Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 21/04/2014 05:30:02

Title: What is the origin of Earth's magnetic field?
Post by: thedoc on 21/04/2014 05:30:02
Wilf James asked the Naked Scientists:
   
One of my sort of hobbies is demolishing astronomical howlers. I often hear about the Earth's magnetic field being produced by moving metal within the Earth. I don't think that the moving metal theory for the Earth's magnetism is valid for two reasons:

Firstly, the metal concerned is NOT magnetic because it is too hot at a temperature well above the Curie Point for all known magnetic materials. This can be checked with a kitchen science experiment. The items needed are a strong button magnet of the sort used at the end of a telescopic pickup tool, and a small selection of small iron objects Medium sized screws and nails will do, preferably with an ordinary     paper clip. A screw or a medium sized nail is attatched to the magnet by     magnetism. The paper clip is attached to the screw or nail in the same way.
Then, as many nails as will stick to the paper clip are attached in a chain to the paper clip.

Then using a cigarette lighter, the paper clip is heated until it is red hot. The nails hanging from the paper clip will fall off. They can be easily re-attached when the paper clip has cooled.

This experiment shows that iron loses its magnetic properties when it is red hot. The metal within the Earth is very much hotter. The paper clip is used because it will heat up quickly in a lighter flame.

This experiment shows that the metal within the Earth is too hot to  be magnetic.

We now come to the second reason why the metal within the Earth does not create a magnetic field. A person at the equator is going around the Earth at just over 1,000 miles an hour. The metal at 2,000 miles depth below the equator is thought to be more or less fluid and travelling around at 500 miles an hour. The axis of the Earth's magnetic field is very near the spin axis. The metal is spinning around the centre of the magnetic field. At any given radius from the magnetic axis the strength of the magnetic field is more or less constant. Only changes in magnetic strength can induce currents in conductors. As the magnetic field will be unchanging for most of the metal, no significant electric currents will be induced in it. The molten metal at 2,000 miles depth will only be convecting at a very low speed when compared with its rotational speed. The electric currents induced by convection will be small and partially counteracted by Lenz's Law. An indirect way to test this is with a button magnet and a metal spinning top. The magnet is attached to the centre of the top.
The top is spun at a known rate and timed to find out when it topples. Then a piece of unmagnetised metal the same size and weight of the button magnet is attached to the top. Then the top is spun and timed as before. There should be no significant difference in the time taken for the top to topple. This will show that the magnet does not slow down the top's rotation.

The next tests are with a toy gyroscope with a non magnetic rotor. The gyro is spun up to a known rotation speed with and without the button magnet held near its rotor.
The magnet will slow down the rotor quickly, showing that an off-centre magnet is generating a current in the rotor.

The two experiments with the top and the gyroscope show that a centred magnet does not generate a current that causes drag. The magnetic field at the centre of the Earth does not cause any significant currents to be induced in the molten metal spinning around it.

There is a magnetic field caused by the ring current around the Earth. The ring current is made of of positively charged particles from the solar wind that are dragged around the Earth eastwards by the Earth's rotation. The particles must be positively charged because only a positive current can produce a magnetic south pole in the vicinity of the Earth's geographic north pole. A south magnetic pole is needed to attract the north ends of navigational compasses. This explanation for the Earth's magnetic field provides a possible answer for the repeating changes in the polarity of the Earth's magnetic field. The sun is presumably emitting predominantly positively charged particles at the moment. Over time this will cause the sun to develop a very strong negative charge that will inhibit the emission of positively charged particles. Then it could begin to emilt predominantly negatively charged particles and cause the ring current to be negative. Then the Earth's magnetic field
will be reversed. The magnetism of the rocks on either side of the mid atlantic ridge provides a sort of record of the changes in the Earth's magnetic field. Presumably the sun emits charged particles of one polarity or the other at irregular intervals on a geological scale.

