i love maps
and i love mountains and i love being on
mountains with a map so i read with some
alarm a warning from the scottish
mountaineering association telling
people to be careful about how they
store their compasses
we carry a lot more magnets around than
we used to
and that can get you into trouble it can
deflect the needle of your compass or in
some cases it can reverse the polarity
altogether so
got your trusty compass here lots of
surprising things have magnets in them
anything with a speaker in it has a
magnet and you can see just by waving
the phone nearby the compass you can
deflect the needle and so if you
inadvertently have a magnet in your
sleeve or as a fastening on your phone
and you have anywhere near your compass
you can get pointed in the wrong
direction and if you store this compass
for a long time next to something with a
strong magnet you could actually reverse
the polarity of the compass needle but
this sort of opened the
floodgates into learning a bit about the
earth's magnetic field as a whole
astronomers don't know a lot about
magnetic fields it's a running joke that
at a conference if someone can't think
of a question to ask they ask have you
considered magnetic fields so no i would
not count myself as an expert on
magnetic fields if you're going out in
the hills and you're going to rely on a
compass which is a good thing to do
because
even if you have a gps on your phone
which isn't going to be using the
earth's magnetic field you never know
when that might conk out so if you ever
look closely at a topological map like
the ones i've got here you see in the
corner often a little diagram that says
hey be careful here what you think of as
north on this map might not match what
your compass is telling you the
direction that the compass points is not
necessarily
almost never exactly at the true north
towards the north pole and so on every
corner of every map you'll see a little
correction that tells you how much you
have to correct your bearings for this
particular
point on the earth so i i can't say that
myself i've ever been reliant on that
level of detail in my navigation
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plus here in the uk the deflections are
fairly small so here you can see three
degrees correction east of north but in
some places particularly near the
magnetic north pole in the north of
canada you could be up by 40 degrees and
you definitely don't want to be relying
on your compass without correcting it in
that case so there are multiple poles on
the earth there's what we think of as
the north pole you know where santa
lives and this is
this is associated with the rotation of
the earth the earth is spinning on its
axis that axis is actually tilted with
respect to the sun but we won't worry
about that today so the earth is
spinning on the axis the axis of
rotation goes up through the surface of
the earth and
subject to some caveats i'm not going to
talk about in this video is more or less
what we call the geographic north pole
that the the maps point to that's pretty
much where your finger and thumb are
right right about there yeah where the
pins would go into a globe the pins
would go in the globe yeah but how would
a compass needle know to point at the
geographic north or south pole in fact
they don't the magnetic north pole and
magnetic south pole that our compasses
point towards are actually in a
different place all together and in fact
that place actually moves so to
understand what's going on here you
really have to understand
how the magnetic field of the earth is
formed in the first place unfortunately
we don't really know at least not in the
details it gets very complicated but
what we do know is that the structure of
the interior of the earth has at its
core just like this plum does a very
solid core of solid iron
and that's surrounded by a sea of liquid
iron and that makes up about half of the
earth's interior so about 3 000
kilometers in radius from the center and
it's this molten iron core that we think
is
generating and keeping this magnetic
field going why would the fact that
there's liquid metal
suddenly say all right well then you can
have a magnetic field
so now we enter the very complicated
field of magneto hydrodynamics
where you're putting together equations
from fluid dynamics like the
navier-stokes equation and you're
pulling together you know the core
equations maxwell's equations from
electromagnetism and you put them all
together and you try to understand how
the behavior of a conducting fluid in
motion through a magnetic field what the
effects of that will be so to produce
the earth's magnetic field you need
three things
first you need a conducting liquid you
need that liquid to be in motion and not
just chaotic motion but fairly ordered
flows
and you get that from two processes
first convection so the the core of the
earth is extremely hot and so much like
a lava lamp it cycles through up and
down and up and down and circulates but
then it gets complicated because it's
also rotating because of the rotation of
the earth and so this sets up some very
complicated flow patterns and then the
third really important ingredient you
need is a seed magnetic field so
something has to be there to start with
and once you've got that you have
magnetic induction
so you've got a conductive fluid moving
across these magnetic field lines in
this ordered flow and when you have a
conducting fluid moving across magnetic
field lines you generate electrical
currents those electrical currents in
turn generate magnetic field lines and
so you have this self-perpetuating
system without the motions of the fluid
across these magnetic field lines we
would expect the magnetic field to just
decay away in about 15 000 years
so but we know it's been there for for
many many geological time scales you can
think of the magnetic field of the earth
in a very simple approximation like a
giant bar magnet and we call that a
dipole so it's got the north and the
south magnetic poles and then you've got
these magnetic field lines that come out
and around and connect them and so
that's the simple approximation and if
you drew a line through that you get to
what we call the geomagnetic north and
south pole which are exactly opposite
each other the geomagnetic north pole is
located slightly off the coast of
greenland and that's where you see all
