planetary magnetic fields

[upriver]

In this link posted by Nereid it shows a pulsar with a sideways magnetic field.
http://nobelprize.org/nobel_prizes/physics/laureates/1993/press.html

The direction of spin of a planetary body has nothing to do with the magnetic field generation?

Planetary and stellar dynamos: challenges for next generation models
http://www.es.ucsc.edu/~glatz/pub/glatzmaier_chp11_2005.pdf

[G O R T]

Have you never seen Glatzmaier's work on Dynamo theory?

http://www.es.ucsc.edu/~glatz/publications.html

[upriver]

Have you never seen Glatzmaier's work on Dynamo theory?

http://www.es.ucsc.edu/~glatz/publications.html

Yes, the paper that I posted is one of his.

In the paper I posted it showed magnetic field lines. I was just wondering if they represented electric current flow?

[tusenfem]

The direction of the magnetic dipole and of the rotation axis are not related in direction, otherwise we would not be seeing pulsars, although the pic in the press release may be a bit exaggerated. Otherwise, the rotation of the planet / star will be necessary for the generation of a magnetic field. And naturally does the presence of a magnetic field in a non-ferromagnetic object involve currents.

[upriver]

The direction of the magnetic dipole and of the rotation axis are not related in direction, otherwise we would not be seeing pulsars, although the pic in the press release may be a bit exaggerated.


That is one of the most amazing things I have ever heard!!!



Otherwise, the rotation of the planet / star will be necessary for the generation of a magnetic field. And naturally does the presence of a magnetic field in a non-ferromagnetic object involve currents.


So your saying the the MHD dynamo effect does not need rotation. So it works on convection? And buoyancy drives the convection? And the leftover heat from the formation of the planets drives that. Is that it?
What determines the direction of the magnetic field. How does the magnetic field "bend" in reality, vs the simulations?

[G O R T]

Some rotation is needed for the Coriolis force to impart necessary cross-vectors on the convection cells.

The necessary heat in a planet is derived from radioactive decay of heavy elements in the core.

Read this one. (http://www.es.ucsc.edu/~glatz/pub/glatzmaier_olson_sciam_2005.pdf)

[tusenfem]

That is one of the most amazing things I have ever heard!!!

So your saying the the MHD dynamo effect does not need rotation. So it works on convection? And buoyancy drives the convection? And the leftover heat from the formation of the planets drives that. Is that it?
What determines the direction of the magnetic field. How does the magnetic field "bend" in reality, vs the simulations?

Why is that amazing? I think the picture of how the "lighthouse of the universe" works is pretty often shown, or did you have your "tongue in cheek" button turned on?

If you would have read what I wrote, no I am not saying that. I said that rotation is necessary for a dynamo to work. From a very very nice paper by Glassmeier (do not mistake him with Glazmeier!!) on the Hermian magnetic field i found the greatest quick description of what happens in an alpha-omega dynamo:



Rapid convective motion in an electrically conduction core is sufficient to overcome magnetic diffusion out of the field generating region. In its simplest form differential rotation transforms poloidal fields into toroidal ones, the omega effect. Coriolis forces cause an alpha-effect, transforming torioidal fields into poloidal ones, and thereby closing the dynamo cycle. The field strength scales as sqrt(rho omega / sigma) where rho, omega and sigma denote mass densitz, rotation rate and the electrical conductivity of the core [e.g. Stevenson, 2003, "planetary magnetic fields", Earth & Planetary Science Letters, 208, 1-11]


So, look up Stevenson I would say. And I have no idea what you mean with how the magnetic fields "bend" in real vs. simulation.

[upriver]

The direction of the magnetic dipole and of the rotation axis are not related in direction, otherwise we would not be seeing pulsars, although the pic in the press release may be a bit exaggerated.



Why is that amazing? I think the picture of how the "lighthouse of the universe" works is pretty often shown, or did you have your "tongue in cheek" button turned on?




To think that the direction of rotation doesn't affect the direction of the magnetic field in a fluid dynamo system. That to me, is amazing.



So, look up Stevenson I would say. And I have no idea what you mean with how the magnetic fields "bend" in real vs. simulation.


That would be Gary A. Glatzmaier

Page 7 of this PDF. the magnetic fields are bending in the simulation. How does that happen? Do they follow the currents in the liquid metal?
http://www.es.ucsc.edu/~glatz/pub/glatzmaier_chp11_2005.pdf

Here is a thesis on the sodium experiments.
http://www.ireap.umd.edu/ireap/theses/Ellingston-Masters.pdf

In this experiment they expect the dipole magnetic field to be a axially aligned in the direction of rotation.
Why would you expect different?

[G O R T]

Page 7 of this PDF. the magnetic fields are bending in the simulation. How does that happen? Do they follow the currents in the liquid metal?
http://www.es.ucsc.edu/~glatz/pub/glatzmaier_chp11_2005.pdf


Please understand that the Earth's macroscale magnetic field is a composite of the fields of myriad individual convection cells. Local melding (bending) of misaligned fields is to be expected. Magnetic fields always bend to present a closed loop, they must. The fields are produced by current and affected by each other. To say magnetic fields are bent by current flow is non-sequitur, they are bent by other magnetic fields. These other fields are caused by current flow partially induced by magnetic fields.




Here is a thesis on the sodium experiments.
http://www.ireap.umd.edu/ireap/theses/Ellingston-Masters.pdf

In this experiment they expect the dipole magnetic field to be a axially aligned in the direction of rotation.
Why would you expect different?

