PBS Space Time

- Earth's magnetic field protects us from deadly space radiation, but what if it were drastically weakened?

For example, as a precursor to it flipping upside down.

I mean, it has before, many many times.

(upbeat techno music) Spaceship Earth has a literal deflector shield, a geomagnetic field.

Lines of magnetic force forged by currents in the planet's molten core erupt from the surface close to the north and south geographic poles, connecting to each other to wreathe the planet in a dipole field like a giant bar magnet.

Magnetic fields exert a force on moving charged particles causing them to spiral around those force lines.

Now, that's helpful because Earth is constantly bombarded by very fast moving charged particles, especially coming from the sun.

Our magnetic field deflects the worst of these.

Not all planets are so lucky; Mars with its solid core has no such shield, and so the Red Planet's atmosphere was stripped away by the solar wind billions of years ago.

So what would happen if the Earth lost its field?

Would we lose our atmosphere?

Would life be extincted by crazy space radiation?

Well, we may get to find out.

The magnetic field is currently undergoing rapid changes, possibly signaling the imminent flipping of its polarity.

The North Pole may become the South, and the South the North.

This is called geomagnetic reversal, and during that reversal we'd be left relatively unprotected for thousands of years.

I'll come back to the current situation and whether we should be concerned, but first, why do we think such a thing could happen?

Well, because it has in the past, many times.

We see it in the geological record, magnetic materials like iron often form with their natural magnetic field aligned with the earth's field.

We can track the direction of earth's magnetic field in sedimentary layers and in old volcanic flows.

Turns out, Earth's field has completely flipped direction 183 times over the past 84 million years.

So, a little more than once per half a million years.

The last full geomagnetic reversal was over 700,000 years ago, so you might say we're past due.

But not exactly.

In fact, these flips seem to be pretty random events.

We may be no more due than we were at any other time in the past half-million years.

Except for the fact that the magnetic field does seem to be acting strangely lately.

But to really understand whether a flip is likely, we need to try to understand how they happen and for that, we need to understand Earth's magnetic field.

Normally we think of magnetic fields being generated in two ways.

In magnetic materials like iron, the sum total of the tiny magnetic fields of their constituent particles are lined to give you a global field.

That's your bar magnet or a fridge magnet.

Alternatively, flows of many charged particles like electrons, so electrical currents, can produce magnetic fields, for example, in an electromagnet.

But Earth's interior is not intrinsically magnetic, its too hot for iron atoms in the core to spontaneously align.

And although the interior is rotating, it's electrically neutral, so there shouldn't be an overall electrical current.

How then does Earth generate such a gigantic and well organized dipole magnetic field.

Let's start with a quick review of what the interior of the Earth looks like.

Beneath the thin crust and 2900 kilometers of solid mantle, lies Earth's core.

The 2400 kilometer thick outer core is molten iron and nickel and some other stuff.

I'm not talking about lava here, I'm talking about liquid metal with the viscosity of water.

Beneath this layer is the inner core, a 1200 kilometer radius ball of solid iron.

It's solid because of the pressure at that depth.

At around 55 Kelvin temperature, it would instantly melt at lower pressures.

Everything is moving down there, the solid core rotates, slightly more quickly that the surface.

The core's day is a few seconds shorter than a surface day.

The outer core has a rotation gradient.

The outer layer actually rotates a bit slower than the surface, but it speeds up as you get deeper.

And that motion gets even messier.

The interior of the Earth is cooling down very slowly, which means the liquid outer core is freezing onto the solid inner core.

As the inner core grows, it releases non-iron impurities that flow upwards, joining convection streams.

These flows are then twisted into helices by the coreolis force, the same effect that produces hurricanes on Earth's surface.

It's all this motion that together produces Earth's magnetic field, through a process called the dynamo effect, or so most scientists accept these days.

And Dynamo Theory not only explains geomagnetism, but also why Earth's field sometimes reverses its polarity.

Here's how it works.

In that motion I just described, electrons and nuclei should all be moving together, so no electrical current.

So where does the magnetism come from?

The key is that the dynamo effect doesn't really create a magnetic field from scratch, instead it amplifies, organize and sustains an existing field.

I'll come back to where that initial magnetic field comes from, for now let's just say we start with some weak dipole field.

That field passes through the liquid outer core which is an electrical conductor.

Conductors have this cool property that they drag magnetic fields with them, so if the entire core is rotating with the earth, then the magnetic field will also rotate.

But remember that the rotation of the outer core gets faster towards the center.

As a result, the starting magnetic field gets wound up into rings around the axis of rotation into a torus shape.

