## what is the evidence for geomagnetic reversal ?

 Quote by rogerharris So if the inner core is 800 miles radius of compressed and cool iron/ nickel would this not be a pretty strong magnetic dipole structure on its own.
The inner core is not permanently magnetised. This is quite simple to see. Curie temperature gets lower with increasing pressure -- that means that at the centre of the Earth the curie temperature will be quite low (the pressure being quite high!). Temperature increases with depth -- that is another way of saying that the temperature of the inner core is really very high. The conclusion of this, if you think about it (and perhaps if necessary check a few numbers,) is that it is impossible for the inner core to be permanently magnetised.

EDIT: I should also mention that above the Curie temperature it is not possible for a body to have a permanent magnetic field.

The geomagnetic field is generated by the geodynamo as modelled by Glatzmeier and others.

 Quote by Evo Have you read any of the papers suggested at the bottom of the NASA article? Like this one? http://es.ucsc.edu/~glatz/geodynamo.html
thanks. Ive seen this material, but problem is every few years the onions sway back and forwards between scientists as to various parameters and models, and i am curious as to whether there is ferromagnetic order at the inner core despite the high temperatures.

e.g. a while back this summary explains opinion of the inner core structure

http://www.psc.edu/science/Cohen_Stix/cohen_stix.html

Prevalent opinion before these calculations held that iron's crystal structure in the inner core was bcc. To the contrary, the calculations showed, bcc iron is unstable at high pressure and not likely to exist in the inner core.

now we have this from japan last year.

http://www.pnas.org/content/107/21/9507.shor

The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth’s rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth’s inner core.

At the top of the inner core as pointed out by other recent research there is supposed to be freezing occurring due massive latent convection.

http://newscenter.berkeley.edu/2010/...agnetic-field/

About 60 percent of the power generated inside the earth likely comes from the exclusion of light elements from the solid inner core as it freezes and grows, he said. This constantly builds up crud in the outer core.

The Earth’s magnetic field is produced in the outer two-thirds of the planet’s iron/nickel core. This outer core, about 1,400 miles thick, is liquid, while the inner core is a frozen iron and nickel wrecking ball with a radius of about 800 miles – roughly the size of the moon. The core is surrounded by a hot, gooey mantle and a rigid surface crust.

BTW anybody have any idea what this crud is ?

Although papers from the 1950's negate the possibility of high pressure overcoming the curie temps at inner core pressure using calculation from the models of that time, curie temps appear to rise in line with pressure when an apparatus is actually devised, as more recently.

http://iris.elf.stuba.sk/JEEEC/data/pdf/8s_106-6.pdf

MAGNETIC MEASUREMENTS UNDER PRESSURE
Mária Zentková* – Zdenĕk Arnold** - Matúš Mihalik*** - Marián Mihalik* – Anton Zentko * -
Jiří Kamarád** - Zuzana Mitróová* - Slavomír Maťaš*
Two different methods were used to demonstrate that high pressure is a useful tool for investigation of magnetic properties. We report
on the effect of high pressure on the ferromagnetic transition in PrNi single crystal. The Curie temperature was found to increase
under pressure up to 0.9 GPa with a positive pressure coefficient Tc/p = 1 K/GPa. Such a behavior has been attributed to
enhancement of ferromagnetic coupling between Pr ions in PrNi due to pressure induced instabilities of the crystal field singlet
ground state of PrNi. The measurement was realized by transformer method. Additionally, the effect of pressure on magnetic properties
of Cr3[Cr(CN)6]2 x 15 H2O has been studied by means of SQUID magnetometry. Observed increase of Curie temperature
with the pressure coefficient Tc/p = 26 K/GPa can be explained by pressure induced increased overlapping of magnetic orbitals.

However i have not found any recent calculation of this effect for earths core to explain how it can have the Bcc structure.

i think this equation is used for this.

http://en.wikipedia.org/wiki/Clausiu...eyron_relation

So im just wondering if there can be ferromagnetism at the surface of inner core. The new models are saying it has a Bcc structure and at this layer temperatures are freezing. Are these linked ?

