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Secular Variation


The term secular variation is commonly used to denote the whole of the geomagnetic field variations occurring on time scales that range from a few years to millennia. The amplitude of Earth’s magnetic field secular variation for a given place of observation fluctuates between a few nT/year to several tens of nT/year for the magnetic intensive components (X, Y, Z, H and F) and from a few minutes/year to several minutes/year for declination and inclination (D and I). Secular variation is clearly seen in geomagnetic observatory data, when one or more field elements are plotted against time for several years (Figure 1, 2).

Figure 1 Secular variation diagram for central Italy. In the diagram D and I time variation is reported on a stereographic projection for years 1600 to the present.

 

Figure 2 Behaviour of the three Cartesian components of the geomagnetic field recorded at L’Aquila observatory.

Secular variation, like the magnetic field itself, originates in the outer core of the Earth and for this reason the interpretation of secular variation has closely followed studies and theories of the Earth’s magnetic field generation. This variation occurs on two time scales which are related to two types of core processes. Changes on time scales of hundreds or thousands years, are related to the main dipole field variation, while those, clearly appreciable also on the shortest time scales, of the order of tens of years, is related mainly to the non-dipole field variations. For this reason, although secular variation shows different behaviour in a range of worldwide observatories, it is typical of the main field, thus being representative of planetary phenomena.

On the basis of observational data (i.e. in the last 400 years) it is possible to say that the secular variation shows the following characteristics:

  • a mean annual decrease of the dipole moment of the order of 0.005% of its average value with a considerable acceleration in the last 30-40 years;
  • a westward precession of the dipole axis of 0.008% yr-1;
  • a northward displacement of the dipole of the order of 2 km yr-1;
  • a westward drift of the non-dipole field, or of a part of it, of 0.2-0.3° yr-1, associated with a possible but not specified southward drift;
  • an intensity variation (increase or decrease) of the non-dipole field at a mean rate of about 10 nT yr-1.

Besides these regular characteristics, secular variation is often denoted by irregular phenomena such as geomagnetic jerks (on the shortest time scales) and polarity reversals of the geomagnetic field (on the longest time scales). The investigation of both phenomena is particularly interesting for the understanding of the dynamical processes occurring inside the Earth that are responsible for the origin of the geomagnetic field.

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