THE EARTH'S MAGNETIC FIELD

 

Silicates known not to have strong magnetic character.  Metals (and particularly ferrous metals) do have known magnetic properties.  It appears reasonable to associate the Earth's magnetic field with the core.  How and why does the Earth have a magnetic field and why is it important?.

 

The Earth's magnetic field is important for several reasons:-

            -           navigation;

            -           traps ionized particles and gives the Van Allen belt that protects life from UV and cosmic rays;

            -           palaeomagnetic studies led to theory of plate tectonics.

 

What are characteristics of field? and what units, etc. are used to describe it?

 

Basic Concepts

 

The magnetic field, B, of a pole of strength m at a distance r is:-

 

                        B          =          µo m / 4π µR r2

 

µo = magnetic permeability of vacuum; µR = relative mag. perm.

 

Units of B are Weber m-2 or Tesla (T), cgs units of B are Gauss (G)

 

                        1 G       =          10-4 T

 

Earth's field is small, use nano T (10-9 T) as unit.

 

                        1 nT      =          10-5 G   =          1 gamma

 

On the Earth B varies from 25,000 nT at the equator to approx. 70,000 nT at poles. 

 

Have daily variation - diurnal changes, due to solar wind, etc., also longer term variations.

 

The Earth's Magnetic Field

 

To a good approximation the Earth's field at its surface is like that of a bar magnet. The Earth's field is dipolar, with the magnetic axis close to (but not coincident with) the rotational poles.

 

Bar magnets are made from iron, but the origin of the Earth's field is NOT the same as that of an iron bar magnet, for several reasons:-

            (1)        The core temperature (4000-7000 K) is much higher than the Curie temperature of iron (or any other material).   Above their Curie temperature, solids lose their permanent magnetisation.

            (2)        The Earth's field has significant departures from that of a simple dipole.  The Earth has regions of magnetic field "highs" and "lows" - non-dipole field component.

            (3)        Both the dipole and non-dipole components of the Earth's field vary in both position and strength as a function of time -SECULAR VARIATIONS.

 

The geomagnetic field at any one point is a vector, and needs 3 quantities to fully describe it. Usually describe the field in terms of its total field strength (F), the angle of dip or inclination (I), and declination (D).

 

At any one spot D, I and F vary on a time scale of 10's years to 106 years. Can produce global maps showing rate of change of magnetic elements (D, I, F) with time.  The product is an ISOPORIC CHART.  Find some centres are undergoing very rapid changes in magnetic behaviour - ISOPORIC FOCI.

 

When compare ISOPORIC CHARTS compiled at different times, note that ISOPORIC FOCI all drift west at about 0.32o per year.

 

If these magnetic effects are related to the core, it suggests that the core rotates about 0.3o per year more slowly than the crust and mantle.

 

The Origins of the Earth's Magnetic Field

 

Any model that attempts to explain the field must not only explain the fact that an essentially dipolar field exists (and has done so for > 3.8 Ga), but that it has secular variations and undergoes reversals.

 

The Earth's dipole field could be reproduced by several models.

            (a)        Dipole - bar magnet at the centre of Earth.

            (b)        A uniformly magnetised core.

            (c)        A uniformly magnetised Earth.

            (d)        An E-W current system flowing along core-mantle boundary, with a current density proportional to the cosine of the latitude (a toroidal current).

 

Models (a) -> (c) ruled out because:-

            (i)         High T of core argues against any permanent magnetic source at depth.

            (ii)        Silicates not sufficiently strong magnets to dominate field (rock magnetism approx. 1% of whole Earth's field).

 

Most current (!) models based on modifications of (d).  Modern theories based on the DYNAMO THEORY.

 

Dynamo theory attributes geomagnetic field to a system of electric currents in the core and lower mantle. The currents must be maintained (if not they would die out in approx. 106 years).

 

The current flow is maintained by fluid motions in the outer core that act as a dynamo (c.f. bicycle light, electricity generated by rotating wheel). The fluid motion is produced by thermal convection in the hot core.

 

It is suggested therefore that the core acts like a self-exciting dynamo. In such a dynamo, a rotating disc cuts a weak field.  This produces an electric potential between the centre and edge of disc.  The potential causes a current to flow through coil. The current in coil gives a magnetic field, which reinforces the electric potential across disc that causes more current to flow, etc.

