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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 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, 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, 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.

Professor Emeritus of the Alfvén Laboratory in Sweden, Carl-Gunne Fälthammar 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."

Connecting Interplanetary Environment to Magnetosphere and Ionosphere
Birkeland currents are a major element of the connection between the interplanetary medium, the planetary magnetosphere and the high latitude ionosphere. The driving force is the merging of the interplanetary magnetic field (IMF) with the Earth’s magnetic field at the dayside boundary of the magnetosphere as first described by Dungey. Dungey illustrated the concept with two field-aligned currents each of which followed a single field line from the magnetopause (the boundary between the magnetosphere and the interplanetary medium. One current flowed down into the high latitude ionosphere near the dawn meridian and the other current flowed up from the high latitude ionosphere near the dusk meridian.  It was later shown that the Birkeland currents extend across the whole high latitude ionosphere.  Each current is a few degrees of magnetic latitude wide and 180 degrees long.  The downward current stretches from the noon meridian to the midnight meridian on the dawn side and the upward current stretches from noon to midnight on the dusk side.  The schematic shows only a portion of the extent current sheets.  These current sheets, referred to as Region 1 currents as shown in the first schematic, connect to the boundary layer between the magnetosphere and the interplanetary medium. A second set of current sheets, referred to as Region 2 currents as shown in the schematic, exist immediately equatorward. The Region 2 currents connect to the Region 1 currents through the E-layer (100 km to 130 km altitude) of the auroral ionosphere and flow into/out-of the inner magnetosphere. The first schematic shows a noticeable gap between the Region 1 and Region 2 current sheet. The second schematic is more realistic because it shows the Region 1 and Region 2 current sheet adjacent to each other. Most of the current from the Region 1 flows into the adjacent Region 2 through the ionosphere, but a portion of the Region 1 current flows into the opposition Region 1 through the ionosphere.

The quantity of current in the Birkland current system depends on the rate of energy transferred from the interplanetary medium to the magnetosphere  and from the geomagnetic tail to the inner magnetosphere. The release of energy from the geomagnetic tail is associated with a magnetospheric substorm.

Pedersen Current and Joule Heat
In the second schematic, a portion of the E-layer of the auroral ionosphere is shown as a green slab. There is an electric field ($$E$$) which drives an electrical current, known as a Pedersen current ($$J_P$$), across the magnetic field lines in the ionosphere in order to complete the circuit between the Region 1 and Region 2 currents. Since $$J_P$$ ⋅ $$E$$ ≠ 0, energy is dissipated from the electric current into the plasma of the ionosphere and in turn into the high latitude, high altitude neutral atmosphere. Auroral Birkeland currents carry about 100,000 amperes during quiet times and more than 1 million amperes during geomagnetically disturbed times. The power dissipated during quiet times is ~1 to 30 GWatts, and during an average geomagnetic storm, the power dissipated is 300 GW or greater. The heated atmosphere rises and increases drag on low-altitude satellites.

Hall Current and Auroral Electroject
The Hall electric current flows perpendicular to the geomagnetic field lines and the Pedersen current. The Hall current is confined to the E-layer of the ionosphere between approximately 100 and 130 km altitude. The current flows where the magnitude ion and electron cyclotron frequencies is comparable to the ion and electron collision frequencies.

History
The history of Birkeland Currents appears to be mired in politics.

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," (1908), his ideas were generally ignored in favour of an alternative theory from British mathematician Sydney Chapman.

In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper 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 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 arguing that Zmuda et al. had indeed detected field align-currents. Even Alfvén subsequently credited 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, used the name "Birkeland currents" for the first time. In 1970, Zmuda, Armstrong and Heuring wrote another paper agreeing that their observations were compatible with field-aligned currents as suggested by Cummings and Dessler, and by Boström, 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 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  , 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.