Talk:Birkeland current

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1 deleted solar image
2 too general?
3 upload
4 Characteristics: forces
5 Characteristics: synchrotron radiation
6 Inaccuracies and errors in the page

[edit] deleted solar image

I removed the solar EUV image of the Bastille Day post-flare arcade. It had been captioned as an example of a "solar Birkeland current", which it is not. Post-flare arcades are formed by line-tied boundary conditions at the photosphere, together with magnetic reconnection (collapse of a current sheet) during a flare. zowie 00:05, 3 October 2005 (UTC)

[edit] too general?

The Most of my colleagues seem to use "Birkeland current" to refer only to the auroral-related electrojets around the Earth, specifically; the article seems to tie the term to the more general concept of field-aligned currents in any astrophysical plasma. Does anyone care to comment on this? If I haven't heard back in a few days, I'll try to get around to finding a more authoritative source. zowie 00:14, 3 October 2005 (UTC)

Indeed, Birkeland currents were originally applied to field-aligned currents in the ionosphere feeding the aurora, and I guesss they are most well-known here, and hence the use of the name here. But all field-aligned currents, no matter where they are, are also Birkeland currents. But field-aligned currents elsewhere are not generally recognised, and the use of the term is limited, but see for example, Synchrotron radiation spectrum for galactic-sized plasma filaments. Perhaps the article should be called Field-Align Currents, with Birkeland currents being the kind found in the ionosphere? --Iantresman 08:11, 3 October 2005 (UTC)

Hmmm.... Some sort of change sounds like maybe a good idea since it reflects current usage in the scientific community, notwithsanding the ADS link you gave. For example, a quick ADS search for "Birkeland current" in the abstract finds loads of references on the Earth, geospace, and planetary aurorae -- but in the first hundred or so references I found no other titles that refered to interstellar or intergalactic currents. Similarly for Google (aside from the Wikipedia entry! :-).

Since, on the large scale, all non-drift currents are field-aligned in the ideal MHD approximation, it doesn't seem to make sense to me to equate field-aligned currents and Birkeland currents. I'm not, myself, a space geophysicist, but many of my friends are :-). Is there any particular reference work or group of folks you polled to find current usage?

BTW, nice work on the history of Birkeland and on finding so many of the original references on space plasma physics. zowie 15:16, 3 October 2005 (UTC)

I have the same objection as Zowie. The name Birkeland currents in the professional world is only used for the Earth and planets, and then for specific currents. In all other cases the term field aligned currents is used. So at the beginning it would be better to state that usually the name field aligned currents is used.--Tusenfem 19:27, 2 April 2006 (UTC)

I wonder whether the article would be better named "field aligned current" (since this is the general term), and include a section on "Birkeland currents"? --Iantresman 20:49, 2 April 2006 (UTC)

[edit] upload

Image:Cygnus-loop.gif Image:Birkeland-anode-globe-fig259.jpg Image:Magnetic-rope.gif Image:Jupiter-aurora.jpg

Will you upload this image in Commons? It was expected that I wrote an article about Birkeland current in Wikipedia Japan. Please cooperate.

Can you upload the images yourself? All the images are in the public domain? --Iantresman 14:49, 31 October 2005 (UTC)

[edit] Characteristics: forces

The article currently states:

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 gravitational force is inversely proportional to the square of the distance); 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 foreces. [Ref].

The gravitational attraction between two line masses also varies in inverse proportion to their separation, so what's the big deal? The reference is non-refereed and a mess on several fronts (e.g. calling the Earth's core a plasma). In particular, it offers no explanation for the claim of short-range repulsion. If this cannot be explained better or if a better reference cannot be found, I would strike it. --Art Carlson 19:50, 31 October 2005 (UTC)

If there was no Birkeland current flowing through a plasma, then there would be a number of electrons, ions and neutral particles whose mutual attraction would be electromagnetic (about up to the Debye sphere), and gravitational forces that varies as 1/r^2 because there is no line mass present. So the question is, can a Birkeland current in a plasma be considered a line mass from a gravitational perspective? And if so, is it significant since EM forces are 10^39 more powerful? I'm still checking.