 Quote by billiards This is not my area and so I do not know the BEST papers. You could try this one (http://www.springerlink.com/content/c5m8212702058nw8/) to see a study of the rocks in the Deccan Traps -- which themselves are extremely interesting for other geological reasons.
thanks for that, looks like i might have to hassle a geologist if i read a load of stuff and its still not clear. Luckily got other exams to to distract me from this momentary obsession for a month or so.

 there are now two recent studies which are saying that the inner core has either a Bcc or Hcp structure which are ferromagnetic. according to this team from 2010 http://www.pnas.org/content/107/21/9507.shor Hemispherical anisotropic patterns of the Earth’s inner core Maurizio Mattesinia,1, Anatoly B. Belonoshkob, Elisa Buforna, María Ramíreza, Sergei I. Simakc, Agustín Udíasa, Ho-Kwang Maod, and Rajeev Ahujae,f + Author Affiliations aDepartamento de Física de la Tierra, Astronomía y Astrofísica I, Universidad Complutense de Madrid, E-28040 Madrid, Spain; bCondensed Matter Theory, Department of Theoretical Physics, AlbaNova University Center, Royal Institute of Technology, SE-10691 Stockholm, Sweden; cDivision of Theory and Modeling, Linköping University, SE-581 83 Linköping, Sweden; dGeophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015; eCondensed Matter Theory Group, Physics Department, Uppsala University, Box 530, SE-75121 Uppsala, Sweden; and fApplied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden Contributed by Ho-Kwang Mao, April 12, 2010 (sent for review March 20, 2010) Abstract It has been shown that the Earth’s inner core has an axisymmetric anisotropic structure with seismic waves traveling ∼3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth’s core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth’s rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth’s inner core. here bcc is ferromagnetic but not fcc, yet bcc is indicative of the iron being below the curie point. http://www.iap.tuwien.ac.at/www/surf...transformation At ambient conditions, pure iron is body-centered cubic (bcc) and ferromagnetic (i.e., it can be magnetized and is strongly attracted by magnets). Above approx. 920 °C, it becomes face centered cubic (fcc). Whereas the bcc phase gains its stability from magnetism (even though it becomes paramagnetic above its Curie temperature of 770 °C), the high-temperature fcc phase is paramagnetic recently on tv last week. http://www.bbc.co.uk/news/uk-14678002 Kei Hirose has created an incredibly powerful vice using the tips of two diamonds. Between them he has pressurised a sample of iron-nickel to three million times atmospheric pressure and heated the sample to about 4,500C. Under these extraordinary conditions, the crystal structure of iron-nickel alloy changed and the crystals rapidly grew in size. "We may have very big crystals at the centre of the Earth, maybe up to 10km," says Hirose. These crystals would all align "like a forest", says Hirose, pointing at the poles. from the research institute http://www.spring8.or.jp/en/news_pub...se/2010/101015 and the paper http://www.sciencemag.org/content/330/6002/359 So even in this Hcp phase nickel is still ferromagnetic. http://prb.aps.org/abstract/PRB/v39/i4/p2526_1 The Stoner theory of ferromagnetism has been applied to 3d transition metals in the hexagonal-close-packed (hcp) phase. The elements Co and Ni (and possibly Cr) are found to be ferromagnetic. A self-consistent calculation of the band structure of paramagnetic hcp Ni revealed the highest-known density of states at the Fermi level of any transition metal in any structure, providing strong evidence for ferromagnetism so there is now two models that propose the iron and nickel could be in ferromagnetic states even at these incredible temps. the first proposing the Bcc state for core surface, the next the hcp state. Another recent article points out chromium could be part of the earths core. Dont know if it sill is. chromium is also ferromagnetic in the hcp state.. http://www.sciencemag.org/content/33.../1417.abstract So... looks like no current consensus as to whats even in there, never mind what state it is ferromagnetic or not.

