Birkeland-áramok

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A Birkeland current generally refers to a system of electrical currents flowing along the direction of the earth’s magnetic field, between the outer regions of the magnetosphere and the ionosphere. 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. They are sometimes referred to as fieldaligned currents. 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. See Fig. 1. 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 Alfv´en Laboratory in Sweden, Carl-Gunn F¨althammar 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.”

Characteristics

Auroral Birkeland currents can carry about 1 million amperes (Germany, et al.). 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 multiterawatt pulsed power generators. See Fig. 2. 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 (Plasma Phenomena: Web Ref.) (similar, but different to the Kelvin-Helmholtz instability), that subsequently leads to filamentation. Such vortices can be seen in aurora as “auroral curls” http://www.phys.ucalgary.ca/trondsen/pai/Curls.html. 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 (The Electromagnetic Force Web Ref). 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 (i.e., light), x-rays, and gamma rays.

Additional Reading Germany, G.A., P.G. Richards, G.K. Parks, M. Brittnacher, and J.F. Spann: Global Auroral Imaging as a Remote Diagnostic of Geospace, Presented at AIAA Plasmadynamics and Lasers Conference, June 25, 1997. http://uvisun.msfc.nasa.gov/GG/ GG15-AIAA97/home.htm.

==Jegyzetek==

==Források==

• Van Nostrands's Encyclopedia

==További információk==

==Kapcsolódó szócikkek==

==Further reading==

(Books)

• Egeland, Alv, Burke, William J.,(2005), Kristian Birkeland, The First Space Scientist, Springer pp. 221, ISBN 1-4020-3293-5
• Peratt, Anthony (1992), Physics of the Plasma Universe, Birkeland Currents in Cosmic Plasma (p. 43-92), Springer-Verlag, ISBN 0-387-97575-6 and ISBN 3-540-97575-6 [1]
• Ohtani, Shin-ichi; Ryoichi Fujii, Michael Hesse and Robert Lysak, editors (2000), Magnetospheric Currents Systems, Am. Geophys. Union, Washington, D.C., ISBN 0-87590-976-0.

(Peer-reviewed journal articles, online in full)

• 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", J. Geophys. Res. , vol. 80, Sept. 1, 1975, p. 3571-3579, doi:10.1029/JA080i025p03571
• Potemra, T. A. "Birkeland currents in the earth's magnetosphere", from a Special Issue of Astrophysics and Space Science" Dedicated to Hannes Alfvén on his 80th Birthday (Astrophysics and Space Science, vol. 144, no. 1-2, May 1988, p. 155-169, doi:10.1007/BF00793179
• Alfvén, Hannes, On the Filamentary Structure of the Solar Corona (1963) The Solar Corona; Proceedings of IAU Symposium no. 16 held at Cloudcroft, New Mexico, U.S.A. 28–30 August 1961. Edited by John Wainwright Evans. International Astronomical Union. Symposium no. 16, Academic Press, New York, 1963., p. 35
• Alfvén, Hannes, Currents in the Solar Atmosphere and a Theory of Solar Flares (March 1967) ) Solar Physics, V 1, 2, pp. 220–228, doi:10.1007/BF00150857
• Alfvén, Hannes, On the Importance of Electric Fields in the Magnetosphere and Interplanetary Space (1967) Space Science Reviews, Volume 7, Issue 2-3, pp. 140–148, doi:10.1007/BF00215591
• Carlqvist, P., Cosmic electric currents and the generalized Bennett relation, Astrophysics and Space Science (ISSN 0004-640X), vol. 144, no. 1-2, p. 73-84. (May 1988) doi:10.1007/BF00793173
• Cloutier, P. A.; Anderson, H. R. Observations of Birkeland currents Space Science Reviews, 17, p. 563-587, Mar.-June 1975, doi:10.1007/BF00718585
• Potemra, T. A. Observation of Birkeland currents with the TRIAD satellite, Astrophysics and Space Science, 58, 1, Sept. 1978, doi:10.1007/BF00645387