Abstract
The Earth's magnetic field is generated mainly by a self‐sustaining dynamo in the fluid outer core. Known as the core or main field, the dynamo's magnetic field is not constant but changes with time, a phenomenon denoted as secular variation. Unfortunately, no common agreement exists about the definition of secular variation: While some use this term for the temporal changes of the core field in general, others use the term only for its linear part (first time derivative).
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