 Quote by rogerharris there are now two recent studies which are saying that the inner core has either a Bcc or Hcp structure which are ferromagnetic. according to this team from 2010 http://www.pnas.org/content/107/21/9507.shor Hemispherical anisotropic patterns of the Earth’s inner core Maurizio Mattesinia,1, Anatoly B. Belonoshkob, Elisa Buforna, María Ramíreza, Sergei I. Simakc, Agustín Udíasa, Ho-Kwang Maod, and Rajeev Ahujae,f + Author Affiliations aDepartamento de Física de la Tierra, Astronomía y Astrofísica I, Universidad Complutense de Madrid, E-28040 Madrid, Spain; bCondensed Matter Theory, Department of Theoretical Physics, AlbaNova University Center, Royal Institute of Technology, SE-10691 Stockholm, Sweden; cDivision of Theory and Modeling, Linköping University, SE-581 83 Linköping, Sweden; dGeophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015; eCondensed Matter Theory Group, Physics Department, Uppsala University, Box 530, SE-75121 Uppsala, Sweden; and fApplied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden Contributed by Ho-Kwang Mao, April 12, 2010 (sent for review March 20, 2010) Abstract It has been shown that the Earth’s inner core has an axisymmetric anisotropic structure with seismic waves traveling ∼3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth’s core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres. The detected travel-time anomalies can only be disclosed by a lattice-preferred orientation of a body-centered-cubic iron aggregate, having a fraction of their [111] crystal axes parallel to the Earth’s rotation axis. This is compelling evidence for the presence of a body-centered-cubic Fe phase at the top of the Earth’s inner core. here bcc is ferromagnetic but not fcc, yet bcc is indicative of the iron being below the curie point. http://www.iap.tuwien.ac.at/www/surf...transformation At ambient conditions, pure iron is body-centered cubic (bcc) and ferromagnetic (i.e., it can be magnetized and is strongly attracted by magnets). Above approx. 920 °C, it becomes face centered cubic (fcc). Whereas the bcc phase gains its stability from magnetism (even though it becomes paramagnetic above its Curie temperature of 770 °C), the high-temperature fcc phase is paramagnetic recently on tv last week. http://www.bbc.co.uk/news/uk-14678002 Kei Hirose has created an incredibly powerful vice using the tips of two diamonds. Between them he has pressurised a sample of iron-nickel to three million times atmospheric pressure and heated the sample to about 4,500C. Under these extraordinary conditions, the crystal structure of iron-nickel alloy changed and the crystals rapidly grew in size. "We may have very big crystals at the centre of the Earth, maybe up to 10km," says Hirose. These crystals would all align "like a forest", says Hirose, pointing at the poles. from the research institute http://www.spring8.or.jp/en/news_pub...se/2010/101015 and the paper http://www.sciencemag.org/content/330/6002/359 So even in this Hcp phase nickel is still ferromagnetic. http://prb.aps.org/abstract/PRB/v39/i4/p2526_1 The Stoner theory of ferromagnetism has been applied to 3d transition metals in the hexagonal-close-packed (hcp) phase. The elements Co and Ni (and possibly Cr) are found to be ferromagnetic. A self-consistent calculation of the band structure of paramagnetic hcp Ni revealed the highest-known density of states at the Fermi level of any transition metal in any structure, providing strong evidence for ferromagnetism so there is now two models that propose the iron and nickel could be in ferromagnetic states even at these incredible temps. the first proposing the Bcc state for core surface, the next the hcp state. Another recent article points out chromium could be part of the earths core. Dont know if it sill is. chromium is also ferromagnetic in the hcp state.. http://www.sciencemag.org/content/33.../1417.abstract So... looks like no current consensus as to whats even in there, never mind what state it is ferromagnetic or not.
I think you have taken some time to make your post, and it is a good post. You are clearly thinking on a higher level than I had given you credit for.

One thing that I had not fully appreciated until I looked into it further (and really it should have been obvious) is that the Curie temperature as a function of pressure is not always the same simple relationship: The relationship between Curie temperature and pressure is different for different materials.

I think that you have slipped up a bit in that you assume that BCC iron is always ferromagnetic -- you seem to think that BCC iron does not have a Curie temperature. I could understand this if BCC iron was not stable unless it was ferromagnetic, however, a little digging reveals that BCC iron does have a Curie temperature. Furthermore the relationship between BCC iron and Curie temperature has been studies, and is known experimentally and from first principle calculations.