Now, cut to the second type of motion in the outer core.

You have these streams of conducting material twisted into helices by the coreolis force.

Those flows grab hold of our toroidal magnetic field and twist them up further into many little loops.

Those loops form magnetic tubes around the Earth's rotational axis.

These in turn generate toroidal electric currents.

Now we have exactly the conditions of an electromagnet.

Organized rings of current which produce our giant dipole field.

Okay, so start with a weak dipole field and you get a strong one.

But where does the initial magnetic field come from in the first place?

Well actually any weak field, even random bits of field, for example, from thermal fluctuations, are enough to initiate this runaway effect.

Once started the field builds to maximum strength.

In fact, any rotating body with a fluid conductor can produce such a field.

The Earth, but also the sun with its flowing hydrogen plasma, or the liquid metallic hydrogen in Jupiter and Saturn's cores.

The field produced by this effect looks pretty organized but it's not as clean as a bar magnet.

It shifts and it moves, in fact Earth's magnetic field is a highly dynamic beast.

The north and south geomagnetic poles are close to the geographic Poles, so close to Earth's rotation axis, but are not quite exactly aligned.

In fact, they move and shift.

The magnetic North Pole is currently moving at around 60 kilometers per year, around five degrees south of the geographic pole, leaving Canadian territory and heading toward Siberia.

The strength of the field across the surface also changes and all of these shifts are due to changing flows within the outer core.

Okay, so what's all this about the magnetic field flipping over?

In fact, how can it flip?

Surely the direction of the magnetic field depends on the direction the earth is spinning.

Actually no.

It depends on the direction of these giant electrical currents, which in turn depend on the direction of small magnetic loops generated by these helical convection flows.

In fact, we expect that if the magnetic field were switched off entirely, it would re-establish itself randomly, with the north and south magnetic poles aligned either one way or the other.

In the geological record there seems to be no pattern to when the field flips nor to which alignment is preferred.

So that's probably how the flip happens.

Earth's magnetic field isn't necessarily switched off, but it's scrambled in some way.

It then builds up again, choosing its direction randomly.

When it does a full flip, we call it a geomagnetic reversal, and when the field just glitches but ends up in the same direction it started, we call it a geomagnetic excursion.

There are a few ideas on how these glitches might happen.

It may be that some event triggers a disruption in the flows within the outer core.

This could be an asteroid or comet impact, and interaction between the core and the mantle, for example the formation of the new magma plume, or the subduction of a continental plate.

There's no clear evidence of any of these triggers however, and so most scientists think these geomagnetic events are just a natural part of dynamo behavior, in which the chaotic motion of outer core fluid causes a tangling of magnetic field lines and a global drop in field strength.

Now the question you should be asking is how do we know all of this?

Well the fact is it's not easy.

Computer simulations show that the dynamo effect should indeed produce a large scale dipole field that spontaneously reverses, although the details are still a little elusive.

For those of you who don't trust computer simulations, how about building our own giant spinning ball of molten metal?

There are several of these liquid sodium experiments in operation, they simulate the dynamo effect in the outer core quite nicely and even reveal spontaneous polarity flips.

Whatever the mechanism for geomagnetic reversals really is, we know it happens and will surely happen again, but is it happening now?

The International World Magnetic Model is a global map of Earth's magnetic field, updated every five years.

In the past, that's been frequent enough to account for small changes, but not anymore.

The WMM scientists found that the Earth's north magnetic pole was moving so quickly that they updated nearly a year early, at the beginning of 2019, and they'll update again at the end of the year.

But does this mean that the field is preparing to flip?

Not really.

I mean, maybe, but we know that the field must fluctuate quite a bit, even when it's not about to reverse.

We'll need to see a lot more disruption before we start to worry.

How bad is that?

Well, a bit bad, probably, but not catastrophic.

There's no evidence of increased extinction rates associated with any of the past reversals.

Like I said, the field weakens but doesn't switch off completely.

So there may be higher incidences of cancer and other mutations from more high energy particles reaching the ground, and probably we'll have to get much better at shielding satellites from the solar wind.

The field also becomes very messy with many north and south magnetic poles popping up across the surface of the planet.

So certain species of migrating bird, as well as old-timey sea captains are gonna be very confused for a while.

As the classic song goes, "Magnets, how do they work?"

Fortunately, scientists have a pretty good idea and they think that Earth's magnetic field is likely to hold out for our lifetimes, and those of some generations to come.

For now at least we remain protected from the worst ravages of solar storms, and of our dangerously irradiated space time.

(light techno music)