It is my conviction that all theories should have an underlying simplicity which grips the imagination without the prior need for complicated mathematical manipulation. Certainly mathematics is often necessary for developing the theory in order to make predictions to test it and to attempt to falsify it, particularly in the case of the physical sciences, yet even there the basic ideas alone, generally have a convincing ring of truth about them. I do not get this feeling about the prevailing theories, in particular the Dynamo Theory popular nowadays.
There are a number of peculiar observations of Earth's magnetism which have to date eluded theoretical explanation. In this article I will propose a novel unifying explanation which will include all the apparently odd aspects of the Earth's magnetic field, but first let me review some of these features.
Furthermore a hypothetical line drawn from one magnetic pole to the other does not actually pass through the Earth's center. This lack of symmetry with respect to the Earth is difficult to explain by the prevailing theories.
Magnetic fields from past geological epochs are sometimes preserved in magnetizable strata and rocks such as lava as they cool below their Curie points, and their intensities can be measured. Variations in intensity above and below that of the present have been found.
Many complete reversals of the Earth's field have been documented over geological ages from studies of the movement of the sea bottom around the gigantic undersea crack that splits the globe, so clearly in fact that they are used routinely by geologists and paleontologists for relative dating of geological formations and fossilliferous strata.
A study was made of intermittently produced Miocene lava formations at Steen's Mountain, Oregon, USA, which gave a very detailed picture of both the intensity and directional changes which occurred during a magnetic reversal. The complete reversal took about 4500 years and the average magnetic field at the surface fell to about 20% of normal during the change. The transitional field was typically non-axisymmetric and there was much meandering, even crossing the equator three times. Three geomagnetic impulses occurred which corresponded to high rates of change of the field. The angular rates of change were approximately 50 +/- 20 degrees per year.
On a much more rapid scale, around about 1969, a so-called 'jerk' occurred in the geomagnetic field, almost synchronously over the whole surface of the earth. It took place in less than 2 years and has not received a satisfactory explanation as yet.
Furthermore to increase the complexity even more, the existence of more than 1 North/South magnetic pole system is known. A number of additional magnetic fields have been measured accounting for about 10 % of the total magnetic field intensity of the Earth.
In recent months Gauthier Hulot of the Institut de Physique du Globe de Paris and his colleagues used satellite data recorded 20 years apart to track changes in the field. In two regions of the boundary between the earth's core and the overlying mantle, the researchers detected a reversed magnetic field. In a section lying beneath the southern tip of Africa, the magnetic field points toward the center of the Earth opposite to the dominant outward-pointing field of the Southern Hemisphere. A second congregation of reversed-flux patches exists near the North Pole.
Such radical changes in direction, position, and intensity in short periods are not dynamically consistent with theories which rely on the rotation of heavy metallic layers in the Earth's interior.
Clearly a more flexible theory is necessary to explain the multitude of seemingly disparate observations.
The heavy metallic core of the planet is usually considered to have been produced at some early period in its history, by a density differentiation process, where the heavier atoms sink to the core under the action of gravity and heat. It is generally believed and reasonably well established by seismic measurements that the Earth's core consists of a solid metal inner core, made up largely of iron and nickel, surrounded by an outer liquid metallic core of similar composition. The radii of the inner and outer cores are estimated to be approximately 1220 km and 3470 km respectively. A pseudo-solid or plastic, non-metallic, electrically non-conducting mantle continues beyond the outer core's radius to the crust.
It is also generally accepted that the core temperature of the Earth is produced by the heat continuously liberated from radioactive decay of long-lived radioactive elements with half-lives of the order of the age of the Earth, such as U 238, U235, and Th232
It is therefore not unreasonable to suppose that these very heavy metallic elements at the upper end of the periodic table have become concentrated preferentially in a sort of kernel at the very inner core, along with other heavy, even more unstable nuclides with much shorter half-lives which have long since decayed.
The three natually occurring nuclides, U238, U235, and Th232, have been present since the beginning of the Solar System and decay through lengthy series to produce stable non-radioactive isotopes of lead. In doing so, extremely high energy alpha and beta particles are expelled from the nuclei with energies in the order of millions of electron-volts (MeV), as well as gamma rays and large amounts of heat energy. Initially, although positively charged, the alpha particles, (Helium nuclei), cannot combine with the abundant free electrons in the vicinity to produce neutral Helium atoms because they have energies greatly in excess of the dissociation energy of that element, and because of their relative bulkiness they cannot penetrate far through the surrounding matrix, so they remain in the vicinity, suffering repeated collisions, and transferring energy to the core.
