Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Omega effect: turning poloidal magnetic field into toroidal field

  1. Nov 26, 2012 #1
    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!
  2. jcsd
  3. Nov 27, 2012 #2
    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.
  4. Nov 27, 2012 #3
    I have attached a picture as it applies to AGN. Also link to solar dynamo:

    Attached Files:

  5. Nov 28, 2012 #4
    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: Nov 28, 2012
  6. Nov 28, 2012 #5
    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.
  7. Nov 28, 2012 #6
    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.
  8. Nov 28, 2012 #7
    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?
  9. Nov 28, 2012 #8
    I guess. My understanding is superficial.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook