In discussions of sun spots and auroras on Earth, magnetic field lines are often described as "snapping" or "breaking", with the result of releasing charged particles very energetically.

My understanding is that field lines are just a visualization tool. I don't understand, intuitively, how a field line could snap or break, or why that would result in a release of energy.

I'm having trouble even framing this question because the concept of a field line breaking just doesn't make sense to me. What is happening when a magnetic field "snaps"?


5 Answers 5


My understanding is that field lines are just a visualization tool showing points of equipotential magnetic moment tangent to the line.

Yes, field lines are just visualization tools we (humans) invented, they are not physical objects.

I don't understand, intuitively, how an equipotential line could snap or break, or why that would result in a release of energy.

This is an unfortunate consequence of simulations and somewhat due to press release descriptions. In the process of magnetic reconnection, the magnetic field topology does indeed change but fields are a continuous construct. What happens is that magnetic flux is being converted to particle kinetic energy. That is, the flux through the reconnecting region is decreasing and that energy must go somewhere. The result is an inductive electric field which accelerates particles. Because magnetic fields do experience something akin to tension like in a wire, when they are bent they experience an effect kind of like a force acting to straighten the field lines, as it were. Again, this is a visualization way of describing things but the physical way is that gradients in fields tend to do work to get rid of themselves in the absence of other forces.

I'm having trouble even framing this question because the concept of a field line breaking just doesn't make sense to me. What is happening when a magnetic field "snaps"?

Your confusion is warranted, as I stated above. Field lines do not snap, break, or move despite the language often used to describe these phenomena. It's an unfortunate choice that one chooses to describe something they know not to be physically true because it's sometimes easier than describing the real thing. Sometimes there are those that actually do not know that field lines are artificial constructs and they genuinely believe them physical objects. I do not agree with either of these, obviously.

So try think of things in the following way. The plasma involved in reconnection flows inward toward the region of interest. We'll ignore regions near a magnetic field source like stars or magnetized planetary bodies. In these cases, the only source of the magnetic field are the currents created by the relative drifts between oppositely charged particles. The magnetic field and plasma are coupled to each other in highly conductive plasmas through what's called the frozen-in condition (i.e., just a form of flux conservation), as desribed at https://physics.stackexchange.com/a/551944/59023. If the plasma in two adjacent regions starts to flow toward each other and the magnetic fields of each region have at least some projection anti-parallel to each other, then the plasma can generate a thin current sheet. If the current sheet gets thin enough and strong enough, it can become unstable to things like the tearing and filamentation instabilities (i.e., a current sheet breaks up into fine strands of current). The end result is the destruction of magnetic flux, radiation of numerous electromagnetic modes, and ultimately energy transfer from electromagnetic fields to particles.

I am intentionally being vague in the last sentence because although we know a lot about magnetic reconnection, there are still lots of unanswered questions. This is one of the many reasons NASA launched the Magnetospheric Multiscale Mission which has helped to illuminate that reconnection is not a fluid concept, as is often presented in MHD discussions of the topic, but a kinetic one with a separation of scales between electrons and ions.

  • $\begingroup$ honest_vivere For me, the magnetic fields in plasma are not of the same kind as those of a permanent or electromagnet in the air. In plasma it’s like many tiny magnets - the oriented magnetic dipoles of the involved subatomic particles - which form a non-permanent magnet. if they come into contact with other "strings", reassambling is of course possible. Maybe ask me a separate question where you can explain that? Or I’m wrongwith my imagination? $\endgroup$ Commented Jun 17, 2020 at 3:11
  • $\begingroup$ @HolgerFiedler - I'm not sure if this is correct. I usually just think of the magnetic field sources in a plasma, in the non-steady state form, as being comprised of many small-scale currents. I've never really felt comfortable with the magnetic field source of permanent magnets (i.e., I learned about all of these things but my understanding stops around the point of there being regions of coherent magnetic moments that one adds up, where the magnetic moments are due to the fundamental particles comprising the atoms making up the material). $\endgroup$ Commented Jun 17, 2020 at 14:27

Consider the following bar magnet, with the unphysical field lines drawn around it. The real magnetic vector field is tangent to these lines and is represented by black triangles (a magnetic vector field always emanates from the north pole to end up at the south pole, though it continues inside the magnet):

enter image description here

Now consider the following picture of two of equivalent magnetic bars and the associated field lines (in which for every field line only one direction of the magnetic vector field vector is shown by a very small triangle):

enter image description here

Field lines are always closed lines. This is easy to see in the single magnet (the lines continue inside the magnet). All field lines between the two magnets are connected (via the lines inside the magnets) with the lines on the far- left and far-right directed away from the magnets (which makes them closed, though that's hard to visualize).
Now when we pull the magnets away from each other (to form two separate bar magnets), the field lines between the magnets (which are actually not separate, but you can't draw an infinity of field lines) move away from each other too, like the lines on the left and right of the double bar magnet arrangement. The field lines on the far left bend up (forming closed lines with the right ones bending up, which makes their closed Nature visible, like the single lines already bending inward are closed) to connect with the field lines on the left of the field lines in the middle (wrt to a vertical line in the middle of the two magnets). So these lines in the middle seem to "snap", just as the closed lines emerging from the left and entering on the right after which they reconnect to form two closed loops in each magnet. The reversed process, i.e. two closed lines forming one closed line (which is also a form of snapping), occurs, as you might have guessed when bringing two bar magnets together, in the same arrangement as depicted, to form one bar magnet.

