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Research | Space Research | Geomagnetic field - Geomagnetic charts
To produce geomagnetic charts and models for different purposes has been one of the main task in the geomagnetism since centuries. The first known chart was the declination map for the year 1700 compiled by Edmund Halley for navigational aid in the Atlantic Ocean. The first quantitative model representing the global geomagnetic field was published by C.F.Gauss in 1833. He introduced the spherical harmonic analysis for describing potential fields like the geomagnetic field. This method is still the basic one when analyzing globally ground-based or satellite geomagnetic data of internal origin. The International Geomagnetic Reference Field (IGRF) and other relevant global models are based on essentially the same formalism as developed by Gauss.
At FMI magnetic field measurements, geomagnetic observatory recordings and magnetic cartography have long traditions. In fact the institute started as a magnetic observatory in 1838. Today Department of Geophysics and Nurmijärvi Geophysical Observatory continue geomagnetic and other relevant scientific and observational traditions at FMI. The responsibility to maintain and complete a magnetic data base for basic magnetic charts of Finland has belonged to FMI since 1930's.
Printed magnetic maps of Finland (1 : 1,000,000) of geomagnetic fied vector components are available at FMI/GEO. The epoch is 1990.0. A numerical model for PC or Mac is also available. This model is updated and gives magnetic field values at a given place and date in Finland since the year 1800. The accuracy of the computed field values is of course rather poor for the first year, but the last 50 years are relatively accurate.
The isolines in the map show the mean deviation of compass needles eastwards from the true North. The numbers corresponds declination values eastwards from the true North. The chart values give the mean value of declination over an area approximately 50 km x 50 km, but real declination values, as measured at a given place, may differ several degrees from the chart values.
The declination is slowly changing from year to year due to the secular variation of the geomagnetic field as whole. The reason to this change is the slow fluid movement in the Earth's liquid core in the depth of 2900 km. The annual rate of the secular variation in declination is 0.1 degrees to the East. FMI's Nurmijärvi Geophysical Observatory is recording constantly variations in the Earth's magnetic field since 1953.
The map shows the mean distribution of the inclination (I) angle of the geomagnetic field vector. Inclination gives how many degrees geomagnetic field vector deviates from the horizontal plane. In Finland inclination varies between 73 deg and 78 deg from south to north and its annual change is 1'.
If I = 90 deg, the field vector is perpendicular to the Earth's surface. If I = 0, the vector lies in the horizontal plane. The former value occur at the northern geomagnetic pole which is located in Canadian Archipelago approximately at 78 N and 100 W, some 1000 km south of the true North Pole. Its southern counterpoint lies near Antartica about 400 km south of the Australian coast. Inclination value I = 0 defines the geomagnetic Equator which is close to geographical Equator.
The map shows the mean distribution of the horizontal component of the geomagnetic field vector in Finland. The unit is nT (nanotesla). The direction of the H-vector determines the orientation of the needle in a magnetic compass. Declination is the angle between the true North direction and that of the compass needle. H in Finland is decreasing annually about 10 nT. This means that the force keeping the needle in a magnetic compass is slowly weakening.
The map shows the mean distribution of the total geomagnetic field (F), that is the scalar value of the geomagnetic flux density in the unit of nT (nanotesla). In Finland the mean F varies between 51000 nT and 53000 nT and its annual rate is about 15 nT. This change in Finland strengthens the geomagnetic field here opposite to what is the global trend in the Earth's dipole field intensity which is decreasing about 1 promille annually.
More information:
, tel. 358 9 1929 4649
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