This is an odd experiment for several reasons. Kick starting should not have been required since the experiment was already in the Earth's magnetic field. Subject material viscosity would have to be orders of magnitude lower to create convective cells properly affected by both coriolis force and spherical couette flow. The forces that would be generated are affected by Earth's gravity (convection in particular). The experiment was a flop anyway.

[tusenfem]

To think that the direction of rotation doesn't affect the direction of the magnetic field in a fluid dynamo system. That to me, is amazing.

That would be Gary A. Glatzmaier
Page 7 of this PDF. the magnetic fields are bending in the simulation. How does that happen? Do they follow the currents in the liquid metal?
http://www.es.ucsc.edu/~glatz/pub/glatzmaier_chp11_2005.pdf


Well, if you would look at the best known example, the Earth, you would find that the rotational north pole in somewhere in the middle of the arctic ice, whereas the magnetic north pole is in Canada, not really aligned.

And that the magnetic field lines are bend is a "Duh!!", what else would you expect? Like G O R T says, and I have said a million times, magnetic field lines bend and that means that there are currents (which kind of currents you can try to figure out for yourself).

Now, a dynamo is a very dynamic process, as you will know the dipolar poles of the Earth are wandering. Take the Sun, the magnetic field changes polarity every so often. Just a slight mis-alignment can do a lot when the star collapses to a neutron star. I am sure there are calculations about it, but look for a start at the neutron star chapters in the wonderful book by Shapiro & Teukolsky "Black Holes, White Dwarfs and Neutron Stars".

[upriver]

Well, if you would look at the best known example, the Earth, you would find that the rotational north pole in somewhere in the middle of the arctic ice, whereas the magnetic north pole is in Canada, not really aligned.





And that the magnetic field lines are bend is a "Duh!!", what else would you expect? Like G O R T says, and I have said a million times, magnetic field lines bend and that means that there are currents (which kind of currents you can try to figure out for yourself).


Let me say this another way. Just before the magnetic field lines come out of the what I assume is the crust they bend. Does the electric current follow the field lines outside of the earth? Does it continue straight while the magnetic field turns? The other interesting thing is that it appears that they think that a single current flow equals a single magnetic field line?



Now, a dynamo is a very dynamic process, as you will know the dipolar poles of the Earth are wandering. Take the Sun, the magnetic field changes polarity every so often. Just a slight mis-alignment can do a lot when the star collapses to a neutron star. I am sure there are calculations about it, but look for a start at the neutron star chapters in the wonderful book by Shapiro & Teukolsky "Black Holes, White Dwarfs and Neutron Stars".

In a laboratory, how do you make a dipole magnetic field? With a solenoid. A torus shaped coil.
Could the Van Allen Belts drive the earths magnetic field?

[tusenfem]

the currents do, usually not follow the magnetic field lines (Birkeland currents, which you like so much only change the already existing magnetic field). to make a magnetic field line bend you need a current that is somehow "perpendicular" to it. If you would take the figure and somehow be able to take the rotation of the field, you would get the, probably highly complex, current system of the core. Ah, at that matter, where you see the dipole come out, that is the boundary of the Earth's core, not the surface crust.

Math Note
Maxwell tells us that curl(B) = J, so when one takes the curl or rotation (depending on your language) of the magnetic fields, you can get the currents that are flowing. Now what is exactly curl or rot? This is an differentiation operator that works on a vector. E.g. the x-component of the curl only depends on the spatial variation of the vector in the y and z components. This would mean that Jx = dBz/dy - dBy/dz, now rotate the x, y and z circularly to get the other current components.
End Math Note

The Van-Allen belts are trapped high energy particles in the Earth's magnetic field. I strongly doubt that the current they might represent would be strong enough to create a field like the Earth's, who's dynamo is inside the core of the Earth.

[G O R T]

Does the electric current follow the field lines outside of the earth?Electric current can only flow in a conductor, and only flows when the magnetic field is moving or changing strength. Current is generated by a field but does not follow it.


Does it continue straight while the magnetic field turns? Electric current travels the shortest and least resistive path through a conductor that is available to close the current "loop". The direction of this return current is controlled by the conductor and only the source (induced current) is controlled by the original field. In a large conducting area eddy currents are produced that just go in localized circles around the field lines. Current induced by one field can be deflected by another field. Simple rules applied to the complicated interior of the planet give very complicated results.



The other interesting thing is that it appears that they think that a single current flow equals a single magnetic field line? Magnetic field lines are magnitude contours, much like the lines on a topographic map. Visual representations of them can be to any scale. For a dipole that can be a short straight line or the familiar many layered apple. Each "line" must be taken in context.




In a laboratory, how do you make a dipole magnetic field? With a solenoid. A torus shaped coil.A helical shaped coil gives a dipole field (which is really a toroidal field). A torus shaped coil has a closed circular field.



Could the Van Allen Belts drive the earths magnetic field? The Van Allen Radiation Belts are areas of high energy plasma caused by the Earth's magnetic field deflecting the solar wind into relatively high densities (for space) where they are beat to death (technical term) by cosmic rays. Some of the electrons follow the magnetic lines down to the Earth's north polar area and cause auroras. These are effects, the magnetic fields is the cause.

[hhEb09'1]

The Van-Allen belts are trapped high energy particles in the Earth's magnetic field. I strongly doubt that the current they might represent would be strong enough to create a field like the Earth's, who's dynamo is inside the core of the Earth.Not only that, but Gauss's original math analysis of the geomagnetic field convinced him that the field was generated internally rather than externally.