Editing Dynamo theory (section)

== History of theory ==

When [[William Gilbert (astronomer)|William Gilbert]] published ''[[De Magnete]]'' in 1600, he concluded that the Earth is magnetic and proposed the first hypothesis for the origin of this magnetism: permanent magnetism such as that found in [[lodestone]]. In 1822, [[André-Marie Ampère]] proposed that internal currents are responsible for Earth's magnetism.<ref>{{cite book |author-last=Ampère |author-first=André-Marie |date=1822  |url=http://catalogue.bnf.fr/ark:/12148/cb37284089w |language=French |location=Paris |publisher=Crochard |title=Recueil d'observations électro-dynamiques : contenant divers mémoires, notices, extraits de lettres ou d'ouvrages périodiques sur les sciences relatifs à l'action mutuelle de deux courants électriques, à celle qui existe entre un courant électrique et un aimant ou le globe terrestre, et à celle de deux aimants l'un sur l'autre}}{{No ISBN}}</ref> In 1919, [[Joseph Larmor]] proposed that a [[dynamo]] might be generating the field.<ref name=Larmor1919>{{cite journal |first=J. |last=Larmor |year=1919 |title=How could a rotating body such as the Sun become a magnet? |journal=Reports of the British Association |volume=87  |pages= 159–160}}</ref><ref>{{cite journal |first=J. |last=Larmor |year=1919 |title=Possible rotational origin of magnetic fields of sun and earth |journal=Electrical Review |volume=85  |pages= 412ff }}  Reprinted in ''Engineering'', vol. 108, pages 461ff (3 October 1919).</ref> However, even after he advanced his hypothesis, some prominent scientists advanced alternative explanations. The [[Nobel Prize]] winner [[Patrick Blackett]] did a series of experiments looking for a fundamental relation between [[angular momentum]] and [[magnetic moment]], but found none.<ref>{{cite journal|last=Nye|first=Mary Jo|title=Temptations of theory, strategies of evidence: P. M. S. Blackett and the earth's magnetism, 1947–52|journal=The British Journal for the History of Science|date=1 March 1999|volume=32|issue=1|pages=69–92|doi=10.1017/S0007087498003495| s2cid=143344977 }}</ref><ref>{{harvnb|Merrill|McElhinny|McFadden|1996|loc=page 17}} claim that in 1905, shortly after composing his [[special relativity]] paper, [[Albert Einstein]] described the origin of the [[Earth's magnetic field]] as being one of the great unsolved problems facing modern [[physicist]]s. However, they do not provide details on where he made this statement.</ref>

[[Walter M. Elsasser]], considered a "father" of the presently accepted dynamo theory as an explanation of the Earth's magnetism, proposed that this magnetic field resulted from electric currents induced in the fluid outer core of the Earth. He revealed the history of the Earth's magnetic field through pioneering the study of the magnetic orientation of minerals in rocks.

In order to maintain the magnetic field against [[ohm]]ic decay (which would occur for the dipole field in 20,000 years), the outer core must be convecting.  The [[convection]] is likely some combination of thermal and compositional convection.  The mantle controls the rate at which heat is extracted from the core.  Heat sources include gravitational energy released by the compression of the core, gravitational energy released by the rejection of light elements (probably [[sulfur]], [[oxygen]], or [[silicon]]) at the inner core boundary as it grows, latent heat of crystallization at the inner core boundary, and radioactivity of [[potassium]], [[uranium]] and [[thorium]].<ref>{{cite news | first=Robert | last=Sanders | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=2003-12-10 | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml | access-date=2007-02-28 }}</ref>

At the dawn of the 21st century, numerical modeling of the Earth's magnetic field is far from precise. Initial models are focused on field generation by convection in the planet's fluid outer core. It was possible to show the generation of a strong, Earth-like field when the model assumed a uniform core-surface temperature and exceptionally high viscosities for the core fluid. Computations which incorporated more realistic parameter values yielded magnetic fields that were less Earth-like, but indicated that model refinements {{which|date=February 2022}} may ultimately lead to an accurate analytic model.  Slight variations in the core-surface temperature, in the range of a few millikelvins, result in significant increases in convective flow and produce more realistic magnetic fields.<ref>{{Cite journal | last = Sakuraba | first = Ataru |author2=Paul H. Roberts | title = Generation of a strong magnetic field using uniform heat flux at the surface of the core | journal = Nature Geoscience | volume = 2 | pages = 802–805 | date = 4 October 2009 | doi = 10.1038/ngeo643 | bibcode = 2009NatGe...2..802S | issue=11}}</ref><ref>{{Cite journal | last = Buffett | first = Bruce | title = Geodynamo: A matter of boundaries | journal = Nature Geoscience | issue = 11 | pages = 741–742 | year = 2009 | doi = 10.1038/ngeo673|bibcode = 2009NatGe...2..741B | volume=2}}</ref>