The National Geomagnetic Initiative (1993) / Chapter Skim
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A4: Main Field and Core Processes
A4: Main Field and Core Processes
Pages 134-154
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From page 134... ...
Three sources contribute to the magnetic field near the Earth: currents in the core, magnetization in the upper lithosphere, and currents outside the Earth and their induced components inside the Earth. The core (or main)
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From page 135... ...
Many natural materials, such as sediments, lava flows, and baked clays, preserve a record of the geomagnetic field existing at the time of the formation or transformation of the material. With appropriate laboratory measurements and procedures, the paleomagnetic record of the geomagnetic field can provide fundamental constraints on the longer time-scale behavior of the geodynamo.
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From page 136... ...
. _7777~ ;:.:.: :.: :~::: :~: :.:: :.:.:.:.:.:.:.-:.:.-:.:.:.:.-:: ~ : secular variations & oscillations ~ ~ ke .: :, ::::::::::::::::::::::::::: : .: .:,.: .:: Y ~ en ~3~ ~:: ~~:~ :~9 ~~ I:: :~ 1~ ~~ ~ ~~ i:: ::t ~~ :: ~ 1 10 1o2 10 3 10 10 1o6 107 1o8 109 time (years)
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From page 137... ...
The first consists of contemporary and historic observations made by satellites, observatories, and surveys; these data are used to construct models of the main geomagnetic field and its secular variation. The second set consists principally of the imprints of past field configurations in a variety of media including baked artifacts, lavas, lake and sea sediments, and magnetized rocks.
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From page 138... ...
High-quality results have been possible only recently, with the availability of accurate, almost perfectly spatially distributed, satellite observations. Without doubt, one of the most crucial needs for main field geomagnetism is to ensure that data are gathered from evenly distributed permanent magnetic observatories and/or satellites, and that this continues in the future on a regular basis, ideally providing continuous monitoring of the field.
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From page 139... ...
Although the existence of many reported geomagnetic excursions is still a matter of discussion, some geomagnetic excursions have enough spatial and temporal consistency that they can be accepted as valid geomagnetic phenomena. This acceptance raises the question: should these excursions be regarded as large-scale secular variations or as aborted polarity transitions?
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From page 140... ...
The time history of the geomagnetic field over the past few hundred years can be determined from a combination of data from long-running observatories and ship-track records. While ship-track records from England have been used successfully to model past secular variation, the reliability of these reconstructions could be significantly increased by "mining" the historic records of other maritime nations, such as the Netherlands, Spain, and Portugal.
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From page 141... ...
An accurate magnetic polarity time scale is imperative for any attempt to place regional events in the context of global change. in addition, an accurate time scale is also a prerequisite to analyses of the statistics of the polarity intervals, which provide information directly relevant to dynamo modeling.
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From page 142... ...
Reliable determination of the temporal evolution of the magnetic field at the core-mantle boundary provides a probe of the dynamics of fluid flow in the core (of obvious importance to dynamo theory) through the determination of the pattern of fluid flow immediately beneath the core surface in much the same way that surface plate motions constrain models of circulation in the mantle.
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From page 143... ...
Thermal and chemical interactions play a central role in determining the convective motions throughout the Earth's interior and are a vital ingredient in the understanding of the thermal and chemical history of the Earth. There are several questions regarding the nature of a polarity transition: · Is the Earth's magnetic field predominantly dipolar at the Earth's surface during a transition?
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From page 144... ...
· Are reversals distinct from normal secular variation or do they represent an end member of a continuous spectrum in secular variation? Recently, paleomagnetic secular-variation data from lava flows have been used to provide valuable indirect information on the magnitudes of the competing dynamo families (dipolar and quadrupolar)
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From page 145... ...
Moreover, there should be several groups engaged in this research groups that will benchmark each other's programs, devise increasingly efficient numerical techniques, and generally stimulate each other. So far, no efficient numerical method has been devised that can cope with the dynamics over periods of the order of the free decay time of field (14,000 years)
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From page 146... ...
Applications Models and charts of the main field and its secular variation have a wide range of practical applications. They provide quick answers to questions about the strength and direction of the geomagnetic field.
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From page 147... ...
Improved Global Models of the Geomagnetic Field The accuracy of any main field model, both in time and space, depends critically upon the quality and distribution of the data upon which it is based. In the absence of global, vector satellite survey data, modelers are dependent on the various types of surface data.
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From page 148... ...
Since satellite data are acquired above the ionospheric currents, such fields are a source of inaccuracy for field models based only on satellite data. The combination of satellite data with a good distribution of surface data permits separation of the measured field according to its three constituent fields: the interior of the Earth, the ionosphere, and the magnetosphere.
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From page 149... ...
Information about the longer-term secular variation of the geomagnetic field can be obtained from the paleomagnetic study of archaeological materials, lava flows, rapidly deposited sediments, and cave deposits. One clear advantage of using archaeological materials is that they can often by placed in a fairly restricted chronological context.
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From page 150... ...
One or more records of the secular variation of the total geomagnetic field would probably provide important new insights about secular variation itself and about the interaction between the dipole and nondipole components of the field. Furthermore, the intensity of the geomagnetic field and the solar wind are the primary modulators of the flux of galactic cosmic rays that control the production of radiogenic isotopes in the Earth's atmosphere.
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From page 151... ...
For example, the results of laboratory rock-magnetic studies are essential to the interpretation of paleomagnetic samples from the field, because we need to know the conditions under which the recorder of the field is reliable. As these data are accumulated, they will extend knowledge of the behavior of the Earth's magnetic field from the time scale of a few hundred years (available from observatory data)
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From page 152... ...
-sponsored cooperative study of hydromagnetic oscillations, and in Japan, a project on The Earth's Central Core. Under the SEDI umbrella, a Cooperative Study of the Earth's Deep Interior is being developed in collaboration with the Earth Sciences Section of NSF.
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From page 153... ...
To our knowledge, no formal coordination occurs between agencies. The initiation of minimal joint planning between agencies, with the participation of a scientific advisory group, would provide guidance so that the data acquisition efforts could be planned both to satisfy needs of individual agencies and to better meet the needs of the modeling community.
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From page 154... ...
Better field models should be generated from the data to provide constraints on geodynamo theory. When compared with the advances made in the gathering and interpretation of data, the development a model of the geodynamo is seen to be lagging.
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