I hope that you will find what I have written interesting

Wilf James BSc
What do you think?
Title: Re: What is the origin of Earth's magnetic field?
Post by: evan_au on 21/04/2014 12:15:28
Studying the source of Earth's magnetic field is a complex problem in magnetohydrodynamics - a chaotic domain requiring complex calculations on the fastest computers - and even small deviations in the conditions can result in large changes in the result.

I think that one factor ignored in the above analysis is that fluid flow in the molten core is driven by randomly changing convection cells (http://en.wikipedia.org/wiki/Earth%27s_magnetic_field#Earth.27s_core_and_the_geodynamo). This induces varying changes in currents in Earth's core, which causes random changes in the Earth's magnetic field.

The solar wind does cause currents in the ionosphere and in the Earth's rocks. Because the electromagnetic force is so incredibly strong compared to gravitational forces, every cubic kilometer of solar wind will be approximately electrically neutral. So I don't think it is correct to assume that the Sun can spend long periods emitting mainly positive particles (protons), followed by other long periods emitting mainly electrons.

Laboratory experiments (http://physicsworld.com/cws/article/news/2007/mar/09/molten-sodium-mimics-earths-magnetic-field-flipping) to emulate Earth's dynamo have produced oscillating magnetic fields in molten sodium (as mentioned in the OP, the liquid sodium is not ferromagnetic, merely conductive). This is initiated by a small "seed" magnetic field, and sustained by rotational cells in the molten sodium. It does not rely on patterns of particles emitted by the Sun.

The sun's oscillating magnetic field is thought to be generated by a similar mechanism to Earth's dynamo - only in the Sun, the conductive fluid is a hydrogen/helium plasma, not liquid nickel/iron. As far as we know, there is no influx of charged particles from the galactic center which is driving the Sun's approximately 22-year magnetic reversal cycle.
Title: Re: What is the origin of Earth's magnetic field?
Post by: alancalverd on 21/04/2014 23:42:19
The liquid core doesn't need to be magnetic to generate a magnetic field. What matters is that it is electrically conductive. Moving charge (i.e. an electric current) deep inside the planet produces a magnetic field that extends through the crust and into space.
Title: Re: What is the origin of Earth's magnetic field?
Post by: AndroidNeox on 19/12/2015 22:28:29
Electrical conductivity isn't necessary to generation of magnetic fields. To produce a field one needs to have net charge flow. If the magnetic field is generated by core rotation then there must be a mechanism separating charges, in bulk, at the core.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Thebox on 20/12/2015 13:20:09
My opinion from an amateur scientist perspective and looking at it theoretical, it is believed it is something to do with earth's inner/outer core, I would have to agree this is a starting point , I would also have to consider several other things though, light, thermodynamics, photon electrical effect, entropy, the mantle and electricity. 

If I remember correctly , if we electrified something, such as a pin, the pin becomes magnetised, so on that basis, if the earth's core produces an electrical field, or the ''ground'' by the suns rays internally produces a photon electrical effect. then any metal content of the earth would become magnetised to a monopole or dipole. The mantle could be the ''magnet''. 



And a weird thing, even magnets have mass and are attracted to the ground.



Title: Re: What is the origin of Earth's magnetic field?
Post by: chiralSPO on 20/12/2015 14:35:09
And a weird thing, even magnets have mass and are attracted to the ground.

That's not weird. Very simple rule that is always true: If it's made of atoms, it has mass.

As far as the source of Earth's magnetic field:

As pointed out above, the temperature of the core doesn't matter because it is behaving as an electromagnet, not a ferromagnet. The fact that it is iron has nothing to do with the magnetism, other than its conductivity. Jupiter's magnetic field dwarfs the Earth's (and Jupiter is so much further from the sun that if solar winds were responsible, it would be much, much less). It is theorized (though I do not believe proven) that a giant liquid metallic hydrogen core is responsible.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Thebox on 20/12/2015 14:43:39
And a weird thing, even magnets have mass and are attracted to the ground.

That's not weird. Very simple rule that is always true: If it's made of atoms, it has mass.

