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. 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 south geographic
poles, connecting to each other to wreath the planet in a dipole field, like a gigantic
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 Earth lost its field? Would we lose our atmosphere? Would life be extincted by crazy space radiation? Well, 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 a 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 fields aligned with 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. Well, 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 the Earth’s
magnetic field. Normally we think of magnetic fields being
generated in two way: In magnetic materials like iron, the sum total of the tiny magnetic
fields of their constituent particles align to give a global field. That’s your bar magnet or 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 the 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 the 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 look like. Beneath the thin crust and 2900 km of solid
mantle lies earth’s core. The 2400 km 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
km radius ball of solid iron. It’s solid because of the pressure at that
depth – at around 5500 Kelvin temperature, it would instantly melt at lower pressures. Everything is moving down there – the solid
core rotates slightly more quickly than 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 messier. The interior of the Earth is cooling down
very slowly, which means the liquid outer core is freezing into 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 helixes
by the coriolis force – the same effect that produces hurricanes on Earth’s surface. It’s all of 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, organizes, and sustains
an existing field. I’ll come back to where that initial magnetic
field comes from. For now, let’s say that 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 coriolis force. Those flows grab hold of our toroidal magnetic
field and twist it up further - into many little loops. Those loops form magnetic tubes around Earth’s
rotational axis. These in turn generate toroidal electrical
currents. Now we have exactly the conditions of an electromagnet
– organized rings of current, which produce our giant dipole field. OK, so start with a weak dipole field and
you get a strong one. But where does that initial magnetic field
come from in the first place. Well, actually ANY weak field – even random
bits of field – for example 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 moves. In fact Earth’s magnetic field is a highly
dynamic beast. The north and south gomagnetic 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 km per year around 5 degrees south of the geographic pole, leaving Canadian
territory and heading to 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. OK, 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 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 reestablish 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,
an interaction between the core and mantle – for example the formation of a 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 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 maps of Earth’s magnetic field updated every 5 years – in the past that’s
been frequent enough to account for small changes. But not any more. The WMM scientists found the 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 the field is preparing
to flip? Mmm... 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 incidents of cancer
and other mutation 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
mini 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 going to be very confused for a while. As the classic song goes, Magnets, how do
they work? Fortunately scientists have a pretty good
idea, and 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. Hey, everyone - Comment Responses will return shortly, but
before we sign off today there are two important things: First, we’d like to thank all Patreon supporters. You guys really help us keep the lights on, and we'd specially like to thank Morgan Hough, for joining the ranks of the of the Big Bang contributors on Patreon. Morgan, we’re going to spend your contribution
on building a house made of millions of refrigerator magnets to protect us once the
Earth’s magnetic field fails. You will be invited. But, until then, Space Time and PBS Digital
Studios want to hear from all of YOU. Every year we do a survey that asks about
what you’re into, your favorite PBS shows and things you’d like to see more of from
PBS Digital Studios. Space Time fans have always been amazing at
taking the time and making their voices heard on the survey and we’d love for that to
continue. We really do take a look at the results and
discuss them because all of this data helps us understand you better and make more of
the stuff YOU want to see. The survey takes about ten minutes, and you
could win a T-shirt. Link's in the description. Thanks!
Ha I'm totally cool, no panic at all... So we should have pretty cool auroras while we possibly irradiate.