As for the Earth's core being a plasma, this does sound rather contentious. On the other hand, the liquid core is often modelled with magnetohydrodynamics, so perhaps plasma-like is more accurate. --Iantresman 09:59, 8 November 2005 (UTC)

[edit] Characteristics: synchrotron radiation

The article currently states:

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

A pinch just requires current. Whether the electron velocities are relativistic is irrelevant. The electrons might spiral (if they figure out how to become oblique to the magnetic field, but the pinch won't. Why go into details on the potential wavelength of synchrotron radiation in an article that isn't even about double layers? I'm not sure on cause and effect: does the double layer cause the current or the current cause the double layers? --Art Carlson 20:00, 31 October 2005 (UTC)

I think I kind of answered this one on another page. Your point about cause and effect I think is relevent, and not obvious. You may have read in Alfvén's Cosmic Plasma about "circuits", in which he considers several aspects to the source and cause of various plasma phenomenon.

My understanding is as follows. Birkeland currents are like electric cables in space, moving ions from one region to another. They do so in response to a load, in this case a double layer who's main features is the ability to accelerate ions. This will result in a beam of ions, which will pinch, and depending on various parameters, may result in a Bennett pinch. And then if the electrons are moving a relativistic velocities, then synchrotron radiation will be emited from the pinch, in the same way that synchrotron radiation is released in a laboratory z-pinch.

So perhaps Birkeland currents do not directly produce synchrotron radiation;, perhaps it's analogous to describing that a light bulb give off radiation, but not mentioning that it is connected to electric wires.

Again I would suggest Peratt's paper for detailed information, Evolution of the Plasma Universe: I. Double Radio Galaxies, Quasars, and Extragalactic Jets [PDF], and Alfvén's Cosmic Plasma.

--Iantresman 20:30, 1 November 2005 (UTC)

[edit] Inaccuracies and errors in the page

I read through the first part of the page, and there are a lot of inaccuracies that need to be changed.

They are sometimes referred to as field-aligned currents. IMHO, in science they are always called field-aligned currents and only in magnetospheric they are called Birkeland currents.
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. Although the first part is okay, that the field aligned currents flow to the Earth at the dawn side, and away at the dusk side, I have trouble with stating that the electrojet is a Birkeland current. Birkeland currents are defined as field aligned currents, the electroject is perpendicular to the magnetic field and thus not a Birkeland current.
They can heat up the upper atmosphere which results in increased drag on low-altitude satellites. Is the drag on satellites truely caused by a heated up atmosphere? I would think that the increased magnetic field strength, caused by an active system in a magnetic storm would be the cause, because of stronger eddy currents in the spacecraft.
Birkeland currents are also one of a class of plasma phenonena called a z-pinch Here cause and result are switched around. A Birkeland current is not a z-pinch. A z-pinch may occur when a field aligned current exists. But there are certain restrictions to that, a pinch can only occur when the field strength increases above a certain level. I am not sure that z-pinches have been observed in the Earth's Birkeland currents.
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. Why does this most closely correspond to a Birkeland current?
parallel Birkeland currents moving in opposite directions will repel with an electromagnetic force inversely proportional to their distance apart. But anti-parallel Birkeland currents will not occur, as the inflow is on the dawn side and the outflow is on the dusk side.
Electrons moving along a Birkeland current may be accelerated by a plasma double layer. Electrons are not moving along a Birkeland current, they 'are' the Birkeland current.
If the resulting electrons approach relativistic velocities (ie. the speed of light) they may subsequently produce a Bennett pinch No, this is not true, relativistic particles do not increase the current, so if the requirement for a Bennett pinch has not been fulfilled before, then making the electrons relativistic will not make any difference.