Re: Earth's Magnetic Field

[brian whatcott <inet@INTELLISYS.NET>]

At 16:04 7/12/99 -0400, Chuck Britton wrote:

> VERY recent seismographic measurements show that the solid core has a
> slightly different angular velocity than does the liquid core. (I
> forget whether the solid velocity is presently greater or less than
> that of the liquid)
>
> When the earth's B field is reversed, the relative motions
> will be different.
>
> The sun reverses its field every 11 years and the picture that is
> often presented is the HPG with spontaneous, chaotic reversals.

> Chuck Britton


Here's how Phil McFadden of the Australian Survey puts it:

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X-Sender: pmcfadde@pcug.org.au
Date: Wed, 14 Jul 1999 15:43:09 +1000
To: inet@intellisys.net
From: Phil McFadden <pmcfadde@pcug.org.au>
Subject: Geomagnetism

Dear Brian,

Trevor Powell passed your query on to me.

Today it would seem that chaos provides a "plausible" model for everything.
Undoubtedly the geodynamo is chaotic in the strict technical sense of the
term. I say this because, from a mathematical point of view, magnetic field
generation in the core is the consequence of several interlinked
differential equations. Thus a small change in initial conditions will
inevitably lead to exponential growth in solution uncertainty.

As has been shown by numerical models of the geomagnetic field (e.g.,
Glatzmaier and Roberts or Kuang and Bloxham), the reversal process is
almost certainly inherent in the equations. Thus it would seem that
reversals occur without any direct mechanical causation, e.g., a big lump
of the mantle falling into the core and disturbing the flow. Our perception
is that the underlying rate at which reversals are attempted is quite high.

The inner core is a large lump of solidified iron which is obviously not
involved in the convective processes of field generation in the core.
Indeed, just the opposite: it takes a long time for magnetic flux to
diffuse into the inner core and so it provides a "flux memory" with a
significant time constant. We think that this stabilizes the reversal
process, that is, many "attempted reversal events", which would probably
lead to reversals in an unmoderated core, do not in fact produce an actual
reversal. Only those reversal attempts that are large enough and persistent
enough to overcome the inner core memory lead to actual reversals.

Generation of magnetic field in the outer core is of course a consequence
of a thermal engine, and therefore the thermal boundary conditions imposed
upon the core by the lower mantle have an effect on the processes. Our
current view is that the imposed boundary conditions (both the amount of
heat allowed out of the core, and the structure of that heat flux [e.g., is
more heat allowed out at high latitudes than low latitudes or vice versa])
has an impact on the underlying rate of reversal attempts and therefore on
the actual rate at which reversals occur. Consequently we see changes in
the reversal rate, the changes occurring on time scales that match those of
the mantle.

You may be interested to read a book that I recently co-authored with Ron
Merrill and Mike McElhinny. It is called The Magnetic Field of the Earth,
published by Academic Press.

Phil McFadden


------------------

    Phil McFadden

    Home:  (preferred address)
         pmcfadde@pcug.org.au
         Phone : +61-2-6288 3264

    Work:
    Australian Geological Survey Organisation
    GPO Box 378, Canberra, ACT 2601, Australia
        Phil.McFadden@agso.gov.au
        Tel: +61-2-6249 9612       Fax: +61-2-6249 9986

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Sincerely
Brian



brian whatcott <inet@intellisys.net>
Altus OK