Thursday, February 16, 2012

"Pinch" Me, I'm Dreaming of "Magnetic Slinkies!"

Silly metaphors require silly titles. Hopefully today we can sort out some astrophysical silliness and come to a better understanding of what is going on from an electrical standpoint. The silliness in question is the "magnetic slinky" that has been metaphorically associated with a molecular cloud in Orion. Put simply, a better physical explanation is the "plasma pinch."

In my introductory post, I exegesis-ized the relationship between... Yes, okay, I know exegesis-ized isn't a word! Sheesh! Fine, I elaborated (some might prefer belabored) the relationship between magnetic fields and electric currents. Hopefully we're all on the same page by now:
"Electric currents are the sole source of magnetic fields in the universe."
Today, we seek to put that notion into practice with a real-world example. Moreover, let's add another bit of electrical physics to the mix.

The topic I want to recap today is an oldie but a goodie. In 2006, astronomers discovered what was subsequently coined a "magnetic slinky" encircling a molecular cloud in Orion. 

But, what does that even mean? Honestly, by itself, not a whole heck of a lot. It's a pretty bad metaphor. Essentially, what was being described is the approximate shape of magnetic 'field lines' encircling the long axis of the molecular cloud. In a later post, I'll cover why we should move away from the concept of talking about 'field lines' (they don't actually exist).

But, again, what does the discovery actually tell us about what's going on? For that I'll actually grab a slightly better quote from the press release:
"The magnetic field lines are like stretched rubber bands; the tension squeezes the cloud into its filamentary shape."
Ahh, action words! Something happened. Finally, we're getting somewhere! Though, the metaphysics is still terrible, since magnetic 'field lines' do not exist thus cannot have 'tension' and cannot 'squeeze' anything. But at least we have some idea now of what has been observed. Something is causing the compression of the molecular cloud into a filamentary shape. It's a start...

So, what do we know? We know there's a 'cloud' out in space. We know that a magnetic field is involved. We know that the cloud is being compressed. We know that the magnetic field is implicated somehow in compressing the cloud.

So, what's actually going on?

Let's back up and head back to the basics. We've already said that magnetic fields are produced by electric currents, because electric currents of some character are the sole source of magnetic fields in the universe.

When a current flows, a magnetic field is generated around it. It can be visualized as a straight line (current, I) surrounded by concentric circles (magnetic field, B).
But, this universe is messy and things are never quite that simple. When a current flows and generates a magnetic field, the magnetic field can also interact back with the current that created it, constricting the current. As the current is constricted the magnetic field increases.
Original Caption: Fig. 10: Physics of the Z-pinch: A current (orange) generates a magnetic field (blue), which causes the current to pinch inwards along the axis by way of the Biot-Savart F = I × B force. This amplifies the magnetic field and accelerates the pinch, heating the plasma and causing it to radiate X-rays (red).
This feedback continues until magnetic pressure reaches equilibrium with the gas pressure in the compressed conductor. This is known as the 'pinch effect.'

Are there any accessible examples of the pinch effect? Yes. But we'll get to those in a moment.

First let's back up one more time and discuss discharge regimes. In a plasma, there are three basic regimes. One might call them 'intensities' of discharge.
  • Dark Discharge
    This regimes is termed a dark discharge because although a current flows, generally, the discharge remains invisible to the eye.
  • Glow Discharge
    The glow discharge regime owes its name to the fact that the plasma is luminous. The gas glows because the electron energy and number density are high enough to generate visible light by excitation collisions.
  • Arc Discharge
    Beyond the glow discharge is the arc discharge regime. Particle energies and current density are higher. The increased current produces an associated increased magnetic field which pinch the current into a filamentary shape. Arc discharges are also known to emit copiously across the electromagnetic spectrum.
So, are there any examples of the pinch effect? Yes!

In the glow mode, one need look only as far as a novelty 'plasma lamp' or 'lightning globe' as they are affectionately known:

Discharge streamers emanate through low density ionized gases (plasma) from a central electrode, compressed into filamentary forms by way of the pinch effect.

In the arc discharge regime, we can easily point to lightning:

The discharge channel is clearly compressed into a filament with a very small radius. Lightning also emits brightly in the visible range and indeed across much of the EM spectrum from radio to x-rays.

Now you know! The pinch effect can be reasonably easily understood and is useful for understanding several relatively mundane processes here on Earth. But, what about in the cosmos at large?

It is this author's opinion that what's good for the goose is good for the gander. What holds true here on Earth in the lab and in nature should also apply to the cosmos. so, we come full circle back to the original inquiry: the molecular cloud in Orion.

To recap what we know: there is a filamentary cloud of material; it is being compressed into its filamentary shape; magnetic fields are involved somehow.