As far as the source of Earth's magnetic field:

As pointed out above, the temperature of the core doesn't matter because it is behaving as an electromagnet, not a ferromagnet. The fact that it is iron has nothing to do with the magnetism, other than its conductivity. Jupiter's magnetic field dwarfs the Earth's (and Jupiter is so much further from the sun that if solar winds were responsible, it would be much, much less). It is theorized (though I do not believe proven) that a giant liquid metallic hydrogen core is responsible.


I thought it weird because a person would think that if a dipole magnet has two polarities , pos and neq, that the Earth's magnetic field would repel one or the other with being magnetic fields. Clearly to me, it seems like the earth's ''electromagnetic'' field, is not really very magnetic compared to magnets, the two seemingly two different things altogether?



Title: Re: What is the origin of Earth's magnetic field?
Post by: Colin2B on 21/12/2015 17:02:32
I thought it weird because a person would think that if a dipole magnet has two polarities , pos and neq, that the Earth's magnetic field would repel one or the other with being magnetic fields.
It does. This is how a compass works, the compass pointer is a bar magnet.

Clearly to me, it seems like the earth's ''electromagnetic'' field, is not really very magnetic compared to magnets, the two seemingly two different things altogether?
You can't decide 2 things are different just based on relative strength.
In fact the earth's field is quite strong, try this.
Put a compass on a table. Place the strongest magnet you can find on the opposite side of the room to east or west of the compass. Go see whether the compass still points north.
You will find most diy magnets have to be quite close to make the compass swing away from north, but the earth's magnetic pole affects it from 1000s of miles away.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Thebox on 21/12/2015 17:24:44
I thought it weird because a person would think that if a dipole magnet has two polarities , pos and neq, that the Earth's magnetic field would repel one or the other with being magnetic fields.
It does. This is how a compass works, the compass pointer is a bar magnet.

Clearly to me, it seems like the earth's ''electromagnetic'' field, is not really very magnetic compared to magnets, the two seemingly two different things altogether?
You can't decide 2 things are different just based on relative strength.
In fact the earth's field is quite strong, try this.
Put a compass on a table. Place the strongest magnet you can find on the opposite side of the room to east or west of the compass. Go see whether the compass still points north.
You will find most diy magnets have to be quite close to make the compass swing away from north, but the earth's magnetic pole affects it from 1000s of miles away.


Thank you Colin, I understand about  a compass and how a compass works, I have experimented with a compass and compass needle, my problem is yes the compass will turn to direct north and a corresponding direct south, but the compass does not turn to the ground, if there is a magnetic field being emitted from the ground, then why doesn't  my needle bend towards the ground, rather than pointing North or South?
Title: Re: What is the origin of Earth's magnetic field?
Post by: chiralSPO on 21/12/2015 17:30:11
I thought it weird because a person would think that if a dipole magnet has two polarities , pos and neq, that the Earth's magnetic field would repel one or the other with being magnetic fields.
It does. This is how a compass works, the compass pointer is a bar magnet.

Clearly to me, it seems like the earth's ''electromagnetic'' field, is not really very magnetic compared to magnets, the two seemingly two different things altogether?
You can't decide 2 things are different just based on relative strength.
In fact the earth's field is quite strong, try this.
Put a compass on a table. Place the strongest magnet you can find on the opposite side of the room to east or west of the compass. Go see whether the compass still points north.
You will find most diy magnets have to be quite close to make the compass swing away from north, but the earth's magnetic pole affects it from 1000s of miles away.


Thank you Colin, I understand about  a compass and how a compass works, I have experimented with a compass and compass needle, my problem is yes the compass will turn to direct north and a corresponding direct south, but the compass does not turn to the ground, if there is a magnetic field being emitted from the ground, then why doesn't  my needle bend towards the ground, rather than pointing North or South?

The compass needle aligns itself in the N-S direction. Unless you are standing at one of the poles, this will not involve pointing at the ground.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Thebox on 21/12/2015 17:32:55


The compass needle aligns itself in the N-S direction. Unless you are standing at one of the poles, this will not involve pointing at the ground.

So it is only the poles where there is a magnetic field then ? 

