Has a minor update about ongoing plasma research just confirmed a decades old idea about the physical origin of 'magnetic reconnection'? The notion of 'snapping' and 'merging' magnetic field lines may be a thing of the past, replaced by an old theory: exploding electric circuits (exploding double layers).
Breaking news promises to revolutionize our understanding of such processes as magnetic substorms and solar flares wherein an unknown process (commonly referred to as 'magnetic reconnection') releases vast amounts of energy very quickly. And by revolution, I mean that it promises to take us a step backward.
"Wait, backward? How can that possibly be a good thing?" you might ask.
It's a long story about a quiet row in astrophysics that nobody likes to talk about... But, this is my blog, so "out with it!"
There is an unstated or very quietly stated assumption in astrophysics that electric currents can't exist in space (except when they're shown to exist by actual observations; how embarrassing!), or if they do exist they don't do anything of note (well, okay, there's that one jet that's the largest electric current in the universe; "but other than that, what's electricity done for me lately?"). And even if electricity does do interesting things out there, gravity is still king so n'yah!
Unfortunately, this notion has straight-jacketed astrophysics, astronomy, magnetospheric physics, heliospheric physics, etc. In terminology, if nothing else. It's a rather queer situation. Astronomers will tell you they 'factor in electric currents,' on the one hand, and then they'll couch things in terms of 'magnetic reconnection' in such a way as to make you think they have no clue about how electricity and magnetism work, on the other..
Why is this bad? Well, there's the notion of the 'pseudo-pedagogical concept.' Why are you giving me a blank look? Ohh right... Not a philosopher. Okay, in layman's terms, sometimes when one starts using a certain set of terminology and ideas to refer to something else (as a sort of analogical placeholder), one starts believing that the placeholder one has invented is equivalent to the actual thing one was referring to and stops thinking in terms of the original idea at all.
So, what is 'magnetic reconnection?' Up until today, it was pretty fair to say 'nobody knows.' That is, in this particular case, a certain formalism has crept in whereby astronomers and astrophysicists seem to prefer talking in terms of magnetic fields and magnetic field lines.
Although some of us think it's bad, it's also somewhat understandable. Here's why: 1) magnetic fields are easier to detect. 2) Magnetic fields can be easier to work with mathematically.
So, it's certainly easier (if lazier) to talk in terms of the thing we can 'see' and treat mathematically with less effort.
But, getting back to the question of 'what is magnetic reconnection,' it arises out of the observation of certain events in space. In particular, 'magnetic reconnection' is a process of interest in solar flares and the magnetic substorms in our magnetosphere that spark the auroras. What we see is that during substorms, the magnetic field changes in particular ways. At the same time vast amount of energy are released very quickly. The relationship between the magnetic field changes and the energy release is what is not well understood and underpins the idea of magnetic reconnection. We'll return to that in a moment.
Now, a magnetic field is visualized using magnetic 'field lines' kind of like the ones you see with bar magnets and iron filings. Though in the case of field line diagrams the lines serve a specific purpose: denoting magnetic field strength and direction. They are a visual tool and that is all.
That brings us to another problem: reification.
I know, another philosophy word. Basically, it means taking some abstract thing and making the mistake of thinking it's a real thing.
In this case, it seems that some astronomers and physicists have goofed and think that 'field lines' are real. They are not. They are lines on a page. There is not a material object (such as a thread, wire, rope or sheet) in 3-dimensional space corresponding to that line. The line merely represents the strength and direction (sometimes thought of as a 'vector') of the force felt by a charged particle placed at that location in the field. Nothing more or less.
So, they've associated observed energy release with observed magnetic field topology changes (changes in the shape of the field lines) and have come to the conclusion that it's the magnetic 'field lines' that are doing something, and in the process energy is released. That is, 'field lines' are thought to be 'twisting,' 'coiling,' 'breaking' and 'reconnecting.' In point of fact that is not even possible, because they do not exist. Morever, magnetic fields are solenoidal (continuous). What does that mean? It means, for one thing, that field lines are ALWAYS drawn as complete loops (except where they run off the page, and then only if the same number of line return to the page as left the page) and cannot 'break,' let alone then 'reconnect' without violating Maxwell's equations.
