A Birkeland current usually refers to the electric currents in a planet’s ionosphere that follows magnetic field lines (ie field-aligned currents), and sometimes used to described any field-aligned electric current in a space plasma.[3] 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 also known as field-aligned currents, magnetic ropes and magnetic cables).

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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.

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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.”

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Characteristics

Auroral-like Birkeland currents created by scientist Kristian Birkeland in his terrella, featuring a magnetised anode globe in an evacuated chamber. Source

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Auroral Birkeland currents can carry about 1 million amperes.[4] They can heat up the upper atmosphere which results in increased drag on low-altitude satellites.

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“Voyager 2 also detected auroras similar to those on Earth, but Neptune’s registered 50 million watts, compared to Earth’s 100 billion watts and occurred over wide regions of the planet’s surface.”[5]

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Birkeland currents can also be created in the laboratory with multi-terawatt pulsed power generators.

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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[6] [7]

(similar, but different to the Kelvin-Helmholtz instability), that subsequently leads to filamentation.

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Such vortices can be seen in aurora as “auroral curls”.[8]

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.

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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.

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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.

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There is also a short-range circular component to the force between two Birkeland currents that is opposite to the longer-range parallel forces.[9]

 

Adjacent Birkeland current filaments tend to be long-range attractive (F ~ 1/r), and short range repulsive (f ~ 1/r3).[14] Credit.

Electrons moving along a Birkeland current may be accelerated by a plasma double layer.

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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.

 

In 2007, the Themis satellite “found evidence of magnetic ropes connecting Earth’s upper atmosphere directly to the sun”.[10] apparently consistent with Kristian Birkeland‘s theory of the aurora, “that a current of electric corpuscles from the sun would give rise to precipitation upon the earth” (See preface to The Norwegian Aurora Polaris Expedition 1902-1903).

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Describing plasma cables, Hannes Alfvén wrote:

“Plasma cables seem to be reasonably stable formations which can be considered as structures important for the understanding of plasma phenomena.

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(Of course, their interior structure should be described by classical theory.)

The plasma cables are either filaments or ‘flattened filaments’ (sheets with limited extent). They carry an electric current parallel to the magnetic field, and this is what gives them their properties.

The cables are often very efficient in transferring electro-magnetic power from one region to another.

They are embedded in passive plasmas, which have essentially the same properties in all directions around the cables.

They are ‘insulated’ from their surroundings by a thin cylindrical electrostatic sheath (or double layer) which reduces the interaction with its exterior.

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In the magnetosphere and upper ionosphere, the density in the cable is sometimes lower than the surrounding passive plasma (Block and Fälthammar, 1968)[11].

In other cases, the density in the cable may be much larger than the surroundings because ionized matter is pumped into the cable from the outside. By selectively doing so, the chemical composition in the cable may differ from that of its exterior (Marklund, 1978, 1979)[12]

(see Marklund convection).

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Besides the cylindrical electrostatic sheath, there are often longitudinal double layers, in which a considerable part of the power which the cable transmits may be converted into high energy particles.

The double layers sometimes explode, and this produces excessively high energy particles.” [13]