Title: Re: What is the origin of Earth's magnetic field?
Post by: chiralSPO on 21/12/2015 18:04:29


The compass needle aligns itself in the N-S direction. Unless you are standing at one of the poles, this will not involve pointing at the ground.

So it is only the poles where there is a magnetic field then ?

No. The field is everywhere. It is only at the poles where the field points at the ground.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Thebox on 21/12/2015 18:18:49




No. The field is everywhere. It is only at the poles where the field points at the ground.


Yes you said that one once, if the field is everywhere then surely the needle points everywhere? 



if the ground is emitting a magnetic field every where,  then surely the compass needle should point to the ground, I don't understand why it wouldn't , can you explain please?
Title: Re: What is the origin of Earth's magnetic field?
Post by: Colin2B on 21/12/2015 19:37:17
Yes you said that one once, if the field is everywhere then surely the needle points everywhere? 

if the ground is emitting a magnetic field every where, 
no, the needle will point towards the poles along the lines of magnetic force. However, because the poles are created by the molten core they are under the surface. So at the magnetic poles the compass will point directly down. As you move away from the poles the compass will still point towards the poles so will be inclined downwards. Most compasses are balanced so they don't point downwards so you need an instrument like a compass on it's side to measure the inclination.
I remember measuring it at school and it was somewhere around 60 deg, but my memory fails so don't quote me.
At the equator it should be horizontal.

Title: Re: What is the origin of Earth's magnetic field?
Post by: DavidWoodhams on 08/02/2019 21:05:12
Curie Temperature decreases linearly with pressure and with an inner core pressure of approx. 360 GPa it would be necessary to determine the Tc at that pressure.

The relative motion between the inner core and the liquid outer core would produce either a lead-synchronicity-lag, resulting in a field in one direction-no field-reversed field.

Taking the inner core rotation to be constant, the cause of relative variation in speed of rotation of outer core to inner core could be due to a variable balance of the earths rotation as a result of plate tectonics. This would result a variation of rotation of the outer core....but what force could increase its rotational speed? Could be our movement around the Sun, Moons orbit.....???
Title: Re: What is the origin of Earth's magnetic field?
Post by: evan_au on 08/02/2019 21:52:22
Quote from: DavidWoodhams
relative variation in speed of rotation of outer core to inner core could be due to a variable balance of the earths rotation as a result of plate tectonics.
There have been suggestions from seismic measurements that the solid core is rotating with respect to the Earth's surface - by about 1 degree per year. This could lead to a magnetic dynamo effect.
See: https://en.wikipedia.org/wiki/Inner_core#Dynamics

Quote
could be due to a variable balance of the earths rotation as a result of plate tectonics.
I am sure that convection plays a big part in the dynamics of the Earth's core, as does crystallisation, concentration gradients  and melting.

These same forces play a part in plate tectonics.

It depends how closely coupled convection in the outer core is to convection in the Earth's mantle.
 
Quote
Could be our movement around the Sun, Moons orbit.
This I find less likely. The Earth is in free fall around the Earth/Sun/Moon barycenter, so the actual gravitational attraction won't affect the Earth's core.

Tidal effects will impact the core, but the core has smaller diameter than the Earth's crust, so tidal influences will have smaller amplitude than they do on the oceans. More importantly, tidal effects will produce alternate tugs in different directions every 12-13 hours. These will cancel out when the viscosity and momentum of the Earth's outer core suggests convection patterns that may last for many millions of years.
Title: Re: What is the origin of Earth's magnetic field?
Post by: syhprum on 08/02/2019 22:15:04
Not all studies of the Earths magnetic field use computer simulations some analogue devices have been built for the purpose.

https://www.universetoday.com/14664/how-do-you-model-the-earths-magnetic-field-build-your-own-baby-planet/
Title: Re: What is the origin of Earth's magnetic field?
Post by: evan_au on 09/02/2019 09:54:26
This scientist suggests that the Earth's solid core may have started forming "only" 500 million years ago...
....around the time of the Cambrian explosion (https://en.wikipedia.org/wiki/Cambrian_explosion), although he hesitates to suggest that the magnetic field was a contributor.

