Author’s Notes:

What generates Earth’s magnetic field?

The riddle began to be solved around the turn of the 20th century, when people realized that electricity and magnetism are two aspects of a single force. It was hypothesized that there’s an electric current flowing inside the Earth. Seismologists found through the study of earthquake waves that Earth has a distinct core, almost like the pit of a peach. They found that the core is very heavy – and so scientists convinced themselves that the core is iron. Earth’s heavy, iron core can carry an electric current – and a magnetic field can be associated with this current. Add in Earth’s rotation – and you have the dynamo theory. According to the theory, Earth’s spin stirs the material in the planet’s core – to sustain the electric current – and generate Earth’s magnetic field.

When astronomers look out into the universe, they “see” evidence for magnetism everywhere.

Virginia Trimble is an astrophysicist at the University of California at Irvine. In a recent interview, she said, “There are magnetic fields that run along the arms of the spiral structure of our own galaxy. There are magnetic fields pervading whole galaxies, whole clusters of galaxies and the gas in them. And to make a large scale field is much harder than to have a little electric current in your kitchen and the field around it. And everything in the universe really is magnetized. Sometimes it’s important, sometimes it isn’t, but magnetic fields are utterly pervasive.”

So how do astronomers “see” these magnetic fields?

Dr. Trimble described how they were originally discovered around 1950: “The magnetic fields line up dust grains. Dust particles aren’t spherical, they tend to be little sort of needly things. The magnetic fields line up dust grains, so the light that’s scattered by those dust grains is polarized. That was discovered by Hall and Hiltner in 1948 and explained in 1951 by Elever Davison and the late Jessie Greenstein as alignment of dust particles in the magnetic field of the galaxy. So that’s one way – they line up dust, the scattered light is polarized.” Another technique involves measuring polarization and spectral signatures in synchrotron radiation. A third technique involves a property of electromagnetism called the Faraday Effect. All three give the same answer.

The strength of the magnetic field in our galaxy is smaller than the Earth’s magnetic field, but because of the sheer size of our galaxy, that represents an enormous amount of magnetic energy.

Excerpts from an interview with Virginia Trimble:

The process by which stars form from interstellar material in the Milky Way has a significant magnetic component – that is, when you go to calculate how stars from, you’ve got to include gravity, and turbulence, and cosmic rays and magnetic fields, or you don’t get the right answer. So magnetic fields are important, they’re utterly pervasive and we do not in truth understand why they are utterly pervasive.

And there are two schools of thought – well, at least two schools of thought – one is that it’s kind of like your kitchen, that is the first things that end up with magnetic fields in the universe are small scale – they’re the early neutron stars and early quasars back when the universe was less than 10 percent of its present age – and the fields get blown out and stirred around and amplified into large scale structures.

The alternative is that the very early universe produced some magnetic field from physical processes that derive from very high power particle physics – from symmetry breaking and quantum chromodynamics and all this good stuff and that the very early universe left the gas all with a very weak magnetic field that has then been amplified as the gas contracted to make galaxies and spun around and spiral discs and so forth. So there’s a top-down and a bottom-up scenario. I don’t know which one is right. And I think in truth nobody really knows. But there are strong proponents on both sides.

-Is there some kind of overall orientation or structure to the cosmic magnetic fields?

The fields in individual galaxies have orientations on scales the size of the spiral arms. But when you look for instance at the faraday rotation of quasars that we see outside the milky way, it is clear that the galaxy as a whole, the fields are chaotic. They’re ordered on the scale of thousands of parsecs – the scale of the spiral arms. But the direction reverses between adjacent spiral arms. And when you go to larger structures, again you will find that the fields – for instance in a quasar, the fields will be aligned with the jets coming out of the black hole toward the lobes of radio emitting material. But if you look from one quasar to the next, there is no correlation of those directions. Just as there’s no large scale rotation of the universe, there’s no net magnetic field direction.

-So what would the universe be like without cosmic magnetic fields?

Well it’s very possible that the whole solar system might never have formed. I mean star formation involves magnetic fields. From the point of view of life on Earth, solar activity on the whole is bad.

[11 year cycle - at the height of activity, over the horizon radar and power grids are endangered - you also have to worry about health risks to astronauts from cosmic rays that are no longer blocked by the magnetic field.]

If there were no magnetic fields at all – as I say again, the solar system might never have formed – there would be no cosmic rays, and therefore there would be no need to protect Earth from cosmic rays and so the Earth’s magnetic field – the fact that Earth had no magnetic field wouldn’t matter. One practical consequence, the beginning of the great age of navigation around 1450 clearly coincided with the mariner’s compass – the magnetic compass reaching Europe from China, courtesy of the arabs who did some voyaging of their own early on. And in the absence of the magnetic compass, one guesses that navigation would not have been successful as it was. And who knows when Columbus would have gotten to the North America – maybe never.

-Is it on the top 10 list of astrophysics questions because it’s so complicated?

If it were something fairly easy like what does the back side of the moon look like, that one knew exactly what to do to find out, it wouldn’t count as an interesting puzzling question. The really good problems are the ones where you aren’t really 100 percent sure you’re asking the right question.

More Resources:

“”Magnetic Anomalies: What are magnetic fields doing in the middle of nowhere?,”":http://www.sciam.com/article.cfm?articleID=000D6F58-EBCD-1C73-9B81809EC588EF21 (By George Musser, Scientific American, August 2000)

“”Cosmic Bubbles Containing Magnetic Fields May Shape Galaxy Clusters,”":http://www.sciam.com/article.cfm?articleID=000AEC1A-B0B6-1CCE-B4A8809EC588EEDF (Scientific American, January 09, 2002)

The following individual was interviewed for today’s show. Our thanks to:

Virginia Trimble
Professor of Physics and Astronomy
Department of Physics
University of California, Irvine
Irvine, CA