 

If such a system is caused to rotate fast enough, it can maintain its own magnetic field. (N.B.: This is not magic - energy is put into the system to rotate the disc.  That energy is converted into the magnetic field.  In the Earth the energy input is from thermal convection.)

 

The self-exciting dynamo and the core appear to differ in one very important way. The dynamo above works because the system is asymmetric (coil attached at one point only).  The core appears to be geometrically homogeneous and would not have this asymmetry.

 

Major question:- can the spherical core behave like a self-exciting dynamo exist?

 

To answer this must study mathematics of the interaction of fluid motion with electromagnetic fields - a branch of mathematics called magnetohydrodynamics. On all criteria this is a very difficult branch of maths.

 

Major contribution from Parker who showed the dynamo mechanism can work as a result of the Coriolis Forces, produced by the rotation of the Earth (- water down the plug hole!), that cause convective upwellings in the core to spiral (c.f. wind in cyclones).

 

The spiralling is in opposite senses in the two hemispheres and hence reduces the symmetry of the system, hence the Earth is not symmetrically homogeneous and so can act as a self-exciting dynamo. Dynamo theory is now well established, and can be used to explain the magnetic fields of other planets (Mercury and Jupiter) as well as the Sun.

 

Secular variations are due to the flow and development of small eddy currents superimposed upon the main convective system. A feature of Parker's spiralling convection is that it is capable of self-reversals of magnetic field, but the details are still poorly understood, however the reversals may be produced by changes in the distribution of the spiralling, "cyclonic" convective upwellings.

 

Since the evolution of convective system in core is random, the changes in distribution of spiralling convective cells will be random, and so no regular pattern to normal and reversed magnetism would be expected - CHAOS.

 

Power and the Geodynamo

 

To generate the magnetic field power is required to keep the currents and convection running.  Without that power input the field would decay to zero in 105 - 106 years.

 

The energy consumed by the magnetic field is approx. 1010 W (N.B.: Small when compared with heat loss through surface of Earth approx. 4 x 1013 W).

 

Thermal energy (via convection, etc.) is widely believed to be major source for generation of magnetic field. It is unlikely that this is an efficient process, and it is estimated that <1% of the core's thermal energy can be converted into the magnetic field. Thus total thermal energy coming from core must be >1012 W - a significant fraction of the total heat loss of the Earth.

 

Where does this heat come from?

            (1)        If 75% of Earth's K was in core, then heat from K40 -> Ar would give approx. 1012 W.

            (2)        If core has cooled by 100-300 K in history of Earth, then with heat capacity of 7 x 102 J kg-1 K-1, a drop of 100 K in 4.5 Ga -> 1012 W on average.

 

Another source of energy may come from the crystallisation of the inner core. As pure Fe or FeNi crystallises from the FeS outer core, it leaves a liquid rich in low density S, which floats and rises within the outer core.  This releases gravitational energy. If the inner core has grown in 4.5 Ga, then the average release of energy would be approx. 2 x 1012 W.

 

 

More detailed notes on the geomagnetic field here.

 

Magnetic Practical here.

 

Power Point Talk here.

Rock Magnetism

Most materials show some reaction to magnetic fields yet most are very weak.

At the atomic scale magnetic fields come from the motion of electrons both around the nucleus and its own axis(orbital & spin).  Therefore, magnetism is dominant in the transition elements where electrons are unpaired.

In natural material Iron is the most important source of magnetism.

(1) 4th most abundant element in the Earth's crust
(2) Along with Al it combines with the two most abundant elements (Oxygen and Silicon) to build many of the common rock forming minerals.


The strongly magnetic minerals make up only a few % of the volume of rocks.

The magnetization M of a material is defined as magnetic moment/unit volume (A/m).
In any material the magnetization is generally made up of two components:

(1) remanent magnetization (magnetization remaining in the absence of a magnetic field) see below
(2) induced magnetization ( magnetization induced by an applied field which disappears when the field is removed)

Mi=cH
Mi is the induced magnetization,
c is the susceptibility (ability to be magnetized)
H is the applied magnetic field
 

Magnetic minerals:
 

Iron Oxides

Magnetite

Fe3O4

Titanomagnetite

Fe3-xTixO4

Haematite

Fe2O3

Iron Sulfides

Pyrrhotite

FeS

Gregite

Fe3S4


 

Iron Hydroxide

Goethite

aFeOOH


 

Palaeomagnetism and the History of the Geomagnetic Field

 

Palaeomagnetism (PM) is the study of geomagnetic field during geological past. It uses the permanent magnetisation (natural remanent magnetisation - NRM) which is acquired by rocks when they are formed.

