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The secular variation of a time series is its long-term, non-periodic variation (see Decomposition of time series).Whether a variation is perceived as secular or not depends on the available timescale: a variation that is secular over a timescale of centuries may be a segment of what is, over a timescale of millions of years, a periodic variation.
Geomagnetic secular variation refers to changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior , while more rapid changes mostly originate in the ionosphere or magnetosphere .
A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2° per year. [35] This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD. [37]
The International Geomagnetic Reference Field (IGRF) is a standard mathematical description of the large-scale structure of the Earth's main magnetic field and its secular variation. It was created by fitting parameters of a mathematical model of the magnetic field to measured magnetic field data from surveys, observatories and satellites ...
Similarly, secular changes to these flows result in slow changes to the field strength and direction at the same point on the Earth. Level curves drawn on a declination map to denote the magnetic declination, described by signed degrees.
, the trend component at time t, which reflects the long-term progression of the series (secular variation). A trend exists when there is a persistent increasing or decreasing direction in the data. The trend component does not have to be linear. [1]
Archaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field at past times recorded in archaeological materials. These paleomagnetic signatures are fixed when ferromagnetic materials such as magnetite cool below the Curie point, freezing the magnetic moment of the material in the direction of the local magnetic field at that time.
The semi-analytic planetary theory VSOP (French: Variations Séculaires des Orbites Planétaires) is a mathematical model describing long-term changes (secular variation) in the orbits of the planets Mercury to Neptune.