Because we pull the magnets apart the potential energy contained in the magnetic fields of two bar magnets is bigger than in a single one (if the two bar magnets were made from a single one by cutting it in two). You can imagine pulling them away from each other, and "snap!", two magnets with higher energy will emerge (actually the energy increases by infinite snaps in a continuous way, but separating them very fast will feel like a single snap).
The magnetic fields around the bar magnets are produced by the spins of unpaired electrons in the outer shell of the atoms. Each spin produces a tiny magnetic field and in ferromagnets (which are the ones we consider here), if the temperature is not too high, all these tiny fields are permanently aligned, which minimizes the internal energy of the ferromagnet.

Now, these kinds of processes (in very distorted ways and on much bigger scales) also take place on the surface of the Sun, but the (closed) magnetic vector fields are produced by huge plasma currents and the magnetic field lines are closed lines around these plasma currents. These plasma currents constantly change and thus the magnetic field lines. This induces electric fields, which accelerate charged particles, mainly protons, electrons, and a relatively small fraction helium nuclei (solar cosmic rays). When two or more closed field lines emerge from one closed field line (for example when one plasma current splits in two or more), the induced electric field becomes suddenly higher and this sudden increase of the induced electric field gives a burst of high energy protons, electrons, and helium (alpha particle).
So just as in the case of two magnets that are separated very fast, thereby increasing the magnetic field energy in a snap, the sudden increase in the magnetic field's energy is converted into a burst of cosmic radiation, which reduces the energy contained in the magnetic fields around the two (or more) emerging plasma currents (because of which the plasma currents are reduced in strength as a reaction). The difference with the case of the two magnets is that the increased energy in the magnetic field of the two magnets stays (approximately) the same, without imparting the increased energy to other stuff.

You can compare it with the lines of equal pressure in the weather developing. These lines are always closed too and they can merge or split to form new closed lines of equal pressure. The associated energies are contained in the winds. When one closed low-pressure line "snaps" into two closed lines, more wind energy will be released than in the case of the one closed low-pressure line.

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    $\begingroup$ The contrast in pictures is a helpful explanation. $\endgroup$ Commented Jun 18, 2020 at 7:51
  • $\begingroup$ @chrylis-cautiouslyoptimistic- Thanks! From a cautiously pessimistic cross-eyed painter (that's what my user's name means). :) $\endgroup$ Commented Jun 18, 2020 at 9:13
  • $\begingroup$ "the magnetic fields around the bar magnets are produced by electrons moving around atomic nuclei" - actually, no. Most of the magnetic field comes from the electrons' spins, which is a different phenomenon than the orbital angular momentum (which is different yet from movement in the sense of time-variant position probability distribution). Imagining the electrons as spinning in place would be closer (but still wrong). $\endgroup$ Commented Jun 18, 2020 at 9:53
  • $\begingroup$ @JohnDvorak That's what I actually meant. An unpaired electron in the outer shell is indeed associated with a time-varying wavefunction (which, I think, produces no noteworthy averaged magnetic moment). Every unpaired electron (which are the electrons I refer to) gives the atom a constant magnetic moment. I'll edit to make it more clear. Thanks! $\endgroup$ Commented Jun 18, 2020 at 19:27

You are right: magnetic field lines can't snap or break because they are not physical objects. They are more analogous to elevation lines on a topographical map, or more precisely to lines perpendicular to elevation lines: to the fall lines on a ski slope. However, they do describe something physical, which is the magnetic field distribution. When the sources of the magnetic field rearrange, the "magnetic field lines" can change discontinuously, and it's the discontinuous change that is referred to as "snapping" or "breaking".