 Quote by Pressure dependence of the Curie temperature in bcc iron studied by ab initio simulations F. Körmann,* A. Dick, T. Hickel, and J. Neugebauer The pressure dependence of the Curie temperature in bcc iron has been studied employing spin-density-functional theory in combination with the Heisenberg model. We show that the results correctly predict an essential independence of the Curie temperature of bcc iron on the external pressure, in agreement with the experimental findings. This behavior is explained as a result of a competition between the decrease in the local magnetic moments and the increase in the magnetic coupling as function of rising pressure.
http://prb.aps.org/abstract/PRB/v79/i18/e184406

The upshot is that the Curie temperature for BCC iron is the same for all pressures! What's the Curie temperature for BCC iron then? Certainly less than 2000 K. What the temperature of the inner core? Certainly more than 4000 K. Therefore we can be certain that if the core is composed of BCC iron it is NOT permanently magnetised.

Now I believe the same conclusion holds for HCP iron, but would be interested to see if you can prove otherwise.

 annoying. google books has all but chapter 4 and the uni library only has reviews. However i recognize the name McElhinny and think they put out a seminal paper which might provide a brief overview. I do recall somebody pointing out that McElhinny can not so confidently extract latitude of the orientation of magnetic domains from the data as has been carried out. finals on other stuff so a come back later for project.
There are a couple of other books available on Google that should answer the original questions about reversals

The magnetic field of the earth: paleomagnetism, the core, and the deep mantle
By Ronald T. Merrill, M. W. McElhinny, Phillip L. McFadden
Chapter 5, but not all is available.

or

Reversals of the earth's magnetic field
By John Arthur Jacobs
Older, but Chapter 2 should give you the back ground.

Some clarification on the core papers....
 Kei Hirose has created an incredibly powerful vice using the tips of two diamonds. Between them he has pressurised a sample of iron-nickel to three million times atmospheric pressure and heated the sample to about 4,500C. Under these extraordinary conditions, the crystal structure of iron-nickel alloy changed and the crystals rapidly grew in size. "We may have very big crystals at the centre of the Earth, maybe up to 10km," says Hirose. These crystals would all align "like a forest", says Hirose, pointing at the poles.
So the most likely Fe phase of the inner core is hcp Fe ($\epsilon$-Fe).
To avoid confusion (in case there is any, maybe not) the poles that Hirose is pointing to are geographic poles they are not related in any way to magnetic poles.

 The Stoner theory of ferromagnetism has been applied to 3d transition metals in the hexagonal-close-packed (hcp) phase. The elements Co and Ni (and possibly Cr) are found to be ferromagnetic. A self-consistent calculation of the band structure of paramagnetic hcp Ni revealed the highest-known density of states at the Fermi level of any transition metal in any structure, providing strong evidence for ferromagnetism
Firstly, here there are no reports of the temperatures at which these elements are ferromagnetic (I haven't read the paper). I would argue that the key point here is that this is referring to Ni, Cr and Co. By wt% they constitute ~5%, ~1% and so small it is not listed in the table! Fe is ~85% [1]. There are relatively minor constituents of the core.

[1] McDonough, W. F. (2007), Core composition, in Encyclopedia of Geomagnetism and Paleomagnetism, edited by D. Gubbins and E. Herrero-Bervera, pp. 77-80, Springer, Dordrecht.

Curie temperatures (Tc)
The temperature of inner core at the inner core boundary (ICB) is estimated to be 5000 K ($\pm$500 k) so let's use that.

 The upshot is that the Curie temperature for BCC iron is the same for all pressures! What's the Curie temperature for BCC iron then? Certainly less than 2000 K. What the temperature of the inner core? Certainly more than 4000 K. Therefore we can be certain that if the core is composed of BCC iron it is NOT permanently magnetised.

While the Tc of Fe in invariant of pressure, there is some evidence to suggest that for Fe-Ni alloys Tc decreases with pressure at a rate which depends on the Fe-Ni ratio (http://prb.aps.org/abstract/PRB/v6/i11/p4250_1) This an old reference, I'll see if I can dig out a more recent one.