of the aurora and northern lights
centered on that region
but what we care about with our compass
is the magnetic north pole and that has
a very practical definition that's the
point on the earth's surface where if
you're carrying a compass and if you
allowed that compass needle to tilt in
all directions if you stood over the
magnetic north pole the compass needle
would point straight down and that's
what we call north
so when i'm you know walking around
england my needle's not like perfectly
flat it's actually always tilting
slightly down it's always tilting
slightly down and it's not necessarily
pointing straight to the magnetic north
pole it's following whatever the local
magnetic field lines are which are
actually quite complicated so i've got
this map here which shows you the world
magnetic field model this is essentially
how much you have to correct your
compass bearing by
depending on where you are on the
surface of the earth so if you're
anywhere near the green line you're
doing okay and that's where we are up
here we only have a deflection called a
declination of a few degrees elsewhere
you can get up to 10 20 30 degrees and
that it's not necessarily in a nice
north and south pattern in line with you
know lines of longitude it gets really
messy
and even more interesting
the magnetic north pole
is not directly opposite the magnetic
south pole either it gets really really
weird on different parts of the earth so
i can show you a map of what's called
the magnetic anomaly you can note one
interesting place which is in here this
is the central african republic a place
called bongi and it's the site of the
bongi magnetic anomaly and it's a place
where you would have no luck with your
compass it would just be pointing all
over the place because the magnetic the
local magnetic field there just goes
weird is that in terms of like there's a
bunch of mountains with metal in it or
the actual fundamental core created
magnetic field is going weird on us we
don't know the exact cause of the
anomaly it's probably some combination
of crust and mantle interaction either
an igneous extrusion material from the
mantle has come up and solidified or
perhaps an impact from a meteorite has
done something to the crust but
something there locally makes the
magnetic field a bit wonky do we know
why
the way that the magnetic poles
turned out is so
non-aligned with the geographic poles it
would seem intuitive that the spinning
liquid and everything that's going on
would kind of roughly line up with the
rotation of the earth yeah and it and it
does i mean when you think about it they
are pretty close to the north and south
pole we're only talking 10 degrees apart
and in fact that's predictable by what
we know about fluid dynamics is that the
fluid in a rotating system will set up
sort of these columns aligned with the
spin axis but what gets really
interesting is to know that the magnetic
north pole isn't even stable in fact it
moves around we've known it moves around
for quite a long time
but it's moving faster these days it's
accelerating so it used to be that the
magnetic north pole was somewhere in the
north of canada in the territory that we
now call the nunavut territory but it's
been racing and accelerating and is now
heading at great speed towards siberia
interestingly while the magnetic south
pole stays more or less where it is
so that's another illustration that
there's some local effect in the fluid
flow underneath the the north pole um
something akin to the jet stream that we
see in the atmosphere that's actually
driving the change in the magnetic north
pole and making it move around
so for the past 400 years we think it
was contained more or less within
northern canada
but in the past 50 years it started to
move around so it was moving around
about tens of kilometers a year back and
forth and in the last 50 years it
started moving northwards and the past
30 years it's accelerated and this year
it's now moving at 50 kilometers over
the course of the year
heading towards siberia do they put a
stake in the ground or anything like
they do at the south pole is the is this
position marked in any way and does do
they have to move the marker
it's marked on maps but it's not going
to be a physical marker partly because
of how fast it's moving and partly
because
the nature of where it is there's no
there's no solid ground up there or
solid ice that's reliable
particularly since the arctic ice is now
melting at an accelerating rate
so no nobody's up there moving markers
around but because of this motion
normally the the magnetic maps are
updated every five years and they've
actually had to do an inter-cycle sort
of emergency update to that because it's
moving so fast it's not going to affect
anybody here using their compass to walk
in the you know the highlands of
scotland but it is going to make a
difference for anybody relying on the
fine details particularly up near the
north uh northern latitudes i don't know
anyone who is more proudly canadian than
you how do you feel about canada
potentially losing the magnetic north
pole well i only actually learned that
fact today and i do feel a bit sad
because i i it was a source of national
pride that that for hundreds of years we
have the magnetic north pole within our
territory um but you know there's
nothing i can do about it i can't hang
on to it
so this is the neat thing the magnetic
north pole might be moving around and
taking a bit of an excursion around
northern latitudes but we also know
that quite regularly on geological time
scales the whole system flips so just
like i was talking about my compass
reversing its polarity the whole
magnetic field of the earth occasionally
flips so that the north pole becomes the
south and the south pole becomes the
north we don't know why this is again in
the details because it's an immensely
complicated scenario but we do know that
it happens and it happens with great
regularity if not periodicity so we know
that there have been 183 reversals in
the past 83 million years
and that's a record that we can
establish really well thanks to
geological measurements of the magnetic
field because we have this really
interesting phenomenon that when molten
mantle material comes out and solidifies
that cooling volcanic rock
retains sort of the fingerprint of the
earth's magnetic field direction at that
