25 lines
5.6 KiB
Plaintext
25 lines
5.6 KiB
Plaintext
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17. How Could a Rotating Body such as the Sun Become a Magnet?
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Joseph Larmor
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(Report for the British Association for the Advancement of Science, 159 160 [1919])
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In this short report, Joseph Larmor addresses the problem of producing the large-scale magnetic fields of the earth and the sun. Because of the high internal temperatures of both earth and sun, Larmor found the hypothesis of a permanent magnet unacceptable. His solution was a self-exciting dynamo with magnetic fields generated by internal motions of conducting material. Later T. G. Cowling1 showed that rotational motion alone, in the axially symmetric case, was insufficient, but in 1939 Walter Elsasser2 demonstrated that the addition of convective motions could maintain the magnetic field. Besides reviving the dynamo theory for the origin of magnetic fields, Elsasser's paper marked the beginning of a wide range of applications of magnetohydrodynamics to cosmic phenomena.
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1. T. G. Cowling, Monthly Notices of the Royal Astronomical Society 94, 39 (1933). 2. W. M. Elsasser, Nature 143, 374 (1939), and Physical Review 55,489 (1939).
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106
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Unauthenticated Download Date | 6/15/16 3:40 PM
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17. H o w COULD THE SUN BECOME A M A G N E T ?
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TH E OBVIOUS SOLUTION by convection of an electric charge, or of electric polarisation is excluded; because electric fields in and near the body would be involved, which would be too enormous. Direct magnetisation is also ruled out by the high temperature, notwithstanding the high density. But several feasible possibilities seem to be open.
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1
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In the case of the sun, surface phenomena point to the existence of a residual internal circulation mainly in meridian planes. Such internal motion induces an electric field acting on the moving matter: and if any conducting path around the solar axis happens to be open, an electric current will flow round it, which may in turn increase the inducing magnetic field. In this way it is possible for the internal cyclic motion to act after the manner of the cycle of a self-exciting dynamo, and maintain a permanent magnetic field from insignificant beginnings, at the expense of some of the energy of the internal circulation. Again, if a sunspot is regarded as a superficial source or sink of radial flow of strongly ionised material, with the familiar vortical features, its strong magnetic field would, on these lines, be a natural accompaniment: and if it were an inflow at one level compensated by outflow at another level, the flatness and vertical restriction of its magnetic field would be intelligible.
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2
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Theories have been advanced which depend on a hypothesis that the force of gravitation or centrifugal force can excite electric polarisation, which, by its rotation, produces a magnetic field. But, in order to obtain a sensible magnetic effect, there would be a very intense internal electric field such as no kind of matter could sustain. That, however, is actually got rid of by a masking distribution of electric charge, which would accumulate on the surface, and in part in the interior where the polarisation is not uniform. The circumstance that the two compensating fields are each enormous is not an objection ; for it is recognised, and is illustrated by radioactive phenomena, thaï molecular electric fields are, in fact, enormous. But though the electric masking would be complete, the two distributions would not compensate each other as
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regards the magnetic effects of rotational convection: and there would be an outstanding magnetic field comparable with that of either distribution taken separately. Only rotation would count in this way ; as the effect of the actual translation, along with the solar system, is masked by relativity.
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3
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A crystal possesses permanent intrinsic electric polarisation, because its polar molecules are orientated: and if this natural orientation is pronounced, the polarisation must be nearly complete, so that if the crystal were of the size of the earth it would produce an enormous electric field. But, great or small, this field will become annulled by masking electric charge as above. The explanation of pyro-electric phenomena by Lord Kelvin was that change of temperature alters the polarisation, while the masking charge has not had opportunity to adapt itself: and piezo-electric phenomena might have been anticipated on the same lines. Thus, as there is not complete compensation magnetically, an electrically neutralised crystalline body moving with high speed of rotation through the aether would be expected to produce a magnetic field: and a planet whose materials have crystallised out in some rough relation to the direction of gravity, or of its rotation, would possess a magnetic field. But relativity forbids that a crystalline body translated without rotation at astronomical speeds should exhibit any magnetic field relative to the moving system.
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The very extraordinary feature of the earth's magnetic field is its great and rapid changes, comparable with its whole amount. Yet the almost absolute fixity of length of the astronomical day shows extreme stability of the earth as regards its material structure. This consideration would seem to exclude entirely theories of terrestrial magnetism of the type of (2) and (3). But the type (1), which appears to be reasonable for the case of the sun, would account for magnetic change, sudden or gradual, on the earth merely by change of internal conducting channels: though, on the other hand, it would require fluidity and residual circulation in deepseated regions. In any case, in a celestial body residual circulation would be extremely permanent, as the large size would make effects of ordinary viscosity nearly negligible.
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107
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Unauthenticated Download Date | 6/15/16 3:40 PM
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