Regardless, the Tc of bcc Fe is ~1040 K so at core temperatures it is paramagnetic. So even at depths well above the ICB Fe cannot hold a permanent magnetization.

As for hcp Fe the main magnetic ordering is reported to be antiferromagnetic, which has a Tc of 69 K (http://prb.aps.org/abstract/PRB/v67/i18/e180405). This is model based.

In addition to all of this there are observation that indicate that as the structure of Fe changes to hcp magnetic ordering is lost.

 The single Mossbauer line indicates that Fe57 may not be aligned in the hexagonal phase. This might indicate a paramagnetic medium...
http://www.sciencemag.org/content/14.../1035.full.pdf

I hope this all helps.

 Quote by billiards The upshot is that the Curie temperature for BCC iron is the same for all pressures! What's the Curie temperature for BCC iron then? Certainly less than 2000 K. What the temperature of the inner core? Certainly more than 4000 K. Therefore we can be certain that if the core is composed of BCC iron it is NOT permanently magnetised. Now I believe the same conclusion holds for HCP iron, but would be interested to see if you can prove otherwise.
well there goes that idea, i guess ill just have to live with the reality of an unstable core !

 Quote by geo101 There are a couple of other books available on Google that should answer the original questions about reversals The magnetic field of the earth: paleomagnetism, the core, and the deep mantle By Ronald T. Merrill, M. W. McElhinny, Phillip L. McFadden Chapter 5, but not all is available. http://books.google.co.uk/books?id=9...page&q&f=false or Reversals of the earth's magnetic field By John Arthur Jacobs Older, but Chapter 2 should give you the back ground. http://books.google.co.uk/books?id=M...page&q&f=false Some clarification on the core papers.... So the most likely Fe phase of the inner core is hcp Fe ($\epsilon$-Fe). To avoid confusion (in case there is any, maybe not) the poles that Hirose is pointing to are geographic poles they are not related in any way to magnetic poles. Firstly, here there are no reports of the temperatures at which these elements are ferromagnetic (I haven't read the paper). I would argue that the key point here is that this is referring to Ni, Cr and Co. By wt% they constitute ~5%, ~1% and so small it is not listed in the table! Fe is ~85% [1]. There are relatively minor constituents of the core. [1] McDonough, W. F. (2007), Core composition, in Encyclopedia of Geomagnetism and Paleomagnetism, edited by D. Gubbins and E. Herrero-Bervera, pp. 77-80, Springer, Dordrecht. Curie temperatures (Tc) The temperature of inner core at the inner core boundary (ICB) is estimated to be 5000 K ($\pm$500 k) so let's use that. While the Tc of Fe in invariant of pressure, there is some evidence to suggest that for Fe-Ni alloys Tc decreases with pressure at a rate which depends on the Fe-Ni ratio (http://prb.aps.org/abstract/PRB/v6/i11/p4250_1) This an old reference, I'll see if I can dig out a more recent one. Regardless, the Tc of bcc Fe is ~1040 K so at core temperatures it is paramagnetic. So even at depths well above the ICB Fe cannot hold a permanent magnetization. As for hcp Fe the main magnetic ordering is reported to be antiferromagnetic, which has a Tc of 69 K (http://prb.aps.org/abstract/PRB/v67/i18/e180405). This is model based. In addition to all of this there are observation that indicate that as the structure of Fe changes to hcp magnetic ordering is lost. http://www.sciencemag.org/content/14.../1035.full.pdf I hope this all helps.
Thanks for these. Its looking unlikely to make a case there could be any ferro or anti-magnetism in the core.

What got me interested in there being a ferro or anti-ferromagnetic order was this freezing layer that is brought up. Ill need to dig it back out. It seemed that what was being said was that there was still such high latent extraction that the surface of the inner core had freezing even today. Hirose talked of drops of frozen crystals here.

I need to extract the papers rather than news articles. I am guessing i must have misunderstood and taken this freezing out of earth evolution time context ?

