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From: Tim Thompson To: All Msg #45, Sep-10-93 04:33PM Subject: Re: Earth's magnetic field and Cowling's (sp?) Organization: Jet Propulsion Laboratory From: tjt@scn1.Jpl.Nasa.Gov (Tim Thompson) Message-ID: <26r6d4$qae@elroy.jpl.nasa.gov> Reply-To: tjt@scn1.Jpl.Nasa.Gov Newsgroups: talk.origins In article dkm@news.mic.ucla.edu, rush@eggneb.astro.ucla.edu () writes: >I don't understand: why is it such a big deal for Barnes to use >Cowling's theory to day that a dynamo isn't generating the earth's >magnetic field? Or, is that even what's he's saying. > >I read the FAQ but couldn't understand this. > >-Brian Well, I'm not sure what constitutes a "big deal", but, in a nutshell, Barnes is wrong. Cowling's Theorem was published in the Monthly Notices of the Royal Astronomical Society about 1934 (I don't recall the reference specifically righ now, but I think it was in the FAQ?). It shows that no magnetic field that is axially symmetric, nor one mathematically similar, could be maintained via axially symmetric current flow. Cowling's intended application was sunspots, but he noted in his paper that his derivation was applicable in the general case. A pure dipole field is axially symmetric, and therefore would not be generated/supported by currents flowing in a planetary core, the dynamo mechanism theorized for the presence of the Earth's magnetic field. This is no problem for the Earth, because it's field is not a dipole, except in the crudest sense; it has copious energy involved in higher order terms, and therefore can be dynamo supported, as is the case for all known planetary or stellar magnetic fields. To get around this problem, Barnes postulates, without any justification, that only the dipole component of the Earth's magnetic field is generated by currents flowing in the core, and that all higher order terms are the result of magnetic features at the surface, or telluric currents generated as a reaction to the solar magnetic field impinging on the Earth's magentic field, solar wind, thubderstorms, and etc. Tossing out the higher order components allows him to invoke Cowling's theorem and deny the existence, or even the possibility, of dynamo maintenance for the Earth's magnetic field. Barnes's method is silly. The verious order terms of a harmonic expression of the Earth's magnetic field are dominated by features of approximately the same physical size as the field component. For instance, and surface feature on the Earth that could influence the quadrapole moment of the Earth's field would have to cover about 1/8 the surface of the Earth. Telluric currents are scaled very much smaller than that, and even the largest concentrations of magnetised rock are nowhere near that large. Add to this the fact that there is a significant westward drift in even the lower order elements of the field (quadrapole, octopole, etc.), and explain how magnetised rocks move over the surface much faster than the continents they are attached to. I know I have jargonized this to some extent, but a detailed explanation in plain language calls for a lot of words, as it depends on the meaning of the language Barnes uses, or mis-uses. The definition of terms is really important here. Hope this helps. --- ------------------------------------------------------------ Timothy J. Thompson, Earth and Space Sciences Division, JPL. Assistant Administrator, Division Science Computing Network. Secretary, Los Angeles Astronomical Society. Member, BOD, Mount Wilson Observatory Association. INTERnet/BITnet: tjt@scn1.jpl.nasa.gov NSI/DECnet: jplsc8::tim SCREAMnet: YO!! TIM!! GPSnet: 118:10:22.85 W by 34:11:58.27 N

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