Neutron stars and magnetic fields

[Forestman]

Since neutrons have no electric charge, how is it that a neutron star is able to produce a magnetic field?

 

[Janus]

Since neutrons have no electric charge, how is it that a neutron star is able to produce a magnetic field?

For the same reason that the Earth, while over-all is electrically neutral, has a magnetic field. Neutrons are made up of quarks, which do have an electric charge.

 

[Forestman]

I see your point, still if the quarks are forming neutrons, than their charge would cancel out. Leaving no charge at all. Plus the Earth has a magnetic field because electric currents run through the outer liquid core. Is it possible that electric currents run through neutron stars?

 

[SpiffyKavu]

A neutron star cannot be 100% neutrons, since that would be very unstable. At the surface, a neutron star is composed of "normal" matter, in that there are atoms and nuclei. Then it smoothly transitions into degenerate neutron matter. At these pressures, the neutron matter becomes a superfluid. In other words, the neutrons flow with zero resistance.

 

[Forestman]

Thanks SpiffyKavu, I think I understand how it works now. I guess that powerful electric currents in the crust of the neutron star could produce its powerful magnetic field. I might be wrong of course.

SpiffyKavu, would it be possible for electric currents to run through the degenerate neutron matter? And if they could, would they too experience superconductivity. I understand that the neutrons are already flowing without resistance, but I don't see how objects that don't have charge could produce magnetic fields. Form what I have learned, which is limited, only objects with charge can produce magnetic fields.

 

[Chronos]

The crust of of neutron stars is believed to be a thin layer of iron.

 

[Forestman]

Thanks Chronos. This is all extremely interesting.

 

[SpiffyKavu]

Yes indeed, Forestman. I have been searching around, and it turns out that current thought is that there is a layer within the neutron star which contains superfluid protons, in other words, superconducting protons! The thickness of this layer depends on the exact equation of state of the neutron star (how internal pressure depends on density and temperature). Unfortunately, the equation of state is not well defined for neutron stars, so the superconducting layer is not terribly well defined.

I will point you to a couple articles if you feel up to a little reading:
http://arxiv.org/abs/nucl-th/0412011

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVB-473NPV7-1KC&_user=961305&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000049425&_version=1&_urlVersion=0&_userid=961305&md5=c731cdf14efc1d5de5125dcc9a04ffe2"

As was indicated by Chronos, the crust of a neutron star is largely iron. As the pressure increase, more and more neutrons are packed into the atomic nuclei, until the pressure becomes high enough that free neutrons are quasi-stable. Even higher pressure yields greater stability, eventually leading to pretty much pure neutron matter.

 

[Forestman]

Thanks SpiffyKavu.

 

[Vanadium 50]

There is a lot of speculation here.

First, there is no need to posit that the magnetic field is trapped in the crust. The bulk of a neutron star is 10% protons (with their associated electrons), so you have conductivity. Possibly superconductivity, possibly not. There is also the magnetic moment of the neutron - even an insulating neutron star can trap the magnetic fields by spin-aligning the neutrons. I don't know of anyone who argues this actually happens - my point is that one can't argue that the crust must be involved.

Second, be careful of arguing that you have a superconducting neutron star. Type I superconductors expel magnetic flux. Again, this doesn't mean that the neutron star is superconducting, just that "make it superconducting" doesn't necessarily solve the problem you are trying to solve. Things may not be simple.