 What got me interested in there being a ferro or anti-ferromagnetic order was this freezing layer that is brought up. Ill need to dig it back out. It seemed that what was being said was that there was still such high latent extraction that the surface of the inner core had freezing even today. Hirose talked of drops of frozen crystals here.
The freezing of the inner core at the inner core boundary (ICB) is completely correct. The inner core is solid and it is growing as heavier elements (mostly Fe) from the liquid outer core solidify at the ICB. Although the core is solid it is at a temperature well above it's Curie temperature and cannot hold any permanent magnetization.

Taking this a little further, the freezing of heavier elements at the ICB creates a compositional gradient in the liquid outer core. As the inner core grows lighter elements are released at the ICB. This drives compositional convection in the outer core and this is one of the fundamental forces that runs the geodynamo and hence creates the geomagnetic field.

 Quote by geo101 The freezing of the inner core at the inner core boundary (ICB) is completely correct. The inner core is solid and it is growing as heavier elements (mostly Fe) from the liquid outer core solidify at the ICB. Although the core is solid it is at a temperature well above it's Curie temperature and cannot hold any permanent magnetization. Taking this a little further, the freezing of heavier elements at the ICB creates a compositional gradient in the liquid outer core. As the inner core grows lighter elements are released at the ICB. This drives compositional convection in the outer core and this is one of the fundamental forces that runs the geodynamo and hence creates the geomagnetic field.
So how thick is this ICB layer where it is actually freezing and what is the temperature there ?

Well you see what im trying to get at. This is really the bit i didnt understand.

 So how thick is this ICB layer where it is actually freezing and what is the temperature there ? Well you see what im trying to get at. This is really the bit i didnt understand.
This layer of crystal and liquid "mush" is on the order of about 150 km, but it has variable thickness. I don't want to complicate things too much, but in some areas the inner core may actually be melting (http://www.nature.com/nature/journal...ture10068.html). The temperature here is ~ 5000 K. In fact this estimate is based on the coexistence of both liquid and solid Fe phases. Strictly, the melting point of Fe at ICB pressures is ~6200-6350 K, but the addition of lighter elements (e.g., O, Si, S are the more likely candidates) will suppress this.

Is it the temperature structure of Earth that you are not too sure about? I don't have a good diagram handy, but if that is the confusion I can find one.

 Quote by geo101 While the Tc of Fe in invariant of pressure, there is some evidence to suggest that for Fe-Ni alloys Tc decreases with pressure at a rate which depends on the Fe-Ni ratio (http://prb.aps.org/abstract/PRB/v6/i11/p4250_1) This an old reference, I'll see if I can dig out a more recent one.
Perhaps this is why I thought the Curie temperature decreased?

 Quote by geo101 As for hcp Fe the main magnetic ordering is reported to be antiferromagnetic, which has a Tc of 69 K (http://prb.aps.org/abstract/PRB/v67/i18/e180405). This is model based. In addition to all of this there are observation that indicate that as the structure of Fe changes to hcp magnetic ordering is lost. http://www.sciencemag.org/content/14.../1035.full.pdf I hope this all helps.
Thanks for that. Good information that helps to complete the picture!

It seems there is no way around it. The inner core is emphatically NOT a permanent magnet!

 As for hcp Fe the main magnetic ordering is reported to be antiferromagnetic, which has a Tc of 69 K (http://prb.aps.org/abstract/PRB/v67/i18/e180405). This is model based. In addition to all of this there are observation that indicate that as the structure of Fe changes to hcp magnetic ordering is lost.
Just to clarify (I hadn't finished my morning coffee when I wrote this).

I say that hcp Fe loses magnetic ordering as it due to the structural change "in addition to" it's low Tc. This is should really be that as Fe transforms to hcp Fe loses magnetic ordering due to the fact that hcp Fe has a lower Tc (these experiments were at room temperature).

 Quote by geo101 Just to clarify (I hadn't finished my morning coffee when I wrote this). I say that hcp Fe loses magnetic ordering as it due to the structural change "in addition to" it's low Tc. This is should really be that as Fe transforms to hcp Fe loses magnetic ordering due to the fact that hcp Fe has a lower Tc (these experiments were at room temperature).
im a believer !! not seeing how there could be a permanent magnet anyway.

here comes the columbo bit tho

whats the core mechanism for reversal ? if these inner cores are massive upward pointing ultra compressed paramagnetic crystals, and the dipole field is created by the liquid sloshing around it, then the crystals inner core still has the dominant paramagnetic order by comparison to the outer core.

how do these highly pressurized crystals get flipped around by outer core fluids which are going to be kind of random, inconsistent and paramagnetically weaker ?

 ere comes the columbo bit tho
Do that mean I am the killer???

