# Does accretion reduce the magnetic field of a neutron star?

1. Dec 17, 2015

### Bernie G

When a large amount of plasma radiates greatly while accreting on a magnetized neutron star, where does the radiation ultimately get its energy from, the magnetic field and/or gravitational field? Without doing the numbers, if the magnetic field does the work against the gravitational field, and assuming the neutron star only has an inherited magnetic field, would the magnetic field strength decrease a little as a result of the accretion?

2. Dec 18, 2015

### Bernie G

I’d like to modify the above question. The radiation gets its energy from the change in gravitational potential energy of the accreted plasma. Does the magnetic field expend energy during the accretion process? Maybe the answer is simply no but any thoughts on this would be appreciated.

3. Dec 18, 2015

### Jonathan Scott

If I remember my classical electromagnetism correctly, a magnetic field doesn't do any work on a moving charge, as the force is perpendicular to the motion.

4. Dec 18, 2015

### Bernie G

Could the pressure from the accretion be large enough to push some of the magnetic field into the surface of the neutron star?

5. Dec 19, 2015

### Jonathan Scott

How do you push a magnetic field with pressure? The question doesn't make sense to me.

6. Dec 19, 2015

### Bernie G

Gravity does not directly affect a magnetic field and normally gravity could not push a magnetic field. But above the neutron star equator gravity would exert great downward force on the plasma and hence the magnetic field lines. Does that make sense?

7. Dec 19, 2015

### Jonathan Scott

Well, perhaps in a vague hand-waving sort of way, but it all seems a bit mixed up.

I'm not an expert on this, but based on what I remember from student days...

Basically, as I presume you know, gravity pulls stuff in, angular momentum speeds it up, friction heats it up to plasma and charged stuff tends to follow magnetic field lines while being accelerated by gravity and coriolis forces, and that current flow can modify the existing magnetic field (as well as leading to jets). Material which is in free fall is not likely to be at high pressure, and if the material initially had significant angular momentum it may initially go into orbit and only gradually be dispersed inwards. The intense magnetic field of a neutron star is mostly created by the initial contraction to form the neutron star which speeds up the rotation rate. The magnetic field may possibly be reduced by incoming material subsequently slowing the rotation. I don't personally know anything about the way in which the motion of plasma is expected to modify the magnetic field.

8. Dec 20, 2015

### Bernie G

The model I normally use for gyrotating plasma is primarily electrons are magnetically contained, the ions are mostly electrostaticaly coupled to the electrons, and gravity has a negligible effect on this ion-electron mix. But near a neutron star gravity is the prime mover on the ion-electron mix. Wouldn’t large perpendicular force be exerted on the magnetic field lines above the equator? There the magnetic field lines are what supports the plasma.

9. Dec 20, 2015

### Jonathan Scott

I'd agree that if material is deflected by the magnetic field, then there is an equal and opposite force somewhere, effectively back on the source of the field. However, a magnetic field isn't something you can "push" on in the mechanical sense. You can obviously modify it, including changing the intensity and distribution, by adding currents and other magnetic sources. I've not studied this particular case and I can't say anything more about it, sorry.

10. Dec 20, 2015

### Bernie G

"A magnetic field isn't something you can "push" on in the mechanical sense."

I disagree at least for now. Gravity can't directly push or pull a magnetic field, but charged particles contained by the magnetic field could push or pull the magnetic field. If these charged particles are affected by another force, say pulled by gravity, gravity should then be indirectly pulling on the magnetic field via the contained particles.

11. Dec 20, 2015

### Bernie G

Above instead of opining "the pressure from the accretion could be large enough to PUSH some of the magnetic field into the surface of the neutron star" it should have said "the gravitational force on the accretion could be large enough to PULL some of the magnetic field into the surface of the neutron star"

12. Dec 22, 2015

### Staff: Mentor

The motion of charges can certainly affect the magnetic field. I don't know that "push or pull" is the best way of describing the effect, though.

Neither of these seem like apt descriptions to me. For example: what effect would either "pushing" or "pulling" the magnetic field into the surface of the neutron star have on the field strength? Your hand-waving model doesn't give any answer to that question, as far as I can see.

You might want to take a step back and think about Maxwell's Equations.