The Earth's magnetic field, through its variability over a spectrum of spatial and temporal scales, contains fundamental information on the solid Earth and geospace environment (the latter comprising the atmosphere, ionosphere, and magnetosphere). Integrated studies of the geomagnetic field have the potential to address a wide range of important processes in the deep mantle and core, asthenosphere, lithosphere, oceans, and the solar-terrestrial environment. These studies have direct applications to important societal problems, including resource assessment and exploration, natural hazard mitigation, safe navigation, and the maintenance and survivability of communications and power systems on the ground and in space.
Studies of the Earth's magnetic field are supported by a variety of federal and state agencies as well as by private industry. Both basic and applied research is presently supported by several federal agencies, including the National Science Foundation (NSF), U.S. Geological Survey (USGS), U.S. Department of Energy (DOE), National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA), and U.S. Department of Defense (DOD) (through the Navy, Air Force, and Defense Mapping Agency). Although each agency has a unique, well-defined mission in geomagnetic studies, many areas of interest overlap. For example, NASA, the Navy, and USGS collaborate closely in the development of main field reference models. NASA, NSF, and the Air Force collaborate in space physics. These interagency linkages need to be strengthened.
Over the past decade, new opportunities for fundamental advances in geomagnetic research have emerged as a result of three factors:
1. well-posed, first-order scientific questions;
2. increased interrelation of research activities dealing with geomagnetic phenomena; and
3. recent developments in technology.
These new opportunities can be exploited through a national geomagnetic initiative to define objectives and encourage coordination of efforts among federal and state agencies, academic institutions, and industry to systematically characterize the spatial and temporal behavior of the Earth's magnetic field on local, regional, and global scales in order to understand the physical processes in the solid Earth and geospace environment, and to apply this understanding to a variety of scientific problems and to technical and societal needs.
Scientific and Societal Issues
Geomagnetic studies are driven by a host of first-order scientific problems as well as by significant societal concerns. These studies have a renewed importance because interdisciplinary investigations have defined critical and solvable research problems. Recent technological advances—including increased computational power, improved instrumentation and observational platforms, and inversion schemes—have improved research capabilities.
Examples of fundamental issues in global dynamics that can be addressed by this proposed geomagnetic initiative include the following:
What are the mechanisms sustaining the Earth's magnetic field?
How does the geomagnetic field reverse?
How are fluids—water phases and/or molten magma—distributed in the deep crust, and what is their role in regional tectonic processes?
What is the magnitude of true polar wander?
What petrological and petrophysical processes are associated with large-scale, systematic differences in the magnetization of the lithosphere?
What are the plate tectonic building blocks?
How do solar-terrestrial interactions disrupt communication links and power-transmission systems, and can these effects be predicted and mitigated?
Main Field and Core Processes
The mechanism for generating the geomagnetic field remains one of the central unsolved problems in geoscience. Investigation of this problem brings together observationalists, who study the present morphology and history of the field; applied mathematicians, who create numerical models (for example, dynamo models); and theorists, who interpret the observations and numerical models. The geodynamo is one of many interrelated problems addressed by studies of the core and mantle of the Earth. Other problems include core and lower-mantle diffusivities; topography of the core-mantle boundary; fluid motion at the surface of the core; the nature and extent of momentum and energy transfer between the core and mantle; and possible thermal influence of the mantle on core dynamo action.
The conductivity of the lower mantle is a focus of research through laboratory measurements of rocks under high temperature and pressure and through interpretation of changes in the geomagnetic field. At present, conductivity is known only to within several orders of magnitude. To model adequately the conductivity structure of the Earth's deep
Page 4interior, it is necessary to characterize long-term external fields produced by currents in the magnetosphere and to understand how these external fields interact with induced fields in the solid Earth. For very deep Earth studies, such as those of interest to the international Studies of the Earth's Deep Interior (SEDI) program utilizing source fields having periods of a few years and longer, it is also necessary to separate the temporal fluctuations in the Earth's core from those in the magnetosphere.