  • $\begingroup$ So does that mean that if two magnetic objects are moving relative to each other at near-relativistic speeds, changes to "magnetic potential" would propagate at the speed of light, and the field lines would "appear" between places where the instantaneous potential crosses certain thresholds? $\endgroup$
    – supercat
    Commented Jun 18, 2020 at 17:06
  • 1
    $\begingroup$ Changes to magnetic fields always propagate at the speed of light. It's not crossing a magnitude threshold of potential that produces "field lines". Instead, it is variations in the arrangement of "peaks and valleys" in the magnitude of the potential that produce "field lines": the field lines correspond to the directions of the slopes between the "peaks and valleys". $\endgroup$
    – S. McGrew
    Commented Jun 18, 2020 at 17:16
  • $\begingroup$ That makes sense. But would it be fair to say that magnetic field lines can appear to move faster than the speed of light because they can move faster than the fields that are represented thereby (much as the spot projected by a rotating searchlight can travel at a rate much faster than any portion of the searchlight)? $\endgroup$
    – supercat
    Commented Jun 18, 2020 at 17:28
  • $\begingroup$ It depends a bit on what definition you choose for "magnetic field line". But you're right. For example, if two transverse magnetic waves are approaching each other from nearly opposite directions, there will be changes in the "magnetic potential contour lines" that move faster than the speed of light. The same can happen with light waves: phase contours can move faster than the speed of light because nothing physical is actually moving. $\endgroup$
    – S. McGrew
    Commented Jun 18, 2020 at 17:47
  • $\begingroup$ I think it might be worth extending your answer to note that because the phase contours don't represent any physical entity, they are not bound by ordinary concepts of conservation or movement. $\endgroup$
    – supercat
    Commented Jun 18, 2020 at 19:05

I simulated two bar magnets with 4 dipoles each and plotted the field lines and field intensity around them.

Here is what I observed.

magnetic field lines around two bar magnets

I noticed that the dipole chain forms a set of null spots in the field on either side of the chain. As the magnets are pulled apart, two of these null spots (green dots) move away from the point of separation, and this is where the field lines "snap". What is actually happening is that the field lines reform, changing from the field lines surrounding the combined bar magnet to the field lines surrounding the two separate bar magnets.

The field lines "snap" as they pass through the null spot. They are not really snapping, but as the field strength vanishes at the null spot, each field line can reform smoothly into two new field lines surrounding the two separate magnets. Field lines are drawn to follow the field direction, but they do not show field intensity, so they appear to snap when they pass through the null spots.

As material moving along a field line encounters a null spot, it will be free of the field, and if the surrounding field is too weak to recapture it, this material will escape into space.

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    $\begingroup$ This is a really helpful visualization. Thank you. $\endgroup$
    – Robert
    Commented Jun 22, 2020 at 19:45
  • 1
    $\begingroup$ That's what I envisioned in my head! Great! I don't agree with the last paragraph though. $\endgroup$ Commented Jun 28, 2020 at 1:24
  • $\begingroup$ It seems to me that the most likely place for material to leave the magnetic field is where the field vanishes. It doesn't seem likely to leave where the field is stronger. $\endgroup$
    – robjohn
    Commented Jun 28, 2020 at 1:51
  • $\begingroup$ I have edited the last paragraph to make its intent clearer. $\endgroup$
    – robjohn
    Commented Jun 28, 2020 at 19:24
  • $\begingroup$ I have added a larger animation from dropbox. Click on the small image to see it. If it stops working, let me know and I will put the smaller image back. $\endgroup$
    – robjohn
    Commented Jan 22, 2021 at 18:33

Do this experiment. (Be prepared for a bit of clean up.)

Place a small bar magnet under a piece of stiff paper. Sprinkle iron filings on top. Those "imaginary" "visualization tools" become quite apparent. Now, turn the paper and magnet over so that the iron filings are under the paper. Now, move the magnet away from the paper and watch the filings drop. Initially the magnetic flux lines and their properties exert enough force to overcome other forces and hold the filings in place. As the magnet is forced away from the filings, at some point that is no longer true and gravity takes over to cause the filings to "fly away" from the paper.

Now, visualize a gravity-infested, swirling, convective, nuclear-furnace, inducing its own electrical eddies and powerful magnetic fields. For simplicity's sake, lets refer to it as a star. As physical convective currents occur, localized magnetic fields develop and manifest as sunspots, flares etc.

Visualize also that our iron-filings are now jets of streaming plasma subjected to many different forces but following continuous paths as our star continues its molten, subsurface, chaotic, electromagnetic dance. As new magnetic hot-spots develop, the flux lines will move smoothly into new paths. (Smoothly does not necessarily mean slowly.) When the inertia of the particles in the stream becomes greater than the forces exerted by rapidly-changing flux lines, the particles become increasingly affected by other forces; e.g. gravity, solar wind, the particles' own inertia... The point being that the moving lines of force might change quickly enough that they suddenly lack the necessary forces to constrain their previously-captive, highly-energized, chromospheric particles.

In our little experiment, the filings likely dropped straight onto a table and made a small mess. Consider what would happen if you had a fan blowing on your setup.

Ponder the vectors and energies of suddenly-released plasma particles that until a moment ago had been circulating at tremendous velocities in an arc extending 10,000 km above our star's surface. The storm of forces at play might make quite a mess of your living room. -- D. R. McClellan


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