Stefan_NGDC
Tourist
Reged: 06/14/07
Posts: 5
Loc: USA
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The Earth is like a giant magnet, surrounded by a magnetic field. This magnetic field, which is a vector with both direction and intensity, is generated by a dynamo process in the fluid outer core of the Earth. Due to the chaotic movement of the core fluid, the Earth's magnetic field gradually changes over the years. The first animation (sample image on left) shows the horizontal direction of the magnetic field lines at the surface of the Earth. The magnetic North and South poles are shown as blue and red stars, respectively (note the change in location of the magnetic poles and the change in the speed of movement). Where the lines are blue, the magnetic field dips into the Earth, where they are red it emerges from the Earth. The transition from red to blue, where the field lines are horizontal, is called the magnetic equator.
When using a magnetic compass for navigation, the compass needle points in the direction of the lines displayed on the left. Obviously, this direction is not equivalent with True North. The compass pointing direction can also differ substantially from the direction to the Magnetic North Pole, since magnetic field lines are not just great circles connecting the magnetic poles. The second animation (middle in sample image) illustrates the orientation of compass needles distributed over the surface of the Earth. True North is indicated by the direction of the black lines.
The angle between the pointing direction of a compass needle and True North is called magnetic declination, or magnetic variation, and is measured in degrees east (positive) or west (negative) of True North. Shown in the third animation (right in sample image) are the lines of equal declination (isogonic lines). On the black, agonic line (declination = 0) True North and Magnetic North are identical. In areas of red lines (positive declination) the compass points East of True North, and in areas of blue lines it points West of True North. Line spacing is 5 degrees. The magnetic North and South poles are indicated by black stars. Erratic looking features at the geographic poles are due to a mathematical complication arising from the definition of declination.
In these animations, the magnetic field from 1590 to 1980 is given by the GUFM-1 model of Jackson et al. (2000), while the field from 1980 to 2010 is given by the 10th generation of the International Geomagnetic Reference Field.
Further infos and related animations:
Historical main field change and declination at geomag.org
NGDC/WDC answers to Geomagnetic Frequently Asked Questions
NGDC World Magnetic Anomaly Map
Earth's magnetism on NOAA's Science on a Sphere
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Hill
Master Guide
Reged: 10/31/04
Posts: 8714
Loc: Los Angeles
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Very nice animations, Stefan. They take a while to load and shut down GE twice now, but that may just be my computer.
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Stefan_NGDC
Tourist
Reged: 06/14/07
Posts: 5
Loc: USA
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Yes, you are right that it takes a while to load. After starting an animation it can take several minutes until all of the frames are loaded and the animation runs smoothly. One can then right-click the button on the left of the time-line to reduce the animation speed.
Regarding the shutting down of Google Earth: I tried it on different computers, and GE consistently crashes after loading about 500 frames, although each frame is only about 60 KB. With about 32 MB in total for all 3 animations, this should not be a problem of limited local computer memory. Changing GE cache sizes also does not help. In order for users to be able to load at least two of the animations in one go, I therefore reduced the frame resolution, and the file 'mainfield.kmz' is now set up in such a way that it loads a frame only for every alternate year. This reduction in size and number of frames also optimizes download times.
In case there is a demand for a further reduced version (one frame every 10 years?), or for a higher resolution version which is less blurred at the geographic poles, please let me know and I will be happy to post it.
-Stefan
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Richard_Fozzard
Tourist
Reged: 03/10/08
Posts: 2
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Stefan,
The problem with loading more than two animations may indeed be memory-related (or Microsoft-related).
On my Linux box, with 4GB RAM, all three animations load beautifully. Google Earth itself is taking more than 2GB of RAM to do it, but it doesn't crash.
PS: I like the altitude for the compass points; you can fly *under* them and look up. They look like giant alien space invaders (remember the movie Independence Day?) ;-)
--Rich
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Richard Fozzard
Computer Scientist, CIRES, University of Colorado
Enterprise Data Systems, National Geophysical Data Center/NOAA
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Richard_Fozzard
Tourist
Reged: 03/10/08
Posts: 2
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Update: I just tried this on my 7-year old Mac iBook laptop (0.7GHz CPU, 0.6GB RAM), and -- amazingly -- all three animations load and work correctly. The ancient CPU has to work hard, turning on the fans, and chewing up virtual memory (about 1.1 GB with all three loaded), but it's remarkable how nice these animations look even on a very old and slow machine.
So I think we can blame Microsoft, not a memory problem, for the crashing on the Windows version. ;-)
--Rich
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jpwade
Builder
Reged: 03/12/06
Posts: 585
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Hello Stefan_NGDC,
The animatins are great!
I noted at geomag.org online calculator the US Historic Declination
dates are from 1750-2010.
Would you have any projections beyond 2012?
maybe I could flip the GE globe upside down?
*****
extended load time for the png images at approx 60kb through NetLinks is expected.
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JP Wade - Click for Permissions
www.czmartin.com/jpw - Contact & list of models submitted to GE Forum
Download Latest Version of Google Earth
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Stefan_NGDC
Tourist
Reged: 06/14/07
Posts: 5
Loc: USA
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Hi Joey,
>I noted at geomag.org online calculator the US Historic Declination
> dates are from 1750-2010.
The reason for the difference in time span is that they are based on different magnetic field models:
The US Historic Declination Calculator is based on a continental magnetic model of North America, compiled by the US Geological Survey.
In contrast, the GUFM model of Jackson et al (2000) which was used for the Google Earth animations is a global spherical harmonic model based on all available historical magnetic field measurements worldwide.
> Would you have any projections beyond 2012?
You can always project the magnetic field outward beyond 2010 from the present generation of the International Geomagnetic Reference Field. We do not allow this in the online-calculators, but you can do it using the programs on our Models and Software page. Due to the chaotic nature of long-term magnetic field changes, the quality of the prediction will deteriorate increasingly, the further you predict into the future.
The reason why we state that the IGRF-10 and WMM-2005 are only valid until 2010 is because there will be new versions of these models available on Jan-1 2010, and the old models should no longer be used when the updated versions are available.
- Stefan
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