A better description of the geomagnetic field and its secular variation has applications in many related areas, one of which is navigation, including directional orientation. An understanding of core processes will improve predictive capabilities for global charting. A large number of civilian and military systems still depend on magnetic directional information for either primary or backup systems. The development and updating of standard magnetic field models, such as the International Geomagnetic Reference Field (IGRF), to describe the main field in space and time are invaluable for identifying anomaly fields associated with lithospheric structures, many of which are important in geophysical exploration for mineral and energy resources.
Electromagnetic Induction in the Solid Earth and Oceans
Improved techniques for imaging the electrical structure of the Earth using naturally varying electromagnetic fields have provided important new insights on interior structures and processes. Electrical conductivity is strongly modulated by large-scale dynamic processes in the Earth, particularly when grain-boundary phases, such as fluids (water and/or melt), minerals (especially sulfides), and graphite are involved. Electrical conductivity imaging has important practical applications, particularly at shallow levels in the crust. These include energy, mineral, and water resource exploration; reliability of electric power grids; ground water protection; and waste management.
Electrical structures in the crust and upper mantle are mapped through regional electromagnetic surveys. It is important to map such structures in order to strip off their effects on measurements of the electrical structure of the middle and lower mantle. In this sense, regional surveys are of interest in global studies and for investigating various mechanisms for core-mantle coupling. Ocean currents generate electric fields through interactions with the geomagnetic field. Electric field measurements on the seafloor are useful for monitoring large-scale ocean currents and for studying the long-term variability of the oceans.
Lithospheric Magnetic Anomalies
Few geophysical methods have had a greater impact on the geological sciences than magnetic methods. Magnetic surveys provide key information concerning the geological, tectonic, and thermal state of the Earth's lithosphere. The use of airborne geophysical surveys is a cost-effective way to map the lithosphere.
The discovery of magnetic anomalies in oceanic crust was critical for development of the plate tectonics theory. Insights into the character and depth of magnetic source regions aid investigations of the mechanical and thermal structure of the lithosphere, crustal and oceanic accretion and evolution, true polar wander, the variation of magnetic field intensity with time, and the process of magnetic field reversals. The large dynamic range of magnetization intensity in rocks enables the detection of otherwise subtle variations in lithology, rock properties, and structure. Persistence of magnetization in the lithosphere makes magnetic methods useful for studying its deep levels. The ultimate goal of interpreting magnetic anomalies is an understanding of the three-dimensional distribution of magnetization from which, together with other geophysical data, the geological and tectonic state of the lithosphere can be deduced.
Magnetic anomaly studies have important societal applications. They can be used to delineate features associated with mineral or hydrocarbon accumulations, such as igneous intrusions, fault zones, salt domes, and anticlines. Because crustal magnetization is sensitive to metamorphism and hydrothermal alteration, magnetic contrasts in the crust reflect variations in thermal and geochemical histories that may be diagnostic for certain energy and mineral deposits. National programs to evaluate earthquake and volcanic hazards, to characterize environmentally contaminated areas, and to permit safe disposal of radioactive waste benefit from magnetic anomaly studies.
Magnetospheric and Ionospheric Processes
The geomagnetic field spans all regions of the Earth, from the core, through the oceans and atmosphere, to the ionosphere and magnetosphere. Spatial and temporal changes in the geomagnetic field provide important information on the physical properties of these regions and their connectivity. These changes also provide warnings of natural hazards in space, such as geomagnetic substorms and storms.
The ionosphere and magnetosphere are a closely coupled system that channels energy and momentum from the solar wind to the atmosphere. A number of coupled current systems flow in the conducting plasmas that fill these regions. These currents are responsible for most of the temporal changes in the geomagnetic field that occur on time scales of seconds to days, including magnetic pulsations. Studies of the ionosphere and magnetosphere seek to obtain a quantitative understanding of the flow of energy and momentum through the solar wind, magnetosphere, and ionosphere systems; understand the physics of magnetic reconnection at the magnetopause, the response of the magnetosphere to changes in solar wind pressure, and the processes responsible for viscouslike interactions;
Page 7and understand the physical mechanisms responsible for generating pulsations and controlling their cross-field transport in the magnetosphere. These studies also address the temporal and spatial morphology of magnetic field transients, particularly the effects of induced fields within the Earth on transients observed on or near the surface. 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.
