A Birkeland current is a set of currents which flow along geomagnetic field line connecting the Earth’s magnetosphere to the Earth's high latitude ionosphere. They are a specific class of magnetic field-aligned currents. Lately, the term Birkeland currents has been expanded by some authors to include magnetic field aligned currents in general space plasmas. In the Earth’s magnetosphere, the currents are driven by the solar wind and interplanetary magnetic field and by bulk motions of plasma through the magnetosphere (convection which is indirectly driven by the interplanetary environment). The strength of the Birkeland currents changes with activity in the magnetosphere (e.g. during substorms). Small scale variations in the upward current sheets (downward flowing electrons) accelerate magnetospheric electrons and when they reach the upper atmosphere, they create the aurora Borealis and Australis. In the high latitude ionosphere (or auroral zones), the Birkeland currents close through the region of the auroral electrojet, which flows perpendicular to the local magnetic field in the ionosphere. The Birkeland currents occur in two pairs of field-lined current sheets. One pair extends from noon through the dusk sector to the midnight sector. The other pair extends from noon through the dawn sector to the midnight sector. The sheet on the high latitude side of the auroral zone is referred to as the Region 1 current sheet and the sheet on the low latitude side is referred to as the Region 2 current sheet.
The currents were predicted in 1903 by Norwegian explorer and physicist Kristian Birkeland, who undertook expeditions into the Arctic Circle to study the aurora. He rediscovered, using simple magnetic field measure instruments, that when the aurora appeared the needles of the magnetometers changed direction, confirming the findings of Anders Celsius and assistant Olof Hjorter more than a century before. This could only imply that currents were flowing in the atmosphere above. He theorized that somehow the Sun emitted a cathode ray[2] [3] , and corpuscules from a solar wind entered the Earth’s magnetic field and created currents, thereby creating the aurora. This view was scorned at by other researchers[4] , and it took until the 1960s before sounding rockets, launched into the auroral region showed that indeed the currents posited by Birkeland existed. In honour of his ideas, these currents were named Birkeland currents. A good description of the discoveries by Birkeland is given in the book by Lucy Jago[5] .
Professor Emeritus of the Alfvén Laboratory in Sweden, Carl-Gunne Fälthammar wrote[6] : "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."
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Auroral Birkeland currents carry about 100,000 amperes during quiet times[7] and more than 1 million amperes during geomagnetically disturbed times[8] . The ionospheric currents which connect the field-aligned currents heat up the upper atmosphere due to the finite conductivity of the ionosphere. The heat (also known as Joule heat) is transferred from the ionospheric plasma to the gas of the upper atmosphere which rises and increases drag on low-altitude satellites.
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 electrons in the form of a circle of vortices, a formation called the diocotron instability[9] (similar, but different from the Kelvin-Helmholtz instability), that subsequently leads to filamentation. Such vortices can be seen in aurora as "auroral curls".[10]
Birkeland currents are also one of a class of plasma phenomena 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.[11]
Electrons moving along a Birkeland current may be accelerated by a plasma double layer. If the resulting electrons approach relativistic velocities (i.e. 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.
The history of Birkeland Currents appears to be mired in politics.[12]
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," [3]), his ideas were generally ignored in favor of an alternative theory from British mathematician Sydney Chapman[13].
In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper[14] 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[15] 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[16] arguing that Zmuda et al. had indeed detected field align-currents. Even Alfvén subsequently credited[17] 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[18], used the name "Birkeland currents" for the first time. In 1970, Zmuda, Armstrong and Heuring wrote another paper[19] agreeing that their observations were compatible with field-aligned currents as suggested by Cummings and Dessler, and by Boström[20], 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, Zmuda and Amstrong[21] did credit Alfvén and Birkeland, but felt that they "...cannot definitely identify the particles constituting the field-aligned currents but … the current is probably carried by electrons …."
It wasn't until 1973 that the U. S. Navy satellite Triad, carrying equipment from A. Zmuda and James Armstrong[22][23][24] , detected the magnetic signatures of two large sheets of electric current. Armstrong and Zmuda's papers in 1973 and 1974 reported "more conclusive evidence" of field-aligned currents, citing Cummings and Dessler but not mentioning Birkeland or Alfvén.
(Books)
(Peer-reviewed journal articles, online in full)
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