 

The NRM is produced by and reflects the Earth's magnetic field, ideally at the time of formation of the rocks.  NRM is carried by small magnetic particles in rock (magnetite, etc.).

 

PM studies are important because:-

 

(1)        show up behaviour of Earth's field not visible on short-time scale studies (e.g. reversals);

(2)        enable palaeolatitude of landmasses to be established for geological past, from which relative continental motions have been inferred.

 

Methods

 

A rock with an NRM can be envisaged as containing a small bar magnet within it. The orientation of the magnetisation reflects the geomag. field when that rock formed as the NRM is aligned along the flux line.

 

A rock formed in the N, would have a steeply dipping remanence. But one which formed near the equator would have its remanent magnetism tangential to the earth.

 

Magnetic Latitude  =  tan-1 (tan I) / 2

 

Measurements of the inclination of the remanent magnetisation enables latitude to be deduced.

 

The approach is as follows:-

 

            (1)        take an orientated specimen from a locality of known age;

 

            (2)        measure the declination and inclination of remanent magnetisation using a MAGNETOMETER.

 

From this the direction of the magnetic field (or AMBIENT FIELD) when the rock formed can be obtained. If take several samples from same location, should average out any secular variation effects.

 

Remanent Magnetisation

 

Carried by small magnetic particles.  Rocks acquire remanent magnetisation in a number of ways:-

 

            (1)        Thermo-remanent magnetisation (TRM) acquired by igneous rocks when they cool through the Curie temperature of their magnetic minerals (see below).

 

            (2)        Depositional remanent magnetisation (DRM) occurs in some sediments when magnetic grains align along ambient field during deposition.

 

            (3)        Chemical remanent magnetisation (CRM) occurs when a new magnetic mineral grows in a rock as a result of diagenesis or metamorphism. The RM reflects field at the time of growth.

 

Biggest problem in any palaeomag study is to define what are the origins of RM and what rocks are suitable for palaeomagnetic study?

 

The past geomagnetic field

 

Secular variation traced back to pre-history by measuring thermo-remanent magnetisation in ancient pots and bricks and by the study of recent lake sediments (C-dating gives age). Conclude that average magnetic pole position coincides with pole of rotation. In palaeomagnetic studies  it is tacitly assumed that this relation has applied for all past times.

 

The palaeo-intensity of a field can be measured by establishing the magnetisation of a pot or rock, and comparing this natural magnetisation with the magnetisation of pots and rocks produced experimentally when cooled in magnetic fields of known intensity.

 

Results indicate that the average palaeo-intensity of the field is about the same as the current field.

 

The oldest rocks on earth display a remanent magnetisation so we know the magnetic field was established by 3800 Ma b.p.

 

An important discovery of palaeomagnetic studies  was the discovery that the Earth's magnetic field has regularly undergone reversals (i.e. N. magnetic pole would be at south geographic pole).

 

The "reversed" field appears to have been as common as the "normal" field in geological time. Palaeomagnetic measurements can be used to track the details of the Earth's field during a transition from a period of normal to reverse magnetism.

 

This can be done by studying magnetic properties of rapidly deposited lake sediments and ocean sediments, or by studying igneous bodies that have been intruded during this reversal period.

 

From studies of an intrusion in Mount Rainier National Park in U.S.A. conclude:-

 

            (1)        Reversal took approx. 4000 years to complete.

            (2)        During reversal, the magnetic pole migrates to its opposite position in an irregular way.

            (3)        The intensity of the field during the reversal drops to approx. 10% of its normal intensity.  The field begins to weaken before reversal starts.

 

It is not clear whether the reversals do occur by the gradual rotation of the dipolar field or if the dipole field dies away and then builds up in the opposite direction. 

 

Pole reversal may be triggered by the weakening of the dipole field. Reversals appear to occur once every 0.3 Ma on average (in last 45 Ma).  Less frequent in early Tertiary and Cretaceous. No regular pattern to reversals.

 

More notes here

 

Palaeomagnetism practical here

 

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