The National Geomagnetic Initiative (1993) / Chapter Skim
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Scientific Issues and Research Opportunities
Scientific Issues and Research Opportunities
Pages 18-50
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From page 18... ...
is it stable or unstable from a convective point of view? What are the mechanisms by which the geomagnetic field varies with time?
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From page 19... ...
The resulting electromagnetic body force exerts a torque on the mantle that may explain decadal fluctuations in the length of day and could conceivably contribute to excitation and damping of the Chandler wobble. Flow of core fluid past seismically detected core-mantle boundary topography may also exert a pressure torque on the mantle.
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From page 20... ...
First, as mentioned above, it would constrain the electromechanical coupling between the core and mantle. Second, the attenuation of secular variations with distance from their sources in the core is strongly dependent on electromagnetic induction in a mantle having a large, though finite, conductivity.
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From page 21... ...
Main Field and Secular Variation Knowledge of the main magnetic field and its temporal changes is fundamental to many basic questions relating to the origin and dynamics of the Earth, as discussed above. In addition, global and regional models of the main field have many practical applications in the commercial sector and the military.
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From page 22... ...
· Can paleomagnetic and archaeomagnetic data be used reliably to improve the understanding of the behavior of the main field by producing models for earlier epochs? Recommendations In order to understand the main field of the Earth, the fluid dynamics of the Earth's outer core, and core-mantle coupling, it is essential to study the geomagnetic field as a function of time on a scale of years to decades, as provided by surface and satellite measurements, and on a scale of hundreds to millions of years, as measured by archaeomagnetic and paleomagnetic techniques.
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From page 23... ...
Order does not imply a priority ranking. Their · Long-term, stable time series should be generated from a global distribution of modern, upgraded observatories and repeat stations, tied together with a long-term, preferably continuous, magnetic field satellite monitoring program.
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From page 24... ...
The magnetic method has many societal applications. Magnetic anomaly studies can delineate features associated with mineral or hydrocarbon accumulations; such features include igneous intrusions, fault
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From page 25... ...
Tectonic Relevance of Magnetic Anomalies Oceanic Anomalies The magnetic source layer in the ocean basins contains a continuous, high-fidelity record of geomagnetic field history and tectonic motion since the Jurassic. Understanding the processes that control the recording of the Earth's magnetic field by the oceanic crust (the crustal "tape recorder")
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From page 26... ...
Paleomagnetic studies of contemporaneous sequences of sedimentary rocks exposed on land or cored on the seafloor will help to distinguish geomagnetic field behavior from the effects of thermal and geochemical processes. Continental Anomalies High-resolution aeromagnetic surveys, such as the statewide survey recently completed by the Minnesota Geological Survey (Figure 3-~)
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From page 27... ...
. Most of the data used to produce this image were acquired under the supervision of the Minnesota Geological Survey; additional data were contributed by the U.S.
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From page 28... ...
Imperfect knowledge of the reference field, lack of anomaly resolution in existing satellite data, and inadequate information on the magnetic properties of the lower crust and upper mantle limit the interpretation of satellite-derived regional magnetic anomalies. External field contamination reduces data quality, especially at the equator and poles.
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From page 29... ...
These techniques include three-dimensional modeling and inversion algorithms, pattern recognition methods, and simultaneous analysis of multiple data sets. Rock Magnetism and Petrology The goal of rock magnetic-petrological studies applied to magnetic surveys is to understand the physical and chemical properties and evolution of rocks responsible for magnetic anomalies.
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From page 30... ...
An especially challenging problem is that magnetizations inferred from satellite measurements are several times larger than those measured in rocks. Better understanding of lithospheric magnetic anomalies will come from experimental and theoretical research on the physical chemistry of mineral assemblages at high temperatures and pressures, as well as experimental studies of temperature and pressure effects on both induced and remanent magnetizations of different rock types.
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From page 31... ...
A second-generation digital magnetic anomaly map should be developed for the United States and its Exclusive Economic Zone (to 320 km offshore)
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From page 32... ...
Experiments at high pressure and temperature are needed to understand deep-seated lithospheric magnetization. Paleomagnetic studies are needed to distinguish geomagnetic field behavior from the effects of thermal and geochemical process.
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From page 33... ...
A number of coupled current systems flow in the conducting plasmas that fill these regions of space. These systems are responsible for most of the temporal changes in the geomagnetic field that occur on time scales from seconds to days.
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From page 34... ...
_ Magnetotai/ 1 Plasma Sheet Neutral Sheet Magnetic Field Lines \ (1 00-1 000km) Magne to tail r
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From page 35... ...
