Shooting Star, 31 Jan. '11

[jjohnson]

If the dusty filaments in the image are magnetized, which radio astronomers can determine using well-known methods, and if they are Birkeland currents, wherein the electric current is coaligned with the magnetic field (hence field-aligned current) then you can only infer that there is current flow there. Electric currents and electric fields are very hard to observe/identify directly at cosmological distances. Maybe impossible unless they are sufficiently excited to operate in a visible mode, end even then might often be called reflection nebula or something.

Nereid, you could help with this discussion — are there any spectroscopic differences between, say, a reflection nebula's spectral characteristics and those of a simple plasma's, if it is excited sufficiently to radiate? Or have astronomers done such comparisons? Which I think they have, because all kinds of spectroscopic examinations are used to identify imaged objects and compare them with one another.

If magnetization can be identified, the "direction" of magnetization is the direction that a small, positive test particle would be accelerated according to the right-hand-rule. You don't get a drift current unless you have an electric field. If there is an electric field — some voltage differential aligned with the magnetic field "lines" and guiding centers (both imaginary but useful constructs to help visualize what's going on) then the negatively charged (polarized) electrons tend to flow counter to the electric field direction, and the heavier ions will go "with" it. The convention in electrodynamics is always (so far as I know and neglecting building electrical systems) the convention to define the "direction" of an electric or magnetic field is its effect (physical and mathematical) on a positive charge.

Sparky, in a simple sense everything glows in infrared! So long as there is any thermal motion, electrons will give off photons at some rate in those lower energy bands. It's the rate that is important to our instruments' ability to detect and create imagery in the IR. An instrument with too much "noise" or too little sensitivity may not be able to extract a usable (low error bars) signal out of the IR background noise in the cosmos. Electrons in matter which has cooled way, way down, say by radiating for a long time, may emit an occasional IR photon only sporadically. Detecting very cold matter is thus very difficult, like using ISO 100 film to create a usable image when you are only getting, say, 20 photons per square cm per hour. Whether or not a Birkeland current is detectable in IR may not even be debatable - if it were, and if that were a unique signature, hardly anyone would be raising questions about where they are and what they are and what phenomena they are associated with.

I'd like to add here that a field-aligned plasma current, if I understand this correctly, is very-low-collision plasma. The lower the collision rate, the less the thermal content due to random, repeated particle collisions. It can have particles with very high kinetic energies, but if they are all at the same velocity (equal amperage) and direction [more or less - I'm not talking perfect conditions here] the charged particles will be "dethermalized" - and likely have a much lower effective temperature than any atoms or dust grains present, which can get knocked about by each other and the ions. Electrons tend to have such tiny cross sections and plasmas are often so rarefied that collision are a rarity compared with those of the larger ions like protons etc, and the little gritty stuff. In fact, the gritty stuff also tends to gather electrons and become electrostatically charged, so it too can eventually reduce its collision rate and become dethermalized. Maybe that's a contributor to why it is hard for astronomers to directly detect electric currents.

Maybe we can talk about how electric fields CAN be detected/inferred on another thread. More applied spectroscopy! Or just google Stark Effect (and Zeeman Effect to broaden your line knowledge) on Wikipedia. This is not recent rocket science. A German physicist and Nobel laureate, Johannes Stark found it in 1913, 4 years before Kristian Birkeland passed away.

Jim

[Influx]

Stephen Smith

"Even though the ISM is extremely diffuse, since charge separation takes placein different regions weak electric fields can develop."

What? This makes no sense.

Spontaneous charge separation?

Someone please clarify. I know it's late and all that, but the above quoted sentence doesn't make any sense to me, no matter how many times I read and reread.

backs away from EU several feet...

[mharratsc]

What's your problem with it? Charge separation takes place in water that is exposed to light. the Universe is 99% plasma- you can't have plasma without charge separation, right? So...?

[Influx]

Universe is 99% plasma- you can't have plasma without charge separation, right? So...?