Now we are getting into the territory of another thread...

For the core you can completely ignore any ferro/ferri/antiferro/para/dia magnetic effects. The geomagnetic field is generated by the geodynamo. Because I am lazy, I'll copy paste from the other thread

 The geomagnetic field is generated by the fluid motion of the electrically conductive fluid outer core around the solid inner core. The fluid motion of the outer core is driven by both thermal and compositional convection (as Fe freezes out of the liquid lighter elements remain creating compositional buoyancy) and is dominated by large scale flow. The Earth’s rotation also plays a big role; it produces convection columns within the outer core that align along the rotation axis. So to answer your question, thermal and compositional convection along with the Earth’s rotation control fluid flow in the outer core..... The flow of fluid is approximately axial-symmetric. The thermal and compositional convection is radial and the Earth’s rotation adds a helical twist to the fluid motion. [So the geomagnetic field aligns with the geographic poles, and not, for example the equator] I guess what you are really asking is why does the geomagnetic field point north or south? First some background. As I mentioned the outer core is conductive and, in the presence of a magnetic field, electric currents will be produce inducing new magnetic. This is the basic premise of the self-sustaining dynamo, which is a big feedback system of convection to electric currents to magnetic fields, which then modify convection currents (through magnetohydrodynamics; http://en.wikipedia.org/wiki/Magnetohydrodynamics ), and so on. Now, imagine you have a convecting Earth-like core system in the absence of any magnetic field, i.e., convection without magnetic induction. Then a seed magnetic field was instantaneous “switched” on; the final stable configuration would depend on the interplay of the strength and direction of the seed field, the configuration of convection before the seed field (and how it changed through time, i.e., magnetohydrodynamics), magnetic diffusion through the solid inner core and its relative scale to that in the outer core (the inner core can act as a breaking system to changes in the magnetic field induced by convection in the outer core), controls on the heat flux through the core mantle boundary, and a host of other details. So those are (some of) the factors that control the orientation of the magnetic field.
So what causes the field to reverse? The short answer is that we don't really know, but basically major changes in the convective regime are likely to cause reversals of the geomagnetic field. The heat flux across the core-mantle boundary, which the main driving force behind the thermal convection, will play a key role in this.

 Quote by geo101 So what causes the field to reverse? The short answer is that we don't really know, but basically major changes in the convective regime are likely to cause reversals of the geomagnetic field. The heat flux across the core-mantle boundary, which the main driving force behind the thermal convection, will play a key role in this.
How important is lateral heterogeneity in the mantle above the core mantle boundary to the geomagnetic field?

To me it seems "obvious" that the temperature gradients in the rocks in the lowermost mantle will control the flow of heat by conduction out of the core. One can envisage more heat flowing towards colder regions of the lowermost mantle (which are colder in the first place because they are the sites of downwelling in mantle convection). How important is this effect in controlling outer core convection -- given that this is to first order controlled by the Earth's rotation axis -- would the thermal gradients exert second order eddies? How important are eddy currents to the geomagnetic field?

Also hot and cold regions in the lowermost mantle seem to be quite permanent features, which to me suggests that heat flux across the core mantle boundary should not vary much in a short span of time. Unless a very slight perturbation in the heat flux can cause the field to reverse I would be surprised that this influence alone could be responsible for the flipping.

I seem to remember Glatzmeier modelling reversal as happening spontaneously. The geodynamo is inherently chaotic, I guess the flipping is an emergent property of the system, there is not a simple north/south switch that is being flicked. It is something less tangible to us.

 Recognitions: Gold Member billiards, do you accept that sea-floor striping and the other evidences/anomalies constitute proof that the whole earth magnetic field has flipped in the past? Respectfully submitted, Steve