Birkeland current

[2] The currents were predicted in 1908 by Norwegian explorer and physicist Kristian Birkeland, who undertook expeditions north of the Arctic Circle to study the aurora.

He rediscovered, using simple magnetic field measurement instruments, that when the aurora appeared the needles of magnetometers changed direction, confirming the findings of Anders Celsius and assistant Olof Hjorter more than a century before.

This view was scorned by other researchers,[5] but in 1967 a satellite, launched into the auroral region, showed that the currents posited by Birkeland existed.

[6] Professor Emeritus of the Alfvén Laboratory in Sweden, Carl-Gunne Fälthammar wrote:[7] "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).

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.

[15] 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,"[4] the story appears to have become mired in politics.

[20] In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper[21] published on the generation of the current from the Solar Wind.

In 1967 Alex Dessler and graduate student David Cummings wrote an article[24] arguing that Zmuda et al. had detected field-aligned currents.

In 1969 Milo Schield, Alex Dessler and John Freeman[26] used the name "Birkeland currents" for the first time.

In 1970 Zmuda, Armstrong and Heuring wrote another paper[27] agreeing that their observations were compatible with field-aligned currents as suggested by Cummings and Dessler and by Boström.

Schematic of the Birkeland or Field-Aligned Currents and the ionospheric current systems they connect to, Pedersen and Hall currents. [ 1 ]
Auroral-like Birkeland currents created by scientist Kristian Birkeland in his terrella , featuring a magnetised anode globe in an evacuated chamber.
The field-aligned current density at its ionopsheric footprints (about110 km altitude) on 4 June 2007, a day under moderate solar wind disturbances, predicted by the open-source MFACE, [ 14 ] [ 16 ] according to the solar wind conditions downloaded from the NASA OMNI serve. [ 17 ] MFACE is an empirical mode extracted from 10-years of CHAMP magnetic observations. The time interval between neighboring frames in this movie denotes 5 minutes. Similar movies but under quiet and active solar wind disturbance levels are available at, e.g., [ 18 ] .
Kristian Birkeland predicted the auroral electrojets in 1908. He wrote p. 95 [ 4 ] "the 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, and thus far come under the second of the possibilities mentioned above". And p. 105, "Fig. 50a represents those in which the current-directions at the storm-centre are directed westwards, and 50b those in which the currents move eastwards".
The complex self-constricting magnetic field lines and current paths in a Birkeland current that may develop in a plasma (Figure 15.3.2, Alfvén and Arrhenius, 1976) [ 28 ]