Birkeland current
A Birkeland current generally refer to any electric current in a space plasma, but more specifically when charged particles in the current follow magnetic field lines. 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.
Characteristics
Auroral Birkeland currents can carry about 1 million amps [Ref.]. They can heat up the upper atmosphere which results in increased drag on low altitude satellites.
Birkeland currents can also be created in the laboratory with multi-terra watt pulse 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 gravitational force is inversely proportional to the square of the distance); 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 Birekeland currents that is opposite to the longer-range parallel foreces. [Ref].
Electrons moving along a Birkeland current will also spiral around magnetic field lines. If the electrons approach relativistic velocities (ie. the speed of light), they will 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 though each other". (See Plasma scaling). Examples include:
Source: Peratt (1992). |
History
After Kristian Birkeland predicted the ionospheric currents in 1908, in 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas and had a paper published on the generation of the current from the Solar Wind. In 1969 Milo Schield, Alex Dessler and John Freeman, published a theory of how the currents might originate, and proposed the name "Birkeland currents. It wasn't until 1973 that the navy satellite Triad, carrying equipment from Alfred Zmuda and James Armstrong, detected the magnetic signatures of two large sheets of electric current that confirmed Birkeland's predictions 70 years earlier.
In 1986, Carl-Gunn Fälthammar(1986) wrote: "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."
References
- Alfvén, Hannes (1939), Theory of Magnetic Storms and of the Aurorae
- Birkeland, Kristian (1908), The Norwegian Aurora Polaris Expedition 1902-1903
- Peratt, Anthony (1992), Physics of the Plasma Universe, "Birkeland Currents in Cosmic Plasma" (p.43-92)
- Rostoker, G.; Armstrong, J. C.; Zmuda, A. J. (1975), Field-aligned current flow associated with intrusion of the substorm-intensified westward electrojet into the evening sector, Journal of Geophysical Research, vol. 80, Sept. 1, 1975, p. 3571-3579
- Schield, M. A.; Freeman, J. W.; Dessler, A. J., (1969) A Source for Field-Aligned Currents at Auroral Latitudes, Journal of Geophysical Research, Vol. 74, p.247