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Some elementary physics Earth Magnetism A Supraconductive Inner Core Cocorico !
Earth magnetism, and Theory

Some reminders about the Earth magnetic field

Earth magnetic field
  1. As a magnet, the magnetic field of the Earth is bipolar.

  2. The geographic north pole of the Earth is a magnetic south pole.

  3. The axis of the magnetic poles is not aligned with the axis of rotation of the Earth.

  4. On the surface of the Earth, the magnetic field intensity is of the order of 0,6 g.

  5. To produce this bipolar field, it is necessary to suppose that the Earth Core is traveled, in a perpendicular plan in the polar axis, by a buckle of constant current between 2 and 6 billion amperes.

  6. The magnetic poles of the Earth suffer constantly drift over the geographic poles.

  7. Over geological time the magnetic poles are reversed on numerous occasions.

Internal structure of the Earth, The theory
Internal structure of the Earth
  1. Upper Mantle (Its viscosity is bigger than that of the lower mantle). It is formed by rocks such as the peridotite (olivine, pyroxene, and garnet).

  2. Mantle (lower) (» 85 % of the Earth volume). IIt behaves as an elastic solid. You should not trust the flowss of lave of some volcanic eruptions. However it is less "hard" than the Upper Mantle.

  3. Fluid Outer Core (Density » 10). Liquid mixture made with 80 % of iron and about 20 % of nickel. Its viscosity is about 1 (That of the water). Its temperature is about 4000°C. This enormous mass of metal in fusion would be shaken by the convection, and also by "Coriolis Forces" generated by the movements of rotation and precession of the Earth. Theses flows of Ferro-Nickel liquid could generate electric currents and, as a consequence, magnetic fields which in return would strengthen electric currents creating so an effect self-excitation in a dynamo. (Positive feedback).
    The liquid Outer Core would thus be at the origin of the Earth magnetic field.

  4. Solid Inner Core, (Density » 13). Its would have the same constitution as the Outer Core. But the pressure is so important that the Inner Core is in the solid state in spite of its temperature greater than 5000°C.
    The Inner Core is not strictly spherical, it presents an anisotropy, and its surface is irregular. We shall talk again of it.
    Inner Core and Outer Core  would represent approximately 17 % of the Earth volume.

Structure of the Earth magnetic field, the Model

Really, the model proposed by the theorists is not that of the dynamo, but indeed that of the Faraday Disc, but in which the generated current is re-injected in a solenoid so that we obtain a positive feedback with the initial magnetic field.
So that it can work, the following conditions have to be respected:

  1. The re-injected field must be directed in the same sense as the initial field.

  2. The gain of the buckle of reaction has to be G ≥ 1.

  3. The losses by joule effect (Resistivity of the ground magma) have to be strictly lower or equal in the energy re-injected. It is an imperative condition so that the condition 2 can be respected.
    For Ferro-Nickel, in ambient temperature the resistivity  =738.10-9 ohm.meters. But if the temperature increases, the resistivity increases, in a linear way for metals. As a consequence the Resistance of the Outer Core at 4000°C does not seem to be able to present favourable conditions to the absence of losses by Joule effect. (Dont forget that the current intensity in the "internal solenoid of the earth" is about 2 to 6 billion amperes).
    The calculation shows that it should entail within a million years the total stop of the rotation of the Earth. It is not what the geology shows us, and if there is indeed a slowing down of this rotation, it is for a totally different cause (Tides).
    Let us note here that we feign to ignore the change of solid-liquid phase. We shall explain it in the following page.

Simulations realized by certain geophysicists show us the structure of the internal magnetic field inside the Earth. They show us the lines of forces deformed by the convection and by the ground rotation, their evolution, and how the field could be reversed. Let us note that the magnetic field is 90 % bipolar. The scenario thus seems rather close to the reality.

  1. Problems:
    unfortunately none of the simulations allows to explain the permanency of the magnetic field during time. (4 billion years).

  2. In fact the abnormality, observed by the archaeologists and the geologists, of brutal inversions of the Earth magnetic polarities during the geologic eras is not really explained in these simulations.

  3. If the Earth magnetism was only provoked by the convection in the outer core then the magnetic field, on its surface, should look like the one is observed on the surface of the Sun. Thus we have to put forward the Coriolis effect caused by the Earth rotation.

Solar Magnetism

experiments were realized with liquid Sodium, between 120 and 160°C. The Fusion point of the Sodium = 97,81°C and the boiling point = 882,95°C. In these conditions of temperature, and considering the dimensions of the tank, the resistivity is very close of 0. In 20° Celsius it is 4,2° μΩ.cm. Thus it is ineffective.
It is about VKS experiment, Who apparently have explained the polarity reversing of the magnetic field. But in certain physical conditions (dimension, temperature and pressure) who are of a quite other rough that those who reign in the Earth Outer Core and especially with a metal whose the resistivity cannot reach the one who has to be for the Ferro-Nickel  in 4000°C.
Knowing that the resistivity of a metal increases linearly according to a temperature law as T1/2, The calculation shows that it would be, in the case of the earth, of the order of 0,2 W For a circumference of the Outer Core about 10.000 Km. We leave you the care of calculating the dissipated energy (Pw=RI2), And thus of the slowing down of the Earth, With a current of 2 to 6 billion ampères. (Let us note however that with the change of solid-liquid phase, this law could be not respected).
VKS Experiment show that we can explain the inversions of the Earth magnetic polarities if the speed of rotation of discs is sufficient and especially if it is artificially maintained constant.

Therefore:

  • The problem of the durability of the magnetic field is not resolved. Because to adapt and to maintain in a artificial way the speed of rotation of the Earth, it is an other thing than to make turning the engines of 75 watts in VKS at the speed which suit us to find what we wish!

  • If the problem of the inversion of polarities, can be explained in the VKS experiments, is not explainned in the real Earth. The resistivities are of a quite other order of height.

  • Not only the experiments (VKS and others) raise multiple problems, but the simulations which were realized with computers, bring up many others. We do not succeed to function the simulations in the time scales which would agree:
    L’effet dynamo, un casse-tête non-linéaire E. Dormy. (French)
    Numerical models of the geodynamo and observational constraints, Geochemistry - E. Dormy, J.P. Valet, V. Courtillot.

Would be the Inner Core a Superconductor?

We shall examine this hypothesis in the next page.

 
Bibliography
and
Documents
  1. Le Champ magnétique terrestre - Théorie & Modèles.

  2. Le champ magnétique terrestre : observations et modèles - Emmanuel Dormy (CNRS)

  3. Le champ magnétique terrestre

  4. http://tel.archives-ouvertes.fr/tel-00011484/en/

  5. Structure of the Earth

  6. Noyau et controverses

  7. La résistivité et la résistance des conducteurs

  8. The VKS experiment: turbulent dynamical dynamos

  9. http://www.psc.edu/science/glatzmaier.html

Creation date : 02/25/07
Last release
: 04/09/16

A Supraconductive Inner Core :

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