Birkeland current

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-aligned 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 1908 by Norwegian explorer and physicist

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."

Characteristics

Auroral Birkeland currents carry about 100,000 amperes during quiet times^[7] and more than 1 million amperes during geomagnetically disturbed times.^[8] Birkeland had estimated currents "at heights of several hundred kilometres, and strengths of up to a million amperes" in 1908.^[3] The ionospheric currents that connect the field-aligned currents give rise to Joule heating in the upper atmosphere. The heat is transferred from the ionospheric plasma to the gas of the upper atmosphere, which consequently rises and increases drag on low-altitude satellites.

Birkeland currents can also be created in the laboratory with multi-

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 will also interact due to Ampère's force law: parallel Birkeland currents moving in the same direction will attract each other with an electromagnetic force inversely proportional to their distance apart whilst parallel Birkeland currents moving in opposite directions will repel each other. 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

Spatial Distribution and Responses to Solar Wind Disturbances

Auroral Birkeland currents are constrained along the geomagnetic field. Therefore, the current’s distribution in 3-dimensional space could be largely described using the 2-dimensional distribution of the current’s footprints at a given altitude in the ionosphere, e.g., 110 km. A classical 2-dimensional description was summarized from satellite observations by Iijima and Potemra.^[12] The footprints of Auroral Birkeland currents exhibit ring-shaped structures. As the currents are driven by solar winds, their spatial distribution and intensity are also dynamically moderated by solar wind disturbances.^[13] Under intensive solar wind disturbances, the rings can quickly shift by 10 degrees in latitude in about 10 minutes. The latitudinal shift takes on average 20 minutes to respond to a solar wind change during the daytime but 70–90 minutes at night.^[14]

History

After Kristian Birkeland first suggested in 1908 that "currents there [in the aurora] are imagined as having come into existence mainly as a secondary effect of the electric corpuscles from the sun drawn in out of space,"

In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper^[20] published on the generation of the current from the Solar Wind. In 1964 one of Alfvén's colleagues, Rolf Boström, also used field-aligned currents in a new model of auroral electrojets.^[21]

Proof of Birkeland's theory of the aurora only came after a probe was sent into space. The crucial results were obtained from U.S. Navy satellite 1963-38C, launched in 1963 and carrying a magnetometer above the ionosphere. In 1966 Alfred Zmuda, J.H. Martin, and F.T.Heuring^[22] analysed the satellite magnetometer results and reported their findings of magnetic disturbance in the aurora. In 1967 Alex Dessler and graduate student David Cummings wrote an article^[23] arguing that Zmuda et al. had detected field-aligned currents. Alfvén subsequently acknowledged^[24] that Dessler had "discovered the currents that Birkeland had predicted" and they should be called Birkeland-Dessler currents. 1967 is therefore taken as the date when Birkeland's theory was finally acknowledged to have been vindicated. In 1969 Milo Schield, Alex Dessler and John Freeman^[25] used the name "Birkeland currents" for the first time. In 1970 Zmuda, Armstrong and Heuring wrote another paper^[26] agreeing that their observations were compatible with field-aligned currents as suggested by Cummings and Dessler and by Boström.^[21]

See also

• Electromagnetism
• Magnetohydrodynamics
• Bursty bulk flows another name for similar phenomena (discontinuous sudden flows as in Aurorae)
• Geomagnetic jerk (Secular geomagnetic variation impulse)
• Telluric currents induced ground currents or ground portion of Birkeland current
• Plasma cosmology claims that Birkeland currents happen at galactic level and are important in the formation of galaxies

References