what is the evidence for geomagnetic reversal ?

billiards

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.

rogerharris

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 ?

rogerharris

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.

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.

billiards

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.

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.

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 ([itex]\epsilon[/itex]-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 ([itex]\pm[/itex]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.

rogerharris

well there goes that idea, i guess ill just have to live with the reality of an unstable core !

rogerharris

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 ?

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.

rogerharris

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.

geo101

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.

billiards

Perhaps this is why I thought the Curie temperature decreased?

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!

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).

rogerharris

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 ?

geo101

Do that mean I am the killer???

Now we are getting into the territory of another thread...
http://www.physicsforums.com/showthread.php?t=518080

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

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.

billiards

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.

Dotini

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