Centered and eccentric geomagnetic dipoles and their poles, 1600–1985

Fraser–Smith, A. C.

doi:10.1029/rg025i001p00001

Cited by 160 publications

(106 citation statements)

References 38 publications

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“…The cutoff concept works like a Heaviside step-function so that all cosmic rays whose rigidity is below the cutoff are not allowed to enter the atmosphere while all particles with higher rigidity can penetrate. This approximation provides a good compromise between simplicity and reality (Nevalainen et al 2013), especially when using the eccentric dipole description of the geomagnetic field (Fraser-Smith 1987). The eccentric dipole has the same dipole moment and orientation as the centered dipole, but the dipole's center and consequently the poles, defined as crossings of the axis with the surface, are shifted with respect to geographical ones.…”

Section: Geomagnetic Shieldingmentioning

confidence: 99%

A history of solar activity over millennia

Usoskin

2017

Living Rev Sol Phys

391

241

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Presented here is a review of present knowledge of the long-term behavior of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the special indices used to quantify different aspects of variable solar activity, with special emphasis upon sunspot number. Over long timescales, quantitative information about past solar activity can only be obtained using a method based upon indirect proxies, such as the cosmogenic isotopes 14 C and 10 Be in natural stratified archives (e.g., tree rings or ice cores). We give an historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that this method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene. A separate section is devoted to reconstructions of strong solar energetic-particle (SEP) events in the past, that suggest that the present-day average SEP flux is broadly consistent with estimates on longer timescales, and that the occurrence of extra-strong events is unlikely. Finally, the main features of the long-term evolution of solar magnetic activity, including the

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Section: Geomagnetic Shieldingmentioning

confidence: 99%

A history of solar activity over millennia

Usoskin

2017

Living Rev Sol Phys

391

241

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“…In the standard definition given by Schmidt [1934], the eccentric dipole has the same moment and orientation as the centered dipole. The coordinates of the ED location can be easily computed from the IGRF's dipole and quadrupole expansion coefficients [e.g., Chapman and Bartels, 1940;Fraser-Smith, 1987]. For the March 2012 epoch, the geographic coordinates of the ED location are r ED = 568 km = 0.08915 R E , λ ED = 22.61°N, φ ED = 139.60°E, while the geomagnetic coordinates are Λ ED = 14.17°N, Φ ED = 210.30°E.…”

Section: Eccentric Dipolementioning

confidence: 99%

Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

2014

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The technique to remotely sense the plasma mass density in magnetosphere using field line resonance frequencies detected by ground-based magnetometers has become more and more popular in the last few years. In this paper we examine the error that would be committed at low and middle latitudes (L < 4) in estimating the equatorial plasma mass density if dipole field lines are assumed instead of the more realistic representation given by International Geomagnetic Reference Field (IGRF) lines. It is found that the use of the centered dipole model may result in an error in the inferred density appreciably larger than what is usually assumed. In particular, it has a significant longitudinal dependence being, for example, greater than +30% in the Atlantic sector and about À30% at the opposite longitude sector for field lines extending to a geocentric distance of 2 Earth radii. This may result in an erroneous interpretation of the longitudinal variation in plasmaspheric density when comparing results from ground-based arrays located at different longitudes. We also propose simple modifications of the standard technique, such as the use of an effective dipole moment or the eccentric dipole model, which allow to keep using the dipole field geometry but with a significant error reduction.

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“…When a separate special analysis is performed to represent the Earth's field so that the dipole coefficients of the multipole representation are maximized and other coefficients brought to a minimum by allowing both the location of the dipole axis center and tilt of the dipole to vary it is found that the Earth's magnetic field is best represented by a tilted dipole whose center is considerably offset from the Earth's center [Cole, 1963;Fraser-Smith, 1987] .This tilted eccentric magnetic dipole has a field line that is not a pole marker, yet it exits perpendicular to the Earth's surface (Figure 1). …”

Section: Forummentioning

confidence: 99%

Comment on “Survey tracks current position of South Magnetic Pole” and “Recent acceleration of the north magnetic pole linked to magnetic jerks”

Campbell¹

2003

EoS Transactions

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In recent months, two articles have appeared in Eos reporting new positions of the north [Newitt et al., 2002] and south [Barton, 2002] magnetic poles from on‐site measurements of locations for vertical magnetic field dip. Readers should be advised that these reported positions are neither magnetic poles nor geophysically important locations. First let us review some facts about our Earth's field. When a magnetic field can be represented by an axially symmetric dipole pattern similar to the electric field from a pair of equal and opposite electric charges, the axis of this magnetic field is called a pole. For the Earth, Gilbert [1958] first called attention to this dipole feature in his 1600 publication De Magnete. With the magnetic dipole center location corresponding to the Earth's geographic center, and the Earth completely spherical, the exit position of the magnetic dipole corresponded to the location of a fully vertical dip of the field. Global maritime exploration that started around the time of the epic 1492 Columbus voyage greatly depended on the ship compass for navigation and on magnetic declination maps for guidance. It was only natural that considerable interest in special expeditions to locate the mysterious Arctic and Antarctic Earth magnetic poles subsequently arose.

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Centered and eccentric geomagnetic dipoles and their poles, 1600–1985

Cited by 160 publications

References 38 publications

A history of solar activity over millennia

A history of solar activity over millennia

Comparison of equatorial plasma mass densities deduced from field line resonances observed at ground for dipole and IGRF models

Comment on “Survey tracks current position of South Magnetic Pole” and “Recent acceleration of the north magnetic pole linked to magnetic jerks”

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