Magnetic reconnection, how does it work?
March 12, 2015 3:37 PM   Subscribe

NASA’s Magnetospheric Multiscale (MMS) mission is set to lift off today, March 12, at 10:44 p.m. EDT from Cape Canaveral Air Force Station in Florida aboard a United Launch Alliance (ULA) Atlas V rocket. (NASA TV launch coverage)

MMS will study magnetic reconnection, a fundamental process that occurs throughout the universe when magnetic fields connect and disconnect explosively, releasing energy and accelerating particles up to nearly the speed of light.
posted by Rob Rockets (9 comments total) 7 users marked this as a favorite
 
No one - no one - understands magnetic reconnection. We wave our hands and talk about rubber bands snapping together under tension, but good luck getting fewer than three interpretations if you ask two theorists about your observations.
posted by RedOrGreen at 5:11 PM on March 12, 2015 [1 favorite]


Thanks for the heads up, I'll watch the launch with my kids tonight.
posted by Dr. Twist at 5:55 PM on March 12, 2015


OK does anyone know what an "Anomaly Chief" (AC) does?

(god help me if i misheard that in some hilarious way)
posted by tigrrrlily at 7:41 PM on March 12, 2015


Literally clicked on this randomly just shy of a minute prior to launch. I don't usually find myself online right around rocket launches, so the novelty of being able to watch a friggin rocket launching into space, from my own computer, live, is still pretty amazing. Thanks for posting this!
posted by cyrusdogstar at 7:49 PM on March 12, 2015


United Launch Alliance, whoa private company owns Atlas rockets? Wow who are they? Once they figure out how it works, will they turn it off at will, yeah right, just some junk energy? Yeah, interferin' with my football reception. I am cynical about some things.
posted by Oyéah at 8:00 PM on March 12, 2015


OK does anyone know what an "Anomaly Chief" (AC) does

Decides if an out-of-tolerance condition is an ignore, hold, hold and fix, or scrub.

In US manned launches, this was the call of the Mission Director, if early in the count, or the Launch Director, if late. The Flight Director didn't take control of the mission until the launch vehicle cleared the tower.
posted by eriko at 8:01 PM on March 12, 2015 [1 favorite]


There sure is some weird shit in the magnetosphere. I don't think we've sorted out LDEs - Long Delayed Echoes - which are radio signals that get reflected or delayed up to 16 seconds (or longer, perhaps) before returning. That's a long journey at lightspeed. but they may be connected with magnetosphere phenomena... or perhaps something else.

Personally, I've never even been sure that 'magnetic lines of force' actually exist, whether they're just convenient fictions like isobars or whether they're actual concentrations of magnetic field strength that are there when you're not measuring them.
posted by Devonian at 4:51 AM on March 13, 2015


I wonder sometimes if we're missing something fairly basic and important, in that large-scale magnetic fields don't really figure into our models for the evolution of galaxies, clusters, and the universe at large.

I hope somebody has a big "Aha!" moment soon.
posted by General Tonic at 8:19 AM on March 13, 2015


We understand some aspects of magnetic reconnection quite well – but the bottom line is that plasmas are complicated.

A plasma is what when you take a gas and start knocking electrons off the atoms, either by heating it up enough, or, as is the case in many space plasmas, having high energy photons zooming around and knocking electrons off in a gas that's so low density that it takes a long time for electrons and ionized atoms to find each other again.

The upshot of this is that you now have a fluid composed more or less entirely of at least two types of charged particles – electrons and ionized atoms (ions). I say 'at least' because you can have more than one type of element making up your plasma, and so more than one kind of ionized atoms, and anything bigger than hydrogen can have different numbers of electrons missing on each atom, and... it can get messy. But fortunately, a lot of plasmas in space are mostly hydrogen. So it's a good start just to pretend it's the only thing there.


So, right. Now you have this fluid of charged particles, which now means they all interact with electric and magnetic fields. Usually at large scales the magnetic fields are most important because freely flowing electrons do a good job of moving around to cancel out any static electric field that tries to build up.

Now magnetic fields affect charged particles by deflecting them. If an electron, say, tries to move perpendicular to a magnetic field, its path gets curved so it ends up moving around in a loop or a spiral in the direction of the magnetic field. If the magnetic field is perfectly straight and uniform, you end up with perfect spirals and perfect trapping. If it varies in direction or strength in any direction, the trapping gets less good, but is still pretty good. What this means is that all the particles are trapped to move along magnetic field “lines.”

