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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.
The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron-rich core. These changes are referred to as secular variation. In most models, the secular variation is the amortized time derivative of the magnetic field B {\displaystyle \mathbf {B} } , B ˙ {\displaystyle {\dot {\mathbf {B} }}}. The second derivative, B ¨ {\displaystyle {\ddot {\mathbf {B} }}} is the secular acceleration.