Similarly the beta particles, (negative electrons), have energies far too high for combination, but unlike the bulky Helium nuclei, instead behave as free electrons in very high energy levels, and are capable of travelling large distances through the lattice before losing much of their kinetic energy via electron/phonon collisions.
By contrast, of course, no motion of the ionic metallic nuclei can occur in the solid inner core, which is undoubtedly in a crystalline condition, and even in the liquid outer core, motion by convection for instance, would be negligible in comparison.
Charge density of the free electron gas is maintained by consumption of cooler electrons at the core by the cooled alpha particles. Consequently an electron current flow is produced and conditions are set up for the generation of current loops throughout the inner and outer core. Once current loops are formed, magnetic fields are produced as a consequence, in accordance with the right hand rule of electromagnetic theory. It is proposed that electrons leaving the kernel will form themselves into semi stable patterns after a time which can be conveniently thought of as cells, somewhat analogous to convection cells in heated fluids. It is the nature of convection cells that sides of adjacent cells move in the same direction parallel to each other, moving up to separate at the top and descend parallel to neighbouring cells, thus eliminating frictional effects between each other. Similarly with the proposed electron cells.
While it is realised that the resulting pattern of cells may be quite complex in reality, I will in the first instance, for the sake of convenience, consider as simple a pattern as possible to illustrate the underlying principles, making the assumption that conclusions derived thereby may be broadly applicable to more realistic conditions. From symmetry considerations the simplest, and perhaps even the most probable pattern of cell formation might be visualized about the X,Y,Z axes of a cartesian coordinate system as being made up of 12 lobes; 2 equal and opposite, along each axis, in both positive and negative directions.
In the absence of a diagram, this can be visualized by imagining outward bound electrons being constrained by the pattern to move from the kernel along each direction of the cartesian axis system until they reach the furthest extent of travel, namely the core/mantle boundary, where they split up in all directions to meet electrons from neighbouring cells, which forces the opposing streams into mutual descent. The descending electrons approaching the kernel are able to combine with sufficiently cooled He nuclei, finally reaching thermal equilibrium with the core temperature.
The cycle continues at a rate set by the rate at which thermal and electrical steady state is maintained. Thermal steady state is in turn established when the rate of heat loss from the core through the mantle by conduction, and convection through the liquid layers of both the outer core and the molten magma of the mantle, balances the heat produced by radioactive decay, a process which is independent of the core temperature.
The steady state core temperature of the Earth is far below the highest recombination temperature of Helium, hence the rate of emission of electrons gives the rate of recombination. Since the kinetic energy of the electrons emitted is known to be in the MeV range and enormous quantities of heat are liberated in maintaining the core temperature we can assume safely that the magnetic fields generated by the putative free electron current loops, where the charges are moving at such high speeds will be enormous.
If this were all, since each cell has an equal and oppositely moving counterpart, within each of which all the electron loops are mirrored by others, all the magnetic fields generated would cancel out and no resultant field would exist.
However an additional factor now comes into play. Due to the rotation of the Earth, centrifugal force causes the Earth's overall shape, and presumably that of the core also, to be oblately spheroidal which distorts the paths that the electrons can take through the core material to favour a flattening of the cells in the axial direction.
This flattening has the overall effect of partially aligning the fields statistically along the axial direction. Even so the fields so produced would continue to add to zero because of the opposite rotations of electrons in opposing cells. However since the fields are so large in the first place, it requires only a minute asymmetry to produce a resultant which is of the correct order of magnitude to produce the observed nett field.
A resultant magnetic field is known to have existed in the Earth at a very early stage, even probably immediately following its formation, hence there must have been some inbuilt primordial asymmetry from the very beginning.
