 Check out the bookstore at IRBS.com |
|
|
|
|||||
|
||||||
From: Trevor J. Kenchington (no email)
Date: Sun Oct 17 2004 - 19:18:06 EDT
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 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
|