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. 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.[1] In most models, the secular variation is the amortized time derivative of the magnetic field , . The second derivative, is the secular acceleration.[2]
Estimated declination contours by year, 1590 to 1990 (click to see variation).
Strength of the axial dipole component of Earth's magnetic field from 1600 to 2020, according to three models.
Secular variation can be observed in measurements at magnetic observatories, some of which have been operating for hundreds of years (the Kew Observatory, for example). Over such a time scale, magnetic declination is observed to vary over tens of degrees.[1] A movie on the right shows how global declinations have changed over the last few centuries.[3]
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 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.[1] 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.[4] However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual.[5]
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.[4] 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.[6]
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 reversals.[7]
Constable, Catherine (2007). "Dipole Moment Variation". In Gubbins, David; Herrero-Bervera, Emilio (eds.). Encyclopedia of Geomagnetism and Paleomagnetism. Springer-Verlag. pp. 159–161. doi:10.1007/978-1-4020-4423-6_67. ISBN978-1-4020-3992-8.
Dumberry, Mathieu; Finlay, Christopher C. (2007). "Eastward and westward drift of the Earth's magnetic field for the last three millennia". Earth and Planetary Science Letters. 254 (1–2): 146–157. Bibcode:2007E&PSL.254..146D. doi:10.1016/j.epsl.2006.11.026.
Merrill, Ronald T.; McElhinny, Michael W.; McFadden, Phillip L. (1996). The Magnetic Field of the Earth: Paleomagnetism, the Core, and the Deep Mantle. International Geophysics Series. Vol. 63. Academic Press. ISBN978-0124912465.
geomagnetic, secular, variation, refers, changes, earth, magnetic, field, time, scales, about, year, more, these, changes, mostly, reflect, changes, earth, interior, while, more, rapid, changes, mostly, originate, ionosphere, magnetosphere, geomagnetic, field,. 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 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 1 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 2 Contents 1 Recent changes 2 Paleomagnetic secular variation 3 See also 4 Notes 5 ReferencesRecent changes Edit Estimated declination contours by year 1590 to 1990 click to see variation Strength of the axial dipole component of Earth s magnetic field from 1600 to 2020 according to three models Secular variation can be observed in measurements at magnetic observatories some of which have been operating for hundreds of years the Kew Observatory for example Over such a time scale magnetic declination is observed to vary over tens of degrees 1 A movie on the right shows how global declinations have changed over the last few centuries 3 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 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 1 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 4 However this strength is about average for the last 7 thousand years and the current rate of change is not unusual 5 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 4 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 6 Paleomagnetic secular variation EditChanges 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 reversals 7 See also EditGeomagnetic jerk Secular variationNotes Edit a b c d Merrill McElhinny amp McFadden 1996 Ch 2 Fournier Alexandre Aubert Julien Lesur Vincent Thebault Erwan December 2021 Physics based secular variation candidate models for the IGRF Earth Planets and Space 73 1 190 doi 10 1186 s40623 021 01507 z Jackson Jonkers amp Walker 2000 a b CGS 2011 Constable 2007 Dumberry amp Finlay 2007 Tauxe 1998 Ch 1References Edit Secular variation Geomagnetism Canadian Geological Survey 2011 Retrieved July 18 2011 Constable Catherine 2007 Dipole Moment Variation In Gubbins David Herrero Bervera Emilio eds Encyclopedia of Geomagnetism and Paleomagnetism Springer Verlag pp 159 161 doi 10 1007 978 1 4020 4423 6 67 ISBN 978 1 4020 3992 8 Dumberry Mathieu Finlay Christopher C 2007 Eastward and westward drift of the Earth s magnetic field for the last three millennia Earth and Planetary Science Letters 254 1 2 146 157 Bibcode 2007E amp PSL 254 146D doi 10 1016 j epsl 2006 11 026 Jackson Andrew Jonkers Art R T Walker Matthew R 2000 Four centuries of Geomagnetic Secular Variation from Historical Records Philosophical Transactions of the Royal Society A Mathematical Physical and Engineering Sciences 358 1768 957 990 Bibcode 2000RSPTA 358 957J doi 10 1098 rsta 2000 0569 JSTOR 2666741 S2CID 40510741 McElhinny Michael W McFadden Phillip L 1998 Paleomagnetism Continents and Oceans Academic Press ISBN 978 0124833555 Merrill Ronald T McElhinny Michael W McFadden Phillip L 1996 The Magnetic Field of the Earth Paleomagnetism the Core and the Deep Mantle International Geophysics Series Vol 63 Academic Press ISBN 978 0124912465 Tauxe Lisa 2010 Essentials of Paleomagnetism University of California Press ISBN 978 0 520 26031 3 Tauxe Lisa 1998 Paleomagnetic Principles and Practice Kluwer ISBN 0 7923 5258 0 Retrieved from https en wikipedia org w index php title Geomagnetic secular variation amp oldid 1136125981, wikipedia, wiki, book, books, library,