I should emphasize that this effect need only be minute, due to the great intensity of the fields with which we are dealing. I propose that it exists due to a postulated crystalline nature of the kernel radioactive material which produces a slightly preferred direction to the release of the decay products of radioactivity. Even should the crystalline material be symmetric in itself, nevertheless if its crystalline symmetry is not also positioned symmetrically with respect to the overall oblate spheroidal symmetry, it could be sufficient to produce the desired slight imbalance of opposing magnetic fields.
Now a nett magnetic field at the strength we know to exist, will be registered on our measuring instruments at the surface of the planet, and a general preference for the observed North/South pole position will be produced by the Earth's rotation, thus giving rise to the major field close to the Earth's rotational axis. Furthermore because of other possible less preferred loop orientations, various other residual fields in different directions may be produced, thus providing an explanation for octopoles, quadrupoles and such, making up the remaining so-called anomalous field intensity.
The theory developed so far is however not yet capable of explaining other observed phenomena, such as complete reversals, large directional changes of fields, and both slow and rapid fluctuations in intensity and direction.
Something more is required, and something that has operated (and continues to operate) over the billions of years of the Earth's history.
I propose that the necessary causes can be found in the existence of multiple enormous variable heat sources scattered throughout the mantle. Such heat sources are well established and referred to by geologists as plumes or hot spots.
Many plumes exist, scattered over the planet. They are apparently responsible for much volcanic activity, both ongoing and in the remote past. The vast lava flows known as the Deccan Traps and the Siberian Floods have been attributed to plume activity in the geological past. Hot spots are responsible for intermittently fuelling many isolated sub-oceanic volcanoes, producing island volcanoes such as the Hawaian chain in the middle of the Pacific tectonic plate and may possibly contribute to the volcanism associated with the great mid-ocean rift crack circling the globe on the sea floors.
What could be the origin of these so-called hot spots?
At present this is a controversial subject.
I propose that plumes are the result of bombardment of the Earth, early in its history, while in a molten condition, by meteorites, many of which were large and contained their own metallic radioactive kernels. These objects penetrated into the less dense hot mantle to various depths where they lodged and remained, as the mantle cooled and attained its present condition.
Over millions of years those meteorites containing significant radioactive kernels produced their own subsidiary heat sources leading to localized hot spots. Heat convecting inwards from these sources modify the core's convection pattern below the meteoritic intrusion, which in turn locally modifiies the electronic cell pattern, but even more significantly for the present theory of magnetism they set up their own localized electronic cell systems with associated magnetic fields. It is obvious that these fields would have disturbing effects on the main core magnetism and consequently provide additional contributions to the required asymmetry necessary for production of the resultant magnetic field in the core. The effects produced on the core electronic cells vary because of factors such as the age of the impact of the meteor and its consequent heat build up time, the size of its radioactive mass, its depth and its position relative to the underlying electronic cell pattern.
In regard to the last mentioned point, an examination of the distribution of plumes over the Earth's surface may be of interest. It is obvious from even a cursory examination that the disposition is non-random, with the bulk of hot spots lying within a band roughly between 45 degrees North and 45 degrees South latitudes, with some heavier clustering on one side of the planet.
Just such a general arrangement would seem to be exactly what is required by my theory to produce the necessary asymmetry of distorting sources consistent with the production of a general N/S magnetic dipole roughly collinear with the geographic spin axis.
An equatorial/sub-equatorial band with clustering would have its primary distorting effect on those electronic cells extending radially into the equatorial direction, leading to differences in magnetic field strengths in opposing loops, with a resultant at right angles in the general direction of the geographic spin axis, while at the same time having little or no effect on opposing cell loops extending into the polar directions.
It is difficult to conceive of a realistic mechanism for the production of such a banded distribution of plume centres based on considerations of internal magma convection. One would expect a more randomly distributed outcome. On the other hand the observed banding with clustering could be consistent with meteorite impacts from objects moving in the ecliptic plane of the solar system if one makes the seemingly reasonable assumption that glancing collisions in the higher latitudes of the Earth are less likely to result in implantation of the colliding bodies than head-on impacts in the more equatorial regions.
I say could be consistent because I think that with the present inclination of the Earth's spin axis, of approximately 24 degrees from vertical to the ecliptic plane, a wider banding might be expected.
This difficulty is removed however if the impacts were made before the Earth's axis was tilted from its presumed original position at right angles to the ecliptic plane.
