Extended Magnetic Model (EMM) and Potsdam Magnetic Model of the Earth (POMME)
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                                  source code


The EMM is an extended geomagnetic field model. It represents not only the
main field from the Earth's core, but also a degree 720 (15 arc minute
resolution) contribution from crustal magnetization, and a simple model 
of the magnetosphere. This magnetospheric contribution includes daily
and annually varying fields which induce electric currents in the Earth,
giving rise to secondary induced magnetic fields. These induced fields
are also represented in this model.

The EMM uses only the first time derivative (secular variation) for the 
extrapolation of the field in time, the most conservative approach. 
In contrast, the POMME models include the 2nd time derivative, also
called secular acceleration. This more accurately reflects the present
behavior of the field, but carries the risk of larger extrapolation
errors in the future, when the acceleration of the true field changes.

The models are more accurate if the present state of the magnetosphere
is provided as an input in the form of magnetic indices. Unfortunately,
these indices are presently not available in real-time, so any real-time
evaluation of the model has to set these indices to zero. 

For previous periods, the E_st and I_st indices are available from:
ftp://ftp.ngdc.noaa.gov/STP/GEOMAGNETIC_DATA/INDICES/EST_IST/Est_Ist_index.lis

The IMF-By is available from:
http://spidr.ngdc.noaa.gov/spidr/index.jsp
or from
http://omniweb.gsfc.nasa.gov/
The merging electric field Em is a parameter derived from these solar wind
inputs by:
Em (in mV/m) = V * BT * sin(beta/2.0) * sin(beta/2.0) / 1000
where V is the solar wind flow speed in km/s, BT is the strength of the
interplanetary magnetic field perpendicular to the Sun-Earth axis and
beta is the clock angle acos(Bz/BT). Use the default value of 0.5 mV/m 
if the value of Em is not known.

The F10.7 solar flux is available from the SPIDR data base at
http://spidr.ngdc.noaa.gov
The model requires the 81-day average value 20 months prior.

This archive contains one main program:
EMM_calc.c is a simple main program that will ask for the location and other
relevant input parameters and compute the magnetic field components. For
testing, it can be set up to replicate the WMM, which can then be compared
with the online calculator at
http://www.ngdc.noaa.gov/seg/geomag/jsp/IGRFWMM.jsp

After adjusting the compiler options in 'Makefile', compile the program using:
make EMM_calc

The executable is then found in the subdirectory ./exe
