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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. [1] The geomagnetic field changes on time scales from milliseconds to millions of years.
Changes in Earth's magnetic field on a time scale of a year or more are referred to as secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees. [13] The animation shows how global declinations have changed over the last few centuries. [34] The direction and intensity of the dipole change over time.
Produced soon after acquisition, 98% of the differences between QDD and definitive data (X-north, Y-east, Z-down) monthly mean values should be less than 5nT. QDD are intended to support field modelling activities during the modern satellite survey era, providing extra constraints on, for example, models of the field secular variation.
Geomagnetic secular variation refers to some changes in the Earth's magnetic field. The field has variations on timescales from milliseconds to millions of years – its rapid ones mostly come from currents in the ionosphere and magnetosphere. The secular variations are those over periods of a year or more, reflecting changes in the Earth's core.
Data collection with proton precession instruments was slow, making high sample density surveys impracticable. Data were manually recorded and plotted. The subsequent introduction of Fluxgate and cesium vapor magnetometers improved sensitivity, and greatly increased sampling speed, making high resolution surveys of large areas practical.
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 ...
The Earth's field is roughly like a tilted dipole, but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the geomagnetic pole stays near the geographic pole , but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses.
The resulting output of the processing program is used as the input for subsequent interpretation. Processing may include the use of remote reference data or local data only. Processed MT data is modelled using various techniques to create a subsurface resistivity map, with lower frequencies generally corresponding to greater depth below ground.