"...fields are solenoidal: that is, they never begin or end..."
But, there are still observations for which to account... And, account for them we must if science is ever to progress to completion. So, where does that leave us?
We know that magnetic field topology changes and we know that copious energy is released at approximately the same time. But we also know that field lines do not exist and can't break or reconnect. This should also give us a clue that field lines are not themselves a primary mover and shaker. It's hard for things that don't exist to do much of anything. Nonetheless things get done...
So, how do we reconcile this conundrum?
While magnetic field lines do not exist, they are helpful visual aids to determine properties of that which does exist, namely magnetic fields (keeping in mind that magnetic fields are actually the expression of a force felt between electric currents).
Hopefully you have already read my very first post. If not: stop what you're doing; don't read any further; read that post first. I'll wait. I'm not going anywhere. I'm just a blog post.
Okay, now that we're sure you've read that, my next statement should no longer come as a cognitively dissonant shocker:
Electric currents are the sole source of magnetic fields in the universe.
To quote myself further:
As electric currents produce magnetic fields, any magnetic fields we observe must be produced by electric currents.
Heady stuff, I know! Simple and to the point... Shocking! And we haven't even gotten to the 'A' material yet...
The above reiterated, the bent of the suggestions in the remainder of this post should not be entirely surprising. Hopefully, it's just pretty cool science.
Let's get to the cutting edge research now and then suggest a better metaphysics to replace the tired old dog known as 'magnetic reconnection'!
In the plasma physics lab (as opposed to 'in a giant supercomputer simulation with 30 adjustable variables'), researchers (Moser & Bellan) have studied the physics of jets (electrical discharges) in plasma in order to ascertain the what, when, where, why and how of 'magnetic reconnection.'
"As in all electrical currents, the flowing electrons in the plasma jet generate a magnetic field, which then exerts a force on the plasma. These electromagnetic interactions between the magnetic field and the plasma can cause the jet to writhe and form a rapidly expanding corkscrew. This behavior, called a kink instability, has been studied for nearly 60 years..."
"The jets in the experiment formed 20-centimeter-long coils in just 20 to 25 microseconds. [Moser] also noticed tiny ripples that began appearing on the inner edge of the coil just before the jet broke--the moment when there was a magnetic reconnection."
"...after months of additional experiments, they determined that the kink instability actually spawns a completely different kind of phenomenon, called a Rayleigh-Taylor instability. A Rayleigh-Taylor instability happens when a heavy fluid that sits on top of a light fluid tries to trade places with the light fluid. Ripples form and grow at the interface between the two, allowing the fluids to swap places."
"What Moser and Bellan realized is that the kink instability creates conditions that give rise to a Rayleigh-Taylor instability ... The plasma tries to swap places with the trailing vacuum by forming ripples that then expand--just like when gravity forces a heavy fluid to try to change places with a light fluid underneath."
"While the coil created by the kink instability spans about 20 centimeters, the Rayleigh-Taylor instability is much smaller, making ripples just two centimeters long. Still, those smaller ripples rapidly erode the jet, forcing the electrons to flow faster and faster through a narrowing channel. 'You're basically choking it off,' Bellan explains. Soon, the jet breaks, causing a magnetic reconnection."
This research puts the answer to the 'magnetic reconnection' mystery squarely back into the court of plasma physics.
Basically, what they're saying is that there is a large-scale electric current flowing. That current produces its own magnetic field. If the current and its self-magnetic field are inhomogeneous, an instability can develop (the kink instability; literally, a kink develops in the current filament). Additionally, a smaller-scale Rayleigh-Taylor instability can develop that causes the primary current to 'pinch' (self-constrict) and 'neck off' (similar to what happens in a 'sausage instability' where a current may self-constrict at many locations along its length making it look a bit like Bratwurst links; don't eat it!). In the end, this 'pinch' can completely disrupt the current.