He says that there was a magnetic field prior to that - they can tell from magnetism frozen in rocks. But the field got rather weak around 500 million years ago.
Listen (11 minutes):  https://www.sciencefriday.com/segments/the-earths-core-might-be-younger-than-scientists-thought/
Title: Re: What is the origin of Earth's magnetic field?
Post by: yor_on on 09/02/2019 10:28:02
Actually the Russians made 'analogue' computers to study fluid systems at the time they were embargoed from getting  'new fangled' computers. They worked very well. I'll see if I can find it later.
=
A little too early perhaps :)
Still.  http://www.digitaljournal.com/article/338106
Title: Re: What is the origin of Earth's magnetic field?
Post by: Bored chemist on 09/02/2019 11:51:46
...it would be necessary to determine the Tc at that pressure.
Not really.
The Curie temperature has to be below the melting point, and we know that the deep core is molten.
However the Earth isn't a permanent "bar magnet" made of iron.
It's an electromagnet; so it doesn't matter what the Curie temperature is.

Actually the Russians made 'analogue' computers to study fluid systems at the time they were embargoed from getting  'new fangled' computers.
The oldest analog computers I know of were water clocks.
It's not clear how long ago they were invented but they certainly predate the existence of Russia as a country.
Title: Re: What is the origin of Earth's magnetic field?
Post by: yor_on on 09/02/2019 12:31:49
Sweet thinking BC. We all must have intuitive ways of doing mathematics. The real problems comes when one want to formalize it into a system. Like adding numbers 34 + 44 + 76 f.ex. The easiest way is to take the 'tens' and then count up the single digits, but that's not the way I saw it presented in school. I guess a lot of us do it this way as it is simpler, and that some even might question why bother to learn a slower system? One good resource I think is the so called vedic mathematics, as they seem pretty intuitive. http://mathlearners.com/

what I'm thinking of is that we must all 'know mathematics' because how else do I know how to lift that cup of tea? And I'm guessing that the first waterclocks came from this intuitive understanding.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Bored chemist on 09/02/2019 18:38:02
Sweet thinking BC. We all must have intuitive ways of doing mathematics. The real problems comes when one want to formalize it into a system. Like adding numbers 34 + 44 + 76 f.ex. The easiest way is to take the 'tens' and then count up the single digits, but that's not the way I saw it presented in school. I guess a lot of us do it this way as it is simpler, and that some even might question why bother to learn a slower system? One good resource I think is the so called vedic mathematics, as they seem pretty intuitive. http://mathlearners.com/

what I'm thinking of is that we must all 'know mathematics' because how else do I know how to lift that cup of tea? And I'm guessing that the first waterclocks came from this intuitive understanding.
There's a story, I don't know if it's true, about a research group trying to make a robot that could catch a ball.
They set out by getting lots of computing power so they could take data from a camera and solve the differential equations to calculate the trajectory. It was very difficult.
Then they asked someone who played cricket how he caught the ball. He thought about it and said
"If the ball looks like it's going up, move backwards. If it looks like it's coming down, move forwards.
If it's to your left, move left. If it's to your right move right.
The ball will hit you in the face."

No maths required.
Title: Re: What is the origin of Earth's magnetic field?
Post by: evan_au on 09/02/2019 21:39:47
It is a mystery why Earth has a magnetic field, and Mars doesn't (any more).
Mars is slightly smaller than Earth, so it would have cooled slightly quicker - but not much quicker.

Perhaps the collision that produced the Moon might have injected a pulse of energy into the Earth's interior, keeping it hotter for longer? The iron core of the impactor would have sunk towards the center of the earth, merging with Earth's core.
See: https://en.wikipedia.org/wiki/Giant-impact_hypothesis
Title: Re: What is the origin of Earth's magnetic field?
Post by: Janus on 09/02/2019 23:06:05
It is a mystery why Earth has a magnetic field, and Mars doesn't (any more).
Mars is slightly smaller than Earth, so it would have cooled slightly quicker - but not much quicker.