We also know that magnetic fields find their genesis in electric currents and can be visualized as field lines wrapped around the central axis of the current.
Original caption: The Orion Molecular Cloud superimposed on the Orion constellation, with the orange star Betelgeuse at the top corner and Rigel at the bottom. The inset shows the Slinky-like coils of the helical magnetic field surrounding the filamentary cloud. (Credit: Saxton, Dame, Hartmann, Thaddeus; NRAO/AUI/NSF)
Diagram shows azimuthal magnetic field (B)
as concentric circles around central current (I).
So, based on what we know of the nature of magnetic fields and the pinch effect, do we have enough information to make an anecdotal case for a hypothesis on what's really going on here? I think we do.

An electric current flowing along the length of the filamentary cloud would generate a magnetic field azimuthal to the cloud (wrapped the short way around the filament). Such a magnetic field could interact back with the current flowing through the cloud causing it to 'pinch' or compress into the observed filamentary shape. Not unlike the filamentary tendrils observed in a plasma lamp (above).

Thus, rather simply, we can back-infer from the magnetic field of the filament, its shape and the implication that the magnetic field is involved in compressing the cloud into its filamentary shape that there is a plausible possibility that a current is flowing there, which would reproduce the features observed.

Would the solution be as simple as drawing a line down the middle of the "magnetic slinky" along the long axis and labeling it (I) denoting a current flowing there (as an analogy to the current [I] vs. magnetic field [B] diagram, above)?

I suggest that it is incumbent upon astronomers to perform some follow up observations and/or modeling to investigate this simple, testable proposition. Moreover, if this hypothesis is found plausible or even probable in this instance, this observation and hypothesis could easily be applied to a number of other structures demonstrating filamentary shapes and strongly associated magnetic fields.

Another pertinent example that jumps to mind is the "Strongest Electrical Current in the Universe" recently derived from observations in an extremely similar manner (by first observing Faraday rotation indicating the presence of a magnetic field then modeling the system to determine the characteristic of the current driving it).

One wonders somewhat emphatically whether all similar jets (such as the "largest stellar jet" stretching over 400 trillion kilometers recently found in the Large Magellanic Cloud or Herbig-Haro objects, generally) may share common features, including an electrical character heretofore not widely acknowledged.

I'd love to see a rigorous investigation with the potential to investigate this very simple hypothesis.

If the above examples are tackled and an answer in the affirmative is obtained, perhaps this hypothesis can then be extended to the investigation of the entire filamentary cosmic web of stars, galaxies and galaxy clusters.

Are any astronomers brave enough to undertake a rigorous voyage of discovery into this wild new frontier? The exploration won't be without its challenges. But the bounties may far outweigh the perils.


  1. Wonderful. Yet again, it's profound to have the concepts rehinged ... to be able to have this door actually swing open when the handle is pulled. To be able to connect the feeling of sunlight to the z-pinch effects from the sun. Photo's of the sun actually look like the z-pich diagram with a "curve" either side (minus field lines of course!).

    I'd love to see an article about Aneutronic fusion ( ) and how discoveries and progress in cosmology can help that emerging field.

  2. Of course, we should recall that Eric Lerner's Focus Fusion concept, I believe, came out of his notions on the plasma cosmology origin of Herbig-Haro objects as pinches & plasmoids...

    His Focus Fusion group has, I think, come out of that research: trying to harness the pinch / plasmoid for fusion & energy production...

    1. I see, of course. The copper bars add current around the edge of the plasma. The resulting magnetic field, as described above, compresses the plasma which creates the huge temperatures required for the fusion.

    2. Animations on DPF operation here:

      One @ top of page and one @ bottom of page (hadn't noticed that one before)...

      Shows in animated terms the formation of a pinch, kink instability & plasmoid, as well as the discharge of the plasmoid in bipolar particle beams.


    3. Seems they've even just now started a wiki:

    4. Yeah, saw that. So how about sending the electricy around in a circuit with the plasma, so something is created that is akin to what is in the crab nebulae ? Focus fusion appears to be more of a "pulsed" technology.

    5. Yes, definitely pulsed power. Or, well, pulsed "experiment" at the moment. ;) Hopefully pulsed "power" eventually...

      If someone has the funding, resources/facilities and plasma know-how to give recreating the crab nebula a go, I'm all for it. :)


  3. Ha ha, on that scale, maybe in a few hundred years. It's just that even as a kid I imagined something like the Star Trek warp core to involve something spinning at a massive speed - possibly superluminal -- this is how a warp core "span up" in my childhood imagination. Now I find that this synchrotron radiation does indeed show that plasma charge is racing around a circuit or "spinning" at a great rate. I expected the focus fusion device to be doing something similiar.