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    Re: Auroras/compass readings
    From: Trevor Kenchington
    Date: 2004 Oct 17, 23:18 +0000

    Charles Seitz posted:
    
    > More than 90% of the geomagnetic field is generated by the
    > Earth's outer core. It is this portion of the geomagnetic field that is
    > represented by the Magnetic Field Models.
    
    
    So, when a chart (Canadian Hydrographic Service chart in my case) shows
    a compass rose with 20 degree West variation, is that drawn from a model
    which ignores 10% of the geomagnetic field, or is it based on empirical
    measurements in the vicinity of the point where the compass rose is printed?
    
    If the former, some of the missing 10% is due to transient currents in
    ionosphere and magnetosphere, which will be too short-lived for their
    effects to be reflected on a chart. Some will be due to magnetized
    bedrock beneath the water, which will usually be too localized to be
    reflected on anything but very large-scale charts. But at least some of
    the 10% must apply over large enough areas and long enough periods to be
    included in charted values of variation. Is it there?
    
      Put another way: How large an error can there be in the variation
    shown on charts, aside from the short-term effects of solar flares, the
    local effects of iron ore, and the like?
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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