Omega effect: turning poloidal magnetic field into toroidal field

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Discussion Overview

The discussion centers on the omega effect, which is proposed to explain the transformation of a poloidal magnetic field into a toroidal field, particularly in contexts such as the sun and black hole accretion disks. Participants explore the mechanisms behind this phenomenon, including differential rotation and the inertia of mass, while seeking clarity and references on the topic.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about the omega effect and request detailed explanations and references.
  • One participant mentions that both purely poloidal and purely toroidal magnetic fields are unstable in heavenly bodies, leading to a mixed field over time.
  • Another participant describes a conceptual model involving a spinning bar magnet to illustrate how magnetic field lines might wrap around a rotating body like the sun.
  • There is a discussion about the concept of magnetic flux being "frozen" into a highly conductive medium, which may explain the twisting of field lines in a differentially rotating environment.
  • Some participants acknowledge their limited understanding of the topic and express uncertainty about the explanations provided.

Areas of Agreement / Disagreement

Participants generally express confusion and uncertainty regarding the omega effect and its mechanisms. Multiple viewpoints and interpretations are presented, and no consensus is reached on the explanations or terminology used.

Contextual Notes

Some participants note that their understanding of the omega effect and related concepts is superficial, indicating potential limitations in the discussion's depth and clarity.

Aziza
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Could someone please explain exactly how the omega effect works? It is supposed to explain how a poloidal magnetic field (of the sun for example, or a black hole accretion disk) becomes toroidal. This is supposed to be due to differential rotation and the inertia of mass. But I just don't see how this can cause field lines to twist into helix...if anyone knows a good reference regarding this subject I would appreciate it, I can't find anything detailed!
 
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Do you have an example of where you heard of this? The terms don't even make sense to me in this context, though I'm no expert on the subject.
 
justsomeguy said:
Do you have an example of where you heard of this? The terms don't even make sense to me in this context, though I'm no expert on the subject.

I have attached a picture as it applies to AGN. Also link to solar dynamo:
http://solarscience.msfc.nasa.gov/dynamo.shtml
 

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Aziza said:
Could someone please explain exactly how the omega effect works? It is supposed to explain how a poloidal magnetic field (of the sun for example, or a black hole accretion disk) becomes toroidal. This is supposed to be due to differential rotation and the inertia of mass. But I just don't see how this can cause field lines to twist into helix...if anyone knows a good reference regarding this subject I would appreciate it, I can't find anything detailed!

I've never heard this name. I do know that in (most? all?) heavenly bodies both purely poloidal and purely toroidal orbits are unstable. Either will decay into a mixed field, and eventually the energy will be divided evenly between the two. It was discovered in 1970 or so. The argument is simple but I never tried to understand it. There is a professor in Bonn named Jonathan Braithwaite who has some presentations about this, with all the detail you might care for.
 
Last edited:
ImaLooser said:
I've never heard this name. I do know that in (most? all?) heavenly bodies both purely poloidal and purely toroidal orbits are unstable. Either will decay into a mixed field, and eventually the energy will be divided evenly between the two. It was discovered in 1970 or so. The argument is simple but I never tried to understand it. The is a professor in Bonn named Jonathan Braithwaite who has some presentations about this, with all the detail you might care for.

I think he's asking the "how" or "why" the magnetic field becomes "wrapped" around the sun. The terminology seemed a bit strange to me, for sure, but if I can reword it, I think this is what he was getting at:

Imagine the sun is a stationary bar magnet. The magnetic field lines are arranged the normal way. As you start to spin the bar magnet, at first the lines rotate with it, but as the pace picks up, the lines start to bend and 'wrap around' the magnet (the sun) until, eventually, any particular field line leaves the pole, wraps around the northern hemisphere a bunch of times in one direction, reverses at the equator, and then unwraps the same number of times before diving into the south pole.

Why/how this happens is a mystery to me as well. I've read the explanation, but I lack the background to really grasp what's happening.

Of course I could be completely wrong in describing the OPs question.
 
justsomeguy said:
I think he's asking the "how" or "why" the magnetic field becomes "wrapped" around the sun. The terminology seemed a bit strange to me, for sure, but if I can reword it, I think this is what he was getting at:

Imagine the sun is a stationary bar magnet. The magnetic field lines are arranged the normal way. As you start to spin the bar magnet, at first the lines rotate with it, but as the pace picks up, the lines start to bend and 'wrap around' the magnet (the sun) until, eventually, any particular field line leaves the pole, wraps around the northern hemisphere a bunch of times in one direction, reverses at the equator, and then unwraps the same number of times before diving into the south pole.

Why/how this happens is a mystery to me as well. I've read the explanation, but I lack the background to really grasp what's happening.

Of course I could be completely wrong in describing the OPs question.

Aha. Well, when you have a magnetic field in a highly conductive environment then the flux tends to get "frozen" to the conductive medium. That is, they move together as one and wherever one goes the other goes too. So if you have a differentially rotating sun, then the flux tends to get twisted like a barber pole.
 
ImaLooser said:
Aha. Well, when you have a magnetic field in a highly conductive environment then the flux tends to get "frozen" to the conductive medium. That is, they move together as one and wherever one goes the other goes too. So if you have a differentially rotating sun, then the flux tends to get twisted like a barber pole.

ohh so it is sufficient to just prove that flux is frozen into plasma in order to see why field lines can get twisted, right?
 
Aziza said:
ohh so it is sufficient to just prove that flux is frozen into plasma in order to see why field lines can get twisted, right?

I guess. My understanding is superficial.
 

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