time so if we go back to our magnetic
anomaly map you can actually see here in
the mid-atlantic or in the mid-pacific
that where material is coming up from
the crust and where these tectonic
plates are being pushed apart you've got
this regular incursion of liquid mantle
coming up and solidifying it's retaining
the direction of the magnetic field of
the earth as it swaps back and forth so
on that map we've got these stripes of
red and blue which indicate that the
field's been switching back and forth
what does a flip look like again we
don't really know so we can do
models
and we can do some quite interesting
models here's computer models of what a
flip might look like done in a super
computer so here's what we think the
magnetic field of the earth might look
like now and so you've got these nice
well-defined dipole magnetic field lines
coming out but a big mess locally
towards the surface of the earth some
point through a reversal you might get a
situation like this where it's just a
big mess you might have the north and
south pole just wandering around they
might disappear you might have more than
one pole pop up chaotically in different
places the magnetic field of the whole
earth might decrease altogether we don't
really know
best estimates are that it might take a
thousand or tens of thousands of years
to happen
some people think it might happen within
a human lifetime does that mean we could
see one happen well it's an interesting
thing to think about that we've seen the
magnetic north pole
go for a wander that wandering seems to
be accelerating since we've been making
measurements since about 1840 there's
been a steady decrease in the strength
of the magnetic field so it's leading
some people to think that we might be in
the early stages of a reversal but
whether that happens next year or in the
next 10 000 years we can't be sure
sometimes the magnetic field just goes
on a bit of a wobble called an excursion
but then it comes back to the the normal
polarity but when it does flip from one
end to the other we mark those
geological time scales in periods called
crons is there any reason to think that
a reversal or this process is dangerous
i mean it's happened so many times that
it feels like it must not be
catastrophic
yeah i mean it does seem to be quite an
alarming thing to happen on the scale of
the planet um but we don't see strong
evidence
that
sort of you know evidence for mass
extinctions in the fossil records are
tied closely to these geological records
then again we also know that some
animals use magnetic fields to navigate
by you have to be concerned about any
small perturbation to something as
complicated as our global ecosystem we
also know that this has happened
since the time that humans have been on
the earth
and we don't have any record of
catastrophic things happening
so it could be that it's just such a
slow and gentle process
that
ecosystems and evolution and everything
can sort of keep up with the change what
we might be concerned about is increased
particle radiation we know that the
magnetic field of the earth protects us
from highly charged particles it might
be that more cosmic rays are able to
reach the ground with a weak magnetic
field but the atmosphere itself does a
pretty good job of protecting us as well
so it might be that we just get
interesting effects like aurora seen
over you know the galapagos islands or
something like that it might not be as
catastrophic as you might fear so of
course the next question is
what are the magnetic fields like on
other planets so all of the gas giants
have strong magnetic fields jupiter
unsurprisingly has the biggest of them
all and you can actually hear the sound
of jupiter's magnetic field if you look
at the recordings from the juno
spacecraft which reached jupiter a few
years ago
recorded bow shock as the instrument
passed through jupiter's magnetic field
so no surprises there
mercury has a weak one venus and mars
don't seem to have any at all and the
relative strength is tied to
what's happening in the core mars has a
solid core we think no conduction no
rotation so that's not going to generate
a magnetic field that might go some way
to explaining why mars has lost the
atmosphere that it used to have the
magnetic field might have some sort of
confining action there but the really
interesting one is uranus of course
uranus is an interesting planet in its
own right because of course its axis of
rotation is tilted at 98 degrees to the
plane of the ecliptic which since we've
been talking about everything being
connected to rotation what does uranus's
magnetic field look like and it's a
complete mess
the axis of the magnetic field in uranus
is actually tilted at 60 degrees from
the axis of rotation and what that means
is the whole field is tumbling as the
planet rotates which means that the
magnetic field lines detach and then
reattach in really chaotic ways which
means that the
the planet actually sort of has a strobe
effect with its magnetic field that it
opens up and lets charge particles in
and then snaps closed again opens and
closes and opens closes presumably the
sun has a magnetic field it does it's
generated in a slightly different way
but of course it has a very large
magnetic field we know that it waxes and
wanes on a quite regular time scale of
11 years and that's tied into solar
activity and sunspot cycles you can see
the beautiful magnetic loops that are
created from that there's still a lot we
don't exactly know like there is like it
is still one of those sort of mystery
subjects there's lots we do know but it
seems like there's lots we don't know
yeah i think that's correct and when you
think about what's going into all of
this the very very complex interplay
between all of the different systems the
difficulty in understanding what's going
on below the surface of the earth 3000
kilometers below the surface of the
earth and making guesswork there plus
the fact that we've only been making
these measurements for a few hundred
years um yeah i think it's a very uh
complicated
field but one i'm sure that's going to
make lots of progress in in the near
future different from the it's not just
a scaled-up version of that
electromagnet it's what's called a
superconducting magnet
so
actually when i applied a current to
that elect that little mini
electromagnet with the orange wire
earlier on