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Geomagnetic secular variation is the changes in the
Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...
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 The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
or
magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
. The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
and
magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
, 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 A model is an informative representation of an object, person, or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin , . Models can be divided int ...
, the secular variation is the amortized time
derivative In mathematics, the derivative is a fundamental tool that quantifies the sensitivity to change of a function's output with respect to its input. The derivative of a function of a single variable at a chosen input value, when it exists, is t ...
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 observatory, astronomical and Terrestrial magnetism, terrestrial mag ...
, for example). Over such a time scale,
magnetic declination Magnetic declination (also called magnetic variation) is the angle between magnetic north and true north at a particular location on the Earth's surface. The angle can change over time due to polar wandering. Magnetic north is the direction th ...
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 expansion (see International Geomagnetic Reference Field). The terms in this expansion can be divided into a dipolar part, like the field around a bar magnet, and a non-dipolar 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 excursions and
geomagnetic reversal A geomagnetic reversal is a change in the Earth's Dipole magnet, dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with North Pole, geographic north and South Pole, geograp ...
s. The Levantine Iron Age anomaly was a fast and spatially localized geomagnetic positive anomaly which took place in the
Levant The Levant ( ) is the subregion that borders the Eastern Mediterranean, Eastern Mediterranean sea to the west, and forms the core of West Asia and the political term, Middle East, ''Middle East''. In its narrowest sense, which is in use toda ...
, with maxima at about 950, 750 and 500 BCE.


See also

* Geomagnetic jerk * Secular variation


Notes


References

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