You'd think that would be where the story ends. That's certainly where that article ends. But this post continues a little further. Read on valiant listener! I implied a solution and I'd hate to disappoint.
What happens when you disrupt such a circuit? Is it like a light switch? Not necessarily...
Here I'll turn to an esteemed colleague, Don Scott, who had already some time ago turned his steely electrical engineering eye toward the notion of 'magnetic reconnection' in an old post of his own, wherein he quotes Nobel prize winner Hannes Alfvén:
"'In the case of the instability leading to the extinction of the current, it should be remembered that every electric circuit is explosive in the sense that if we try to disrupt the current, a release of the whole inductive energy at the point of disruption will occur.' - H. Alfvén, Cosmic Plasma, Reidel, Holland, Boston, 1981, p.27."
"Alfvén extrapolated his findings about terrestrial power lines to the study of magnetized cosmic plasma. In the case of the disruption of an electric current within such a plasma, he said, 'If the current disruption is caused by an instability in the plasma, the inductive energy in the circuit will be released in the plasma. … The disruption of a current through a plasma is often caused by a double layer becoming unstable.'"
"Astrophysicists ignore Alfvén's work. They attempt to arrive at a de novo explanation for the release of such energy by embracing the notion that the motion and interaction of magnetic field lines is its root cause. They expound on the (basically false) idea that magnetic fields are 'frozen into' plasma, and by moving and breaking, these lines carry the plasma along and spew it out into space."
So, it would seem that the disruption of a current in plasma isn't quite the same as flipping a light switch. Rather, when the circuit is disrupted, the inductive energy of the circuit explodes out of the point where the circuit is broken.
It is here that we find our impulsive energy release at the same time as the apparent change in the shape of the magnetic field. When a circuit is broken, there is an electrical explosion. Sometimes the explosion of a double layer of separated electrical charges is involved.
In fact, a mechanism such as this was proposed as far back as 1986 or even earlier:
"According to Bostrom (1974) and Akasofu (1977), an explosion of the transverse current in the magnetotail gives an attractive mechanism for the production of magnetic substorms (see Fig. 11). Bostrom has shown that an equivalent magnetic substorm circuit is a way of presenting the substorm model. The onset of a substorm is due to the formation of a double layer, which interrupts the cross-tail current so that it is redirected to the ionosphere."
Although a double layer may not always factor in, the notion of a current disruption and its attendant energy release has apparently circulated for some time. It seems the idea's time has finally come.
The sequence of events appears (in this instance), to this author, to begin with an electric current (as it should since magnetic fields are involved). At some point the current filament develops an instability such as a kink instability. Beyond that the Rayleigh-Taylor instability develops, causing the current to pinch, neck off and break the circuit. At the point of the break in the circuit, the entire inductive energy of the circuit pours out impulsively. Since the underlying current system has changed and a new impulsive event occurs, it would naturally follow that the 'field lines' denoting the magnetic field shape and strength must be redrawn.
Finally, we can return to a valid causal metaphysics based on real-world entities doing things they are well-known to do in the lab (and we should expect no less in space). We now have a pretty good understanding of what's going on. Currents flow. Currents can become unstable. Instability sometimes breaks the circuit. An electrical explosion may occur at the point of the break in the circuit (not unlike the electrical arcs that happen occasionally at power substations). Magnetic fields change when the underlying current system changes. 'Field lines' do nothing.
'Magnetic reconnection' is dead! Plasma physics hath killed it. Long live plasma physics!
Many kind thanks to Bellan and Moser for this innovative work, even though they have never heard of me (to my knowledge, anyway) and have in no way sanctioned or approved this post. So, if anyone must take the blame for this, I shall happily bear full responsibility for my own contributions and pass on responsibility to Don Scott and Hannes Alfvén those portions which I have borrowed in good faith...