Perhaps the collision that produced the Moon might have injected a pulse of energy into the Earth's interior, keeping it hotter for longer? The iron core of the impactor would have sunk towards the center of the earth, merging with Earth's core.
See: https://en.wikipedia.org/wiki/Giant-impact_hypothesis
Earth's radius is ~1.87 times that of Mars,  This gives it ~3.5 times the surface area, but  ~6.5 times the volume of Mars.  Earth also has a density of 5.52 g/cc to Mars 3.93g/cc,  which makes the Earth over 9 times more massive.   All in all, Earth's mass to surface area ratio is ~2.66 times that of Mars.   Surface area determines how fast the heat can radiate away and mass determines the total heat contained for objects of the same temperature and like make up.  Thus Mars should be expected to lose its heat fairly quickly compared to the Earth
Title: Re: What is the origin of Earth's magnetic field?
Post by: yor_on on 10/02/2019 00:14:33
Well BC. I would say that is mathematics, same as with lifting something, judging a distance, etc. I would also call it intuitive.
Title: Re: What is the origin of Earth's magnetic field?
Post by: evan_au on 10/02/2019 10:54:26
Quote from: Janus
Earth also has a density of 5.52 g/cc to Mars 3.93g/cc
This density difference may be another clue as to why Earth's magnetic field is longer-lasting than Mars.
- Mars has a density closer to rock (basalt: 3g/cm3)
- Earth has a density which is closer to iron (iron: 8g/cm3)
- This suggests that Mars has a fairly small iron core, while Earth has a much larger iron core in relation to its size
- Some versions of the impact hypothesis have much of Earth's rocky crust ending up in the Moon, and much of the Mars-sized impactor's iron core joining the Earth's iron core.
Title: Re: What is the origin of Earth's magnetic field?
Post by: yor_on on 10/02/2019 11:37:35
Actually I seem to remember experiments done with analog small  'robots' that seemed to adapt much easier to different milieus than those defined by binary logic handling it. Which should put the type of mathematics the brain use as 'non linear' adjusted by inputs from the environment, as you described there BC. https://en.wikipedia.org/wiki/Nonlinear_system
=

The wiki and I disagree on one thing though. "  It follows that some aspects of the dynamic behavior of a nonlinear system can appear to be counterintuitive, unpredictable or even chaotic. Although such chaotic behavior may resemble random behavior, it is in fact not random. For example, some aspects of the weather are seen to be chaotic, where simple changes in one part of the system produce complex effects throughout. This nonlinearity is one of the reasons why accurate long-term forecasts are impossible with current technology.  "

I'll put a dollar on that this seemingly expected 'technology' accurately describing and predicting the outcome of every 'chaotic system' never will exist.
Title: Re: What is the origin of Earth's magnetic field?
Post by: Bored chemist on 10/02/2019 13:31:53
I'll put a dollar on that this seemingly expected 'technology' accurately describing and predicting the outcome of every 'chaotic system' never will exist.
Nobody suggested otherwise.
Title: Re: What is the origin of Earth's magnetic field?
Post by: yor_on on 11/02/2019 03:57:42
Heh, seems we're not the only one able to intuitively 'calculate'. "  honeybees can apparently understand the concept of zero, and learn to correctly indicate which of two groups of objects is the smaller.  But now they say insects can learn to carry out exact numerical calculations such as adding and subtracting a given number. “Their brain can manage a long-term rule and applying that to a mathematical problem to come up with a correct answer,” said Dr Adrian Dyer, co-author of the research from RMIT University in Australia. “That is a different type of number processing to spontaneous quantity judgments.”

If the team are right, the insects are in good company. While it was once thought that only humans could manage such calculations, the authors note recent research has revealed a veritable menagerie of creatures can keep track of numbers or even add or subtract. “[There was] evidence that other primates could do it and then an African grey parrot, Alex, famously could do it, but also some spiders could do it,” said Dyer.