Operational Aspects and Data Availability
Observational programs have led to important advances in geomagnetic research and to the application of these research results to other geophysical disciplines. Comprehensive magnetic surveys by ship were begun more than 200 years ago. Permanent magnetic observatories were established around the world more than 150 years ago. Magnetic surveys by aircraft were begun about 50 years ago. Initial surveys by satellites were undertaken about 20 years ago. Measurements made directly on the ocean floor are now becoming available. These data represent a rich national resource for both present and future generations of scientists.
Advances in geomagnetic research require observational programs and the timely availability of data derived from:
land and ocean floor measurements;
marine and aircraft measurements;
satellite measurements; and
prehistorical reconstructions, historical data, and laboratory measurements.
The basic issues regarding availability of geophysical data have been addressed in several National Research Council reports, including those of the Committee on Data Management and Computing (CODMAC)
Page 8(1982, 1986, 1988), Geophysical Data: Policy Issues (1988), and Solving the Global Change Puzzle (1991). The present report emphasizes the importance of effective availability of geomagnetic data and associated data products through national data centers, World Data Centers, and distributed data centers.
Overview of Recommendations
The following discussion summarizes the essential recommendations presented in the body of this report. Because it is a summary, however, it is not intended as a substitute for the specificity of the recommendations given in more detail elsewhere. The order of the recommendations does not indicate a priority ranking.
1. A national geomagnetic initiative should be undertaken to define objectives and encourage coordination among federal and state agencies, academic institutions, and industry to systematically characterize the spatial and temporal behavior of the Earth's magnetic field on local, regional, and global scales in order to improve the understanding of the physical processes in the Earth and the geospace environment, and to apply this understanding to a variety of scientific problems and to technical and societal needs.
2. This initiative should include a plan with both short-range and long-range objectives to characterize the magnetic field. For studies at the Earth's surface, the objectives should include the following:
better distribution of standard geomagnetic observatories with modern digital equipment;
improved mapping of the crustal magnetic field at high spatial resolution; and
better characterization of the electrical conductivity of the Earth utilizing both magnetic variation and magnetotelluric arrays.
For space studies, the objectives should include continuous monitoring of the following:
the Earth's main field;
the state of the magnetosphere; and
For laboratory studies, the objectives should include the following:
improved measurements of magnetic properties and electrical conductivities of rocks and the geological processes that control them; and
reconstruction of prehistorical variations of the Earth's magnetic field using archaeomagnetic and paleomagnetic measurements.
The objectives should also address the issues involving the preservation and release of existing and future data, including:
the continuation of observational programs and the preservation of resultant data;
the arrangement, where possible, for the release of relevant proprietary and classified geomagnetic data to the scientific community; and
the maintenance of national centers to serve as repositories of geomagnetic data, with emphasis on the management of and access to existing and future data.
3. Responsibility for implementation of the national geomagnetic initiative rests with the scientific community, which should develop a
Page 10mechanism to carry the initiative forward. This mechanism should involve the relevant federal and state organizations, academic institutions, industry, and national scientific societies, especially the American Geophysical Union, the Society of Exploration Geophysicists, and the Geological Society of America. It should also take account of pertinent international programs and activities. These diverse elements of the scientific community concerned with geomagnetism—and international bodies concerned with relevant international scientific programs—have a clear opportunity to make their own activities more effective through the kind of cooperation and coordination envisioned for this national geomagnetic initiative.
4. As part of this initiative, special attention should be given to maintaining and improving communication and coordination among the diverse activities in geomagnetism in federal and state agencies, the academic community, and industry—with a view toward encouraging improved efficiencies and fulfillment of goals in research and applications. This effort should include a provision for ongoing discussions of the needs and activities of the geomagnetic community, especially the programs of government agencies. It should also include a provision for establishing interdisciplinary task groups involving the scientific and engineering communities that will organize, design, and implement specific research programs.