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Normal Fields


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Magnetic data obtained during measurement surveys can be used to determinate the so called normal field, an analytical expression in latitude and longitude, that best interpolates, for the region of interest, the longest wavelength part of the measured geomagnetic field. Such a normal field doesn’t have a well defined physical meaning, since it can contain deep crustal origin contributions superimposed to the core origin field; it has however the general purpose to furnish an analytical level, with respect to which, for difference, the local magnetic anomalies can be derived.

For what concerns the normal field computation for the Italian region, a polynomial of the second order, in latitude φ and longitude λ, is chosen. The proposed normal field is able to represent the characteristics of the field up to 700 km wavelengths, allowing us to consider that it reproduces with a good approximation the main field with origin in the Earth’s core.

A generic magnetic element E is therefore calculated through the following expression:

E = a0 + a1 φ + aλ + a3 φ2 + a4 λ2 + aλ φ

where φ indicates latitude, λ longitude and a0, a1, a2, a3, a4 and a5 are the coefficients for E determined through the least squares method on the basis of the repeat stations values reduced at a certain epoch and to the sea level.

In the following table normal field coefficients from epoch 1979 to 2005 for each element has been reported. The latitude and the longitude are referred to the values 42° N and 12° E respectively, with values expressed in arc minutes.

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