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Geomagnetic secular variation refers to changes in the
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic ...
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 A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field. The disturbance that d ...
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 Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...
. These changes are referred to as ''secular variation''. In most models, the secular variation is the amortized time
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
of the magnetic field \mathbf, \dot. The second derivative, \ddot is the ''secular acceleration''.


Recent changes

Secular variation can be observed in measurements at magnetic observatories, some of which have been operating for hundreds of years (the
Kew Observatory The King's Observatory (called for many years the Kew Observatory) is a Grade I listed building in Richmond, London. Now a private dwelling, it formerly housed an astronomical and terrestrial magnetic observatory founded by King George III. T ...
, for example). Over such a time scale,
magnetic declination Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) an ...
is observed to vary over tens of degrees. A movie on the right shows how global declinations have changed over the last few centuries. To analyze global patterns of change in the geomagnetic field, geophysicists fit the field data to a
spherical harmonic In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields. Since the spherical harmonics form ...
expansion (see
International Geomagnetic Reference Field 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 terms in this expansion can be divided into a dipolar part, like the field around a bar magnet, and a nondipolar part. The dipolar part dominates the geomagnetic field and determines the direction of the geomagnetic poles. The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century. At this rate of decrease, the field would reach zero in about 1600 years. However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual. A prominent feature in the non-dipolar part of the secular variation is a ''westward drift'' at a rate of about 0.2 degrees per year. 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.


Paleomagnetic secular variation

Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as ''paleomagnetic secular variation'' or ''paleosecular variation (PSV)''. The records typically include long periods of small change with occasional large changes reflecting
geomagnetic excursion A geomagnetic excursion, like a ''geomagnetic reversal'', is a significant change in the Earth's magnetic field. Unlike ''reversals'', an excursion is not a "permanent" re-orientation of the large-scale field, but rather represents a dramatic, typic ...
s and geomagnetic reversals.


See also

*
Geomagnetic jerk In geophysics, a geomagnetic jerk or secular geomagnetic variation impulse is a relatively sudden change in the derivative (mathematics), second derivative of the geomagnetic field, Earth's magnetic field with respect to time. These events were no ...
* Secular variation


Notes


References

* * * * * * * * {{Refend Geomagnetism