New software for the World Magnetic Model (WMM) Adam Woods(1, 2), Manoj Nair(1,2), Stefan Maus(1,2), Susan McLean(1) 1. NOAA’s National Geophysical Data Center, Boulder, CO 2.CIRES, University of Colorado, Boulder, CO

The WMM is a large spatial-scale representation of the Earth's magnetic field. It consists of a degree and order 12 spherical-harmonic expansion of the magnetic potential of the geomagnetic main field generated in the Earth’s core. Apart from the 168 spherical-harmonic “Gauss” coefficients, the model also has an equal number of spherical-harmonic Secular-Variation (SV) coefficients predicting the temporal evolution of the field over the upcoming five-year epoch. The WMM is produced by the U.S. National Geophysical Data Center (NGDC) in collaboration with the British Geological Survey (BGS). The current WMM is valid up to December 2009. We present the new software for the next-generation WMM (2010-2015). The software and WMM2010 model are available for download at: http://www.ngdc.noaa.gov/geomag/WMM/

•The new WMM software has been written from scratch in C

•The new software exactly follows the algorithm outlined in the technical report

•Uses a modular programming approach to facilitate reuse in other applications

•Conforms to industry standards.

•The core functions are optimized for speed and efficiency

Geoid implementation in WMM software:

The new version of the WMM software allows the user to enter their altitude above the mean sea level (MSL) rather than their height above the WGS-84 ellipsoid. Conversion between heights referenced to MSL and WGS-84 ellipsoid has been implemented using the EGM-96 geoid. This correction results in a difference of magnetic field strength of about 1-3 nT. The largest differences in magnetic field strength coincide with the largest deviations of the geoid from the WGS-84 ellipsoid.

Use of WMM software for high degree geomagnetic models:

The software accurately computes the associated Legendre functions up to degree 2800, making it adaptable for high-degree geomagnetic models. Here we demonstrate the use of the new WMM software to compute NGDC’s new degree 720 Enhanced Magnetic Model (EMM). The figure shows the difference between EMM and WMM for the year 2010. The majority of the differences are due to the difference in spatial resolution between the WMM and EMM. The EMM covers about one third of the crustal field strength. Typical radii for the magnetic deviations are on the order of 50 km. These small deviations represent the contribution of frozen-in magnetic fields in the Earth’s crust, compared to the large scale contribution of the internally generated magnetic field of the Earth.

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GP13A-0766

The new WMM software and WMM2010 model is available for download at:

http://www.ngdc.noaa.gov/geomag/WMM/

IDisplay WMM

information, limitations

Continue ?

Read WMM

Model file, Geoid file, set the

Default parameters

User input :

Location & time

Valid ?

Time change the

coefficients

Compute Spherical harmonic Variables

At poles ?

Spherical Harmonic

summation

Secular variation

summation

Spherical Harmonic

Summation - special

Secular variation

Summation -special

Convert geodetic

Locations to geocentric

Convert height above MSL

To height above WGS84 ellipsoid

Convert the geomagnetic

field elements to geodetic coordinates

Calculate all the geomagnetic field

Elements and their time variations.

Latitude above 55º ?

Grid variation in polar

stereographic projection

Grid variation in Universal

Traverse Mercator proj.

Print results

More points ?Compute Associated Legendre Functions

Use high

degree ALF

Use low

degree ALF

Degree > 16 ?

End of the program

Free memory

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no yes

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no

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Flow chart of WMM Software to compute magnetic field elements at a single point

Magnetic declination at sea level for 2010

Total magnetic field strength at sea level for 2010

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