Aga, and what causes the charge separation to take place. Let me guess the invisible magic currents that power the universe.

[Sparky]

Not really magic, except to those who do not have the basic understanding of current science, such as primitive tribes and certain fundamentalist ideologies. But, any of these can learn science to some degree, even basic plasma physics, unless they prefer the magical perspective over science, and if they do, there is no reasoning with "magical belief".

[jjohnson]

Charge separation in space plasma? No mystery there, even in a good contemporary plasma physics text such as (Cal Tech's) Dr. Paul Bellan's Fundamentals of Plasma Physics, University Press, Cambridge, UK, 2006. The reason I quote him (besides his knowing what he's about, in plasma) is that you can reference mainstream authors later on.

Under Chapter 1, Basic Concepts, Bellan brings up Debye shielding in 1.6, and asks what happens if you have a net neutral plasma (+ and - charges distributed equally throughout) and introduce a single positive test charge to the body of plasma. He notes that the charge perturbs the balance of the charges, and that (so long as things move slowly) the net force exerted by the additional electric field causes the charges to take up new positions. Primarily, nearby negative charges are attracted a little closer to the positive charge, and nearby positive charges will be slightly repelled.

Seen from a vantage point some distance removed, "the slight displacements resulting from these attractions and repulsions will result in a small, but finite, potential in the plasma. The potential will be the superposition of the test particle's own potential and the potential of the plasma particles that have moved slightly in response to the test particle." Potential means a voltage, or a net electric force exerted upon the nearby particles. (In fact its extent is alleged to be indeterminately far, but practically speaking its measurable force may not extend too far.)

The cloud of attracted negative charges around the test particle tends to shield farther particles from its influence - this is the phenomenon known as Debye shielding. This is the slow, nearly electrostatic case, with a "Boltzmann-like plasma response".

The more real-life response in a plasma has much more particle motion, and any charge can be considered a test charge, in isolation, and there are very large numbers of particles involved, with different species of charge having different temperatures. Still, and particularly in magnetized, or field-aligned plasmas, collisions are statistically rare. 1.6 concludes that "a selected particle is no longer assumed to have a random thermal velocity, in which case its effective potential results from the summation of the non-random. static potential associated with its its own charge and a screening potential (or Yukawa potential), which is the time-average of a rapidly changing potential associated with the fleeting, random thermal motions of all other particles."

In 1.7, Quasi-neutrality, Bellan follows up with a "proof" (his words) that plasma tends to be quasi-neutral as a result of this natural shielding tendency: "It is found that the electrostatic electric field associated with any reasonable configuration is easily produced by having only a tiny deviation from perfect neutrality." [bold type mine for emphasis]

This is precisely the point made by the EU authors, as well as the IEEE engineering society, when they assert that plasma conditions are readily made in space, and that well over 99% of all the observed matter in the Universe exists in the plasma state. It only takes a little fraction to promote charge separation, and all else follows: double layers and relativistic acceleration; synchrotron radiation; z-pinching and galaxy/stellar/planetary formation; Marklund ionic elemental separation; current flows and energy transport across immense distances of space, etc.

There are other conditions, too, that create plasma, or more plasma. electron stripping by EM radiation at sufficiently high energies (ultra-violet and higher) from nearby stars, or cosmic rays, and stellar "winds". Alfvén notes, in his Cosmic Plasma, that ionization is also caused by "a hydromagnetic conversion of kinetic or gravitational potential energy into ionizing electric currents" - the auroras observed on many planets being one clear case uf such currents ionizing atmospheres and inducing them to react to the electrical input by radiating in glow mode.

Alfven also introduced the concept of critical ionization velocity, whereby an atom accumulates sufficient energy to become ionized when a magnetized plasma interacts with a neutral gas. The plasma is able to transfer kinetic energy from ions to the electrons in a number of efficient ways including electron heating by electrostatic instabilities.

So there you have some basics and a number of documented methods, confirmed by experiment and observation, whereby charge separation is plainly effected and observed in plasmas in both lab and cosmic conditions.