The space-time morphology of the ring current, its closure in the magnetosphere, and its relation to field-aligned currents during all levels of disturbed conditions are of fundamental importance in magnetospheric physics and solid-Earth induction studies. Charge-dependent drifts of particles in the Earth's magnetic field and streaming of electric charges along field lines produce electrical currents.
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From page 36... ...
Magnetic Pulsations and Rapid Temporal Variations Geomagnetic pulsations are variations in the geomagnetic field on time scales of 0.l to 600 seconds. The amplitudes of these variations range from a fraction of a nanotesla to tens of nanoteslas.
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From page 37... ...
Other, less structured waves are focused along magnetic field lines directly into the polar cusp and cleft regions and, on the nightside, into the auroral zone. Pulsations having periods in the range 10 to 150 seconds (commonly known as Pc 3,4 pulsations)
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From page 38... ...
These transients can produce large potential drops and associated current surges that can cause serious damage to large-scale power distribution systems and communications networks. Satellite studies coordinated with arrays of ground-based magnetometers are needed to make progress on the research problems described above both at high latitudes, where magnetic field lines from the dayside magnetospheric boundary are focused to the cusp and cleft regions and where auroral substorms originate on the nightside; and at middle and lower latitudes, where pulsations and equatorial currents constitute the main components of the external field.
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From page 39... ...
Its primary goal is understanding the flow of mass, momentum, and energy from the solar wind, through the magnetosphere to the ionosphere and atmosphere. The ultimate energy source for all external magnetic field variations is the solar wind and its embedded interplanetary magnetic field.
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From page 40... ...
Recommendations Magnetospheric physics is primarily concerned with study of the interaction of the solar wind with the Earth's magnetic field to create the various current systems described above. The study of magnetic fields and currents in the magnetosphere and ionosphere requires a suite of simultaneously recording instruments on spacecraft and on the ground.
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From page 41... ...
When new scientific and operational missions in this region of space are developed, they should include a research-grade magnetometer to support modeling of the magnetospheric field as a function of TMF direction and subs to activity. Ground-based instrumentation: · Existing permanent observatories should be upgraded to record the magnetic field digitally at sampling rates of at least one vector per minute for normal operation and up to two vectors per second (0.5 hertz)
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From page 42... ...
The Earth's oceans have a significant influence on climate through long-term storage and transport of heat. Ocean water is a good electrical conductor; it generates easily measurable electric fields as it moves through the geomagnetic field.
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From page 43... ...
method (also called geomagnetic deep sounding, or GDS) measures magnetic fields; the magnetotelluric (MT)
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From page 44... ...
The resulting electromagnetic fields contain information about a variety of oceanographic processes: surface waves, internal waves, and steady flows. In the case of large-scale ocean currents, for instance, the electric field at the deep seafloor is closely related to mass transport of the water column above the point of measurement.
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From page 45... ...
to support solar-terrestrial physics campaigns to study the temporal and spatial morphology of current systems in the magnetosphere and ionosphere; and (2) for reconnaissance of electrically conducting features in the Earth's interior, such as sediment-fi~led basins, anomalies associated with fluids in the deep crust, and thermal anomalies in the upper mantle.
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From page 46... ...
Upper Mantle The conductivity observed in the outer 200 km of the mantle through long-period MT and MV studies is controlled by such intrinsic factors as the composition, temperature, and mineralogy of the crystalline matrix, as well as by such extrinsic factors as the composition, quantity, and connectivity of interstitial pore fluids and the presence of intergranular partial melts, graphite, and sulfides. Given this plurality of effects on conductivity, what independent constraints are required to make unique interpretations?
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From page 47... ...
Extending the low-frequency limit requires improved separation of temporal fluctuations from the Earth's core and magnetosphere. In turn, knowledge of the conductivity structure of the deep mantle can contribute to studies of the magnetic fields associated with hydromagnetic processes
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From page 48... ...
. Material Properties Laboratory studies of electrical conductivity provide the fink between models of conductivity and the physical and chemical processes occurring within the Earth.
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From page 49... ...
Some of these instruments should have the capability for electric field recording to enable collection of low-frequency MT data. The limited number of ocean bottom electrometers should be augmented to support water motion, ocean dynamics, and mantle studies.
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From page 50... ...
Effects of the presence and distribution of other conductivity-enhancing phases such as carbon, magnetite, sulfides, and partial melts should be investigated using advanced experimental techniques that can carefully control thermodynamic variables. Conductivity and complex impedance measurements linked to physical properties such as porosity, permeability, and acoustic velocity in porous, water-saturated crustal rocks should also be collected.
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