processes to occur (waves, instabilities, fine structure formation). These in turn lead to consequences such as acceleration of charged particles, both positive and negative, and element separation (such as preferential ejection of oxygen ions). Both of these classes of phenomena should have a general astrophysical interest far beyond that of understanding the space environment of our own Earth."
Additional recommended knowledge
Birkeland currents can also be created in the laboratory with multi-terawatt pulsed power generators. The resulting cross-section pattern indicates a hollow beam of electron in the form of a circle of vortices, a formation called the diocotron instability (similar, but different to the Kelvin-Helmholtz instability), that subsequently leads to filamentation. Such vortices can be seen in aurora as "auroral curls".
Birkeland currents are also one of a class of plasma phenonena called a z-pinch, so named because the azimuthal magnetic fields produced by the current pinches the current into a filamentary cable. This can also twist, producing a helical pinch that spirals like a twisted or braided rope, and this most closely corresponds to a Birkeland current. Pairs of parallel Birkeland currents can also interact; parallel Birkeland currents moving in the same direction will attract with an electromagnetic force inversely proportional to their distance apart (Note that the electromagnetic force between the individual particles is inversely proportional to the square of the distance, just like the gravitational force); parallel Birkeland currents moving in opposite directions will repel with an electromagnetic force inversely proportional to their distance apart. There is also a short-range circular component to the force between two Birkeland currents that is opposite to the longer-range parallel forces.
Electrons moving along a Birkeland current may be accelerated by a plasma double layer. If the resulting electrons approach relativistic velocities (ie. the speed of light) they may subsequently produce a Bennett pinch, which in a magnetic field will spiral and emit synchrotron radiation that includes radio, optical (ie. light), x-rays, and gamma rays.
Cosmic Birkeland currents
Plasma physicists suggest that many structures in the universe exhibiting filamentation are due to Birkeland currents. Peratt (1992) notes that "Regardless of scale, the motion of charged particles produces a self-magnetic field that can act on other collections of charged particles, internally or externally. Plasmas in relative motion are coupled via currents that they drive through each other". (See Plasma scaling). Examples include:
Source: Peratt (1992).
The history of Birekland Currents appears to be mired in politics.
After Kristian Birkeland suggested "currents there are imagined as having come into existence mainly as a secondary effect of the electric corpuscles from the sun drawn in out of space," (1908), his ideas were generally ignored in favour of an alternative theory from British mathematician Sydney Chapman.
In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper published on the generation of the current from the Solar Wind. One of Alfvén's colleagues, Rolf Boström, also used field-aligned currents in a new model of auroral electrojets (1964).
In 1966 Alfred Zmuda, J.H. Martin, and F.T.Heuring reported their findings of magnetic disturbance in the aurora, using a satellite magnetometer, but did not mention Alfvén, Birkeland, or field-aligned currents, even after it was brought to their attention by editor of the space physics section of the journal, Alex Dressler.
In 1967 Alex Dessler and one of his graduates students, David Cummings, wrote an article arguing that Zmuda et al had indeed detected field align-currents. Even Alfvén subsequently credited (1986) that Dessler "discovered the currents that Birkeland had predicted" and should be called Birkeland-Dessler currents.
In 1969 Milo Schield, Alex Dessler and John Freeman, used the name "Birkeland currents" for the first time. In 1970, Zmuda, Armstrong and Heuring wrote another paper agreeing that their observations were compatible with field-aligned currents as suggested by Cummings and Dessler, and by Bostrom, but again made no mention of Alfvén and Birkeland.
In 1970, a group from Rice University also suggested that the results of an earlier rocket experiment was consistent with field-aligned currents, and credited the idea to Boström, and Dessler and his colleagues, rather than Alfvén and Birkeland. In the same year, Zmudu and Amstrong did credit Alfvén and Birkeland, but felt that they "...cannot definitely identify the particles constituting the field-aligned currents."
It wasn't until 1973 that the navy satellite Triad, carrying equipment from Zmuda and James Armstrong, detected the magnetic signatures of two large sheets of electric current. Their papers (1973, 1974) reported "more conclusive evidence" of field-aligned currents, citing Cummings and Dessler but not mentioning Birkeland or Alfven.
It had taken 65 years to confirm Birkeland's original predictions.
(Peer-reviewed online in full)
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Birkeland_current". A list of authors is available in Wikipedia.|