It is known, from evidence on the moon and elsewhere, that a relatively short period of massive bombardment occurred, very early in the evolution of the solar system, possibly within a few hundred million years after the origin. This could well be the period during which the bulk of the plume sources were implanted in the proto-Earth at a time before the Earth's axis was tilted, with the actual tilting perhaps produced by a massive collision towards the end of the bombardment period.
The above obviously has implications for the origin of the impacting bodies and this is a matter with which I will deal in a future article.
Furthermore these considerations provide a mechanism for the temporal variations observed in the Earth's magnetic field.
Those meteorites containing significant radioactive kernels release immense amounts of heat into their surroundings, which builds up over long periods of time, causing local liquefaction of the mantle in their vicinity. The molten magma rises towards the lithosphere and is extruded through holes burnt in the crust. Enormous heat release occurs with the ensuing volcanism until the activity is eventually extinguished. Cooler mantle material convects down around the radioactive source once more and the cycle commences again. Depending on the conditions of the source, the cycle may take hundreds or many thousands of years. During the cycle, as the mantle material is heated, its density decreases and the radioactive source may sink lower, only to rise again with the eventual influx of the cooler and denser convecting mantle material, thus a slow oscillation in depth of the heat source is produced, which in turn produces an oscillating distortion on the core electronic cells below. With a multitude of such heat sources operating within the Earth's lower mantle, each at its particular stage in its cycle, it is clear that this could account for the apparently random occurrences of long stable periods of the Earth's magnetic field, as well as sharp changes in intensity and direction, as smaller variations in heating are superimposed on the longer term effects, depending on fortuitous circumstances in the cycles of the affecting heat sources. Larger and longer lasting distorting sources could conceivably be capable of temporarily over-riding the directionalizing effect of the centrifugal force produced by the Earth's spin and/or effects due to the inherent asymmetry of the plume sources, causing wild excursions of the magnetic poles from even approximate alignment with the geographic rotation axis and leaving imprints of their travels in lava flows and elsewhere. Complete field reversals could obviously be brought about by relatively small distortions in one or other of the large opposing magnetic fields which are responsible for the main resultant magnetic field of the Earth. Thus, should the slightly larger field become instead, slightly smaller than its opposite number, a complete reversal would be observed without the need for any enormous non-physical changes in the angular momentum of the Earth's core for instance. Since only negligibly small changes in momentum are required, namely those of a few of the light electrons in an electronic cell, such changes could occur in very short times.
The observation that the magnetic dipole axis does not pass through the Earth's centre can be accounted for also by a large enough, and stable enough, asymmetric offset of the particular opposing dipole producing cells, whose vector addition is responsible for the main nett magnetic dipole field.
This theory has obvious implications for geophysicists who may be attempting to trace paths of presumed drift of the continents in past geological epochs by associating variations in magnetic field direction and declination, fossilized in rock strata on the continents, with movements over the globe. Continental drift appears to be well established by plant and animal palaeontology and also by satellite measurement of the actual movements of the continents, however attempting to base a history of continental movement on fossil magnetic fields, which are themselves independently subject to more or less random variations over time, could obviously lead to wild and inappropriate apparent motions of the land masses.
It should be noted that, with suitable modifications, the principles developed here can be extended to the other solar system bodies possessing magnetic fields.
This, and the extension to stellar magnetism will be dealt with in future communications.
Since my retirement from academic life in the Department of Chemistry at Sydney University in Australia, and with the pressure of advancing years, I find it more difficult to access library facilities, so I have not attempted to provide references to the scientific literature, nor to rigorously dot all the i's and cross all the t's, preferring rather to place my thoughts quickly on record for any who may feel interested enough to comment upon them. I feel that my haste may well be of the essence because unfortunately at this stage in life, procrastination could well result in no result at all.
Now however, notwithstanding the above, I would nevertheless like to make thankful acknowledgement of an article by C. Johnson which can be found at: http://www.mb-soft.com/public/tecto2.html,
for introducing the concept of two large opposing magnetic fields with slightly different magnitudes producing an observed resultant.
First developed, August 2002,
First published on the Web: November 30, 2002
This page is at: http://www.users.bigpond.com/ernestmcfarlane/geomagnetism.html
E-mail to: scientius@bigpond.com