Magnetic reconnection: a prominent mystery, part 1…
May 31, 2012 Leave a comment
So you might remember a few weeks ago, in my post “A lifetime goal accomplished: astroian, the NASA intern…“, I told you that I was working with the Magnetospheric Multiscale (MMS) team here at NASA Goddard Space Flight Center. I also gave you a brief introduction to what MMS was and how it worked. Now I’d like to explain the main phenomenon which MMS is planning to investigate: magnetic reconnection.
You might be saying, woah, that sounds too sciencey for me. Never fear, it’s not as intimidating as it sounds. First off let’s learn more about magnetic fields. So what is a magnetic field? Well it’s exactly what it sounds like, it’s a field– or region of influence– occupied by a magnetic force. So you know that refrigerator magnet that you bought on your last trip to Florida? You know, the one with the empty beach chair and the Sun wearing the sunglasses, yeah that’s the one. Well that magnet (and any other magnet you know of) has its own magnetic field. And when you bring that magnetic field close to something metal (like your refrigerator), you can feel the magnetic force acting on the magnet, pulling it towards the metal. You remember that whole “opposites attract” thing from grade school, right? Good. Now you know about magnetic fields.
So what causes a magnetic field? Oho, now you’re asking the big questions! There are several ways that magnetic fields can be generated. The first, and most basic is part of the foundation of the physics field known as electricity and magnetism— or electromagnetism. Electromagnetism basically deals with the properties and effects of charges, both electric and magnetic, both in free space and in materials, and is governed primarily by a basic set of four equations known as Maxwell’s Equations (shown in all their scary mathematical glory below- just breathe, it’s going to be fine).
The group of equations is named after 19th-century Scottish physicist James Clerk Maxwell, who didn’t really come up with the equations (as you can tell by the names of those other guys: Carl Gauss, Michael Faraday, and André-Marie Ampère), but grouped them all together, made some tweaks, and realized that they were all inter-related and could explain electromagnetism in their entirety. Eventually Einstein’s theory of special relativity would cause a commotion, but with some adjustments to the equations, they still work! It was really a stupendous realization by Maxwell. Anyways, back to magnetic fields, so looking at Maxwell’s equations, the last equation (Ampère Law) is the one that really governs how magnetic fields are generated. So let’s try to understand what Ampère’s Law is saying. On the right hand side we have two terms, the second one is has that δE/δt, that means the change in E (the electric field– which is given off my a charge) over t (time). The electric field (E) changes whenever you have a moving charge, or current. So think about where you’ve heard the word current before regarding electricity– in wires, exactly. So wires have moving charges, usually negatively-charged electrons, that cause a current and as Faraday’s Law shows us on the left-hand side, generates a magnetic field (H). So every wire that has current running through it generates a magnetic field. Now most of the time that magnetic field is pretty weak and most of the time builders try their best to insulate those wires so that you don’t have rogue magnetic fields leaking all over your house (not like it would really cause you any harm if you did).
So a moving electric charge causes a magnetic field, and conversely Faraday’s Law tells us that a changing magnetic field causes an electric field. You can see that intrinsically then, magnetism and electricity are interrelated. Inside our planet and most others in the solar system, we have molten metal rock, which is spinning around the planet’s rotational axis, in what we call a dynamo. This can cause what we call a current loop, and thanks to Mr. Ampère, we know that a current loop can cause a dipole magnetic field. And that’s almost exactly what we see when we measure Earth’s magnetic field (see picture below).
And boy is that magnetic field a good thing for Earth, because without it we would be exposed to all of the might of the Sun’s magnetic field and all of the harmful radiation associated with it (See: Mars). Earth’s magnetic field is the same as would be generated by a simple bar magnet, with a north and south pole. Now, electric fields lines are defined by convention to illustrate how a positive charge would move in that field, so from positive to negative charge, and the “poles” of a magnet are always defined so that the “North” pole is the positive side and the “South” pole is the negative side, meaning that the field lines start in the north and travel south. Did you follow that? Good, now I’m going to confuse you. Research of Earth’s magnetic field has revealed that Earth’s magnetic “North” pole, aka the positive end of the bar magnet and beginning of its field lines, is actually at the Earth’s southern pole (in Antarctica) and its “South” pole, aka the negative end of the bar magnet and the end of its field lines, is in the Northern hemisphere (currently in Northern Canada but moving towards Russia). But of course, scientists now know that this north-south magnetic field alignment actually flips roughly every 200,000 years. We’re actually overdue for this phenomenon to happen again.
Above is what Earth’s magnetic field actually looks like. The region influenced by the Earth’s magnetic field is known as the magnetosphere, hence the Magnetospheric Multiscale (MMS) mission. Okay so now we have all our necessary background information about magnetic fields, but we still don’t know about magnetic reconnection! Well I think that will have to wait for another post…