Now lines turn out to be a really good way to visualize magnetic fields. At any given point in time you can draw a line following the direction of a magnetic field and be assured that its never going to run into a line you start drawing from a different point, or end, or do anything weird or crazy. And if you make sure each line is equally spaced at one magnetic field strength, you can make a good visual map of the magnetic field direction and strength if the lines end up closer together, that means a stronger field. And since the particles are going to generally be spiralling around in the direction of the magnetic field, you end up with a good map of where your plasma is trapped as well. And even more than that, in a plasma you end up with a net force from the magnetic field that tries to pull curves inward and creates outward pressure where magnetic field lines bunch up – so very much like rubber bands indeed.

Notice that the forces I mentioned are on the plasma, not the magnetic fields. However, not only do magnetic fields affect charged particles, currents of charged particles affects the magnetic field. Take an electromagnet, for example. Coil up a wire and run an electric current through it and you have a magnetic field. In plasmas you end up with a similar, but messier, process since nothing's confined to a wire. So it turns out that in addition to the trapping of particles to the magnetic field, you have a trapping of a magnetic field to the plasma fluid - as long as you have an “ideal” plasma. This is called the “frozen-in flux condition” because the magnetic field (flux) is frozen to the plasma. Ideal here means that the particles making it up aren't colliding too much and that things are happening slowly enough and on large enough scales that the fact that the ions are a thousand times more massive than the electrons doesn't really matter.

So, finally, we can get to magnetic reconnection. This is what happens when you have flows in an ideal plasma that smoosh two regions of magnetic field together that are pointed in different directions. They want to cancel out but the plasma doesn't want to let them so the region between them gets smashed thinner and thinner until something gives. Often pretty violently since the energy that was in those magnetic fields that just cancelled out has to go somewhere. And that process is called magnetic reconnection.

And this is where Devonian's skepticism about the existence of magnetic field lines comes in. Now notice that when talking about magnetic field lines earlier I said “at any given point in time” you can start drawing lines. The problem is that these lines don't necessarily retain their identity at different points in time. Now if you have an ideal plasma, you can just track the fluid. If you start drawing a line from one bit of fluid and then again at a later time then you'll end up connecting bits of plasma. But if you have a non-ideal plasma, all bets are off. If you draw a magnetic field line from one fluid element and it ends up going through one of these reconnecting regions where everything is non-ideal, the next moment you try to draw it, it could end up going somewhere completely different. That is what it means when people say magnetic field lines “break” and “reconnect.”

To give an example relevant to the article, one moment your bit of space plasma has a magnetic field line connecting it to the sun, the next moment it could be connecting to the Earth. Now since the particles in a plasma tend to be trapped along field lines, this has implications as to how many particles streaming out from the sun end up trapped in our magnetosphere messing with satellites and causing auroras and stuff. Which is why we want to study this stuff.

Now the tricky part of magnetic reconnection is figuring out what, exactly, gives when you push these two regions with different magnetic fields together. I'd say we have a pretty good handle on resistive reconnection. Resistance in plasmas means that it's hard to drive an electrical current, because a current means that the ions and electrons with their opposite charges are going opposite directions and run into each other. If the plasma is dense enough, then this happens a lot. If this is the thing driving reconnection, it's easier to figure out what's going on because at least you know that the non-ideal forces are in the same direction as the current. Because they come from electrons running head on into ions.

However, in space, there's not a high density of particles and not a lot of collisions, so you end up having what is called collisionless reconnection, which is a lot more tricky to sort out. There are so many different ways electrons and ions can move and respond to pressure that make a plasma non-ideal on the really small scales of magnetic reconnection that it gets pretty hard to sort out. Which is why having satellite that has really good instruments, but is really tiny relative to what's going on helps. It's not going to be big enough to affect the reconnection process and the reconnection's happening in a big enough region we can record it in detail as the probe passes through one of these regions. This will give us a better picture of how the Earth's magnetic field and the solar wind's magnetic field end up reconnecting.

Well that turned out a lot longer than I had wanted, but I hope it's intelligible enough that someone gets something out of it. But plasmas are kind of this crazy awesome mess, and reconnection is cool because its what happens when things get squished together in one of the ways where you actually have to deal with the messiness.
posted by Zalzidrax at 9:40 AM on March 13, 2015 [3 favorites]


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