The IGRF Model

The IGRF is a global model of the geomagnetic field. It allows spot values of the geomagnetic field vector to be calculated anywhere from the Earth's core out into space. The IGRF is generally revised every five years by a group of modellers associated with the International Association of Geomagnetism and Aeronomy (IAGA).

The IGRF program prompts for a number of input parameters. It requires the latitude and longitude of the position where you want geomagnetic field values. These must be supplied in either degrees and minutes (two integers) or decimal degrees. For negative latitudes and longitudes it is necessary to only supply the figure for degrees as negative, (except for zero degrees). The date must also be given in decimal years. The 11th Generation IGRF will accept dates in the range 1900 to 2020.

The program asks whether you are supplying geodetic or geocentric latitude and longitude. The 'usual' values defining positions on the Earth's surface are geodetic coordinates. If you specify geodetic coordinates then the height in kilometres above sea-level is also required. If you specify geocentric coordinates then the geocentric distance in kilometres is required.

The program then calculates the geomagnetic field values, and their rates of change (secular variation) according to the options ticked, at the position and time specified and displays these on the screen. The input parameters are listed to enable you to check that they were entered correctly. The field elements, which can be displayed are:

• Declination (the difference between true north and magnetic north). Declination is positive if magnetic north is east of true north.
• The horizontal intensity of the field.
• Inclination or dip, the angle the field vector makes with the horizontal, positive below the horizontal.
• The north component of the field.
• The east component of the field.
• The vertical component of the field (positive downwards).
• The total intensity of the field.