The Global Perspective
The Earth's magnetic field, through its variations on a wide variety of scales in space and time, carries fundamental information on a variety of dynamic systems in the Earth's interior and the geospace environment. The magnetic field originates from chaotic fluid motions in the core and is significantly distorted by its interaction with the solar wind. The Earth's lithosphere, asthenosphere, deep mantle and core, oceans, and solar-terrestrial environment—either as primary sources or as induced secondary sources—contribute magnetic “signals” that must be separated and decoded to obtain an “image” of the underlying physical processes. Recent technological advances present unique opportunities for studies of the geomagnetic field to contribute to the understanding of a variety of dynamic processes in the solid Earth and geospace environment.
Historically, geomagnetic studies have been at the leading edge of geophysical research. The first application was in the field of navigation, then in the monitoring of the Earth's changing geomagnetic environment through global magnetic observatories, next in geophysical exploration and regional surveys from aircraft and ships, and finally in the refined charting of dynamic processes in the Earth's magnetosphere, lithosphere, and core by modern space vehicles. Of particular note, the first quantitative evidence for plate tectonics was derived from precision magnetic charts of the ocean basins.
The future promises even greater contributions from the national and international geomagnetic community. The following are examples of the
Page 12fundamental questions in global dynamics that could be addressed by the proposed initiative:
What are the mechanisms sustaining the Earth's magnetic field?
Is the secular variation a fundamental part of the dynamo mechanism?
How does the geomagnetic field reverse?
What is the distribution of electrical conductivity in the mantle?
What role does the magnetic field play in coupling the core and the mantle, contributing to changes in the length of the day over time scales of decades?
How are fluids—water phases and/or molten magma—distributed in the deep crust, and what is their role in regional tectonic processes?
How are physical discontinuities within the Earth sustained—petrologically, thermally, and dynamically—on regional and global scales?
What is the magnitude of true polar wander?
What are the plate tectonic “building blocks” of the lithosphere and how were they assembled?
What petrological and petrophysical processes are associated with large-scale, systematic differences in the magnetization of the lithosphere?
What are the fundamental processes through which plasma and radiation from the sun interact with the geospace environment?
How are magnetospheric and ionospheric processes electrodynamically coupled?
Do solar-terrestrial interactions modify short-term weather and long-term global climate change?
How do solar-terrestrial interactions disrupt communication links and power-transmission systems, and can these effects be predicted and mitigated?
New opportunities in geomagnetic research are driven by the confluence of these well-posed, first-order scientific questions with recent developments in technology. Among the technological advances are the use of satellites for navigation, communications, and magnetic field monitoring; the application of new computer technology—work stations, supercomputers, and optical disk storage media for large data bases—to geophysical modeling and interpretation; and the implementation of low-noise, high-resolution instrumentation.
The scientific issues that can now be addressed through implementing this technology include the following:
the recovery and interpretation of low-amplitude, long- and short-wavelength magnetic anomalies from airborne, marine, and satellite surveys;
the nature of the coupling of ionospheric and magnetospheric processes;
an understanding of dynamic processes in the core as they affect global magnetic field modeling and long-term prediction of secular variations; and
regional and global electromagnetic induction phenomena, including those processes associated with the dynamics of the oceans themselves.
An initiative to capitalize on the new opportunities and to address questions related to geomagnetism can be readily and cost-effectively mobilized within the traditional research establishment (federal and state agencies, academia, industry, and scientific societies). Many aspects of this initiative are under way or have already been planned. Interagency coordination would minimize duplication, establish priorities at the highest levels, and assure that the required facilities were in place when needed. Such a broad coalition would not only improve the quality and competitiveness of the basic research enterprise in this country (with its concomi
Page 14tant improvement in the quality of higher education), but it would provide direct benefits to the public at large, including a better understanding of solar-terrestrial influences on the biosphere (that is, the impact of phenomena related to geomagnetism on biological systems), a refined assessment of natural resources and natural hazards of the continental and oceanic crust, improved and safer navigation, and more reliable satellite-based communications.
The Dynamics of the Global Geomagnetic Environment
Contemporary views of the Earth's magnetic field emphasize its variable nature on a wide range of scales in space and time. Although it is sometimes convenient to think of the “main field” of the Earth as having a static, dipole-like quality, this approximation is valid only at time scales significantly less than 104 years. Perhaps a third of the core field is nondipolar, and much of that fluctuates significantly over periods of a few years to a few centuries; this is called secular variation.
The main magnetic field is created by complex fluid motions in the Earth's core that sustain a hydromagnetic, self-excited dynamo (the geodynamo), which arises from processes connected with the chemical and thermal evolution in the core. The geomagnetic field exhibits remarkably rapid variation relative to other deep-seated geophysical phenomena. For example, the speed of westward drift of the geomagnetic field is 106 centimeters (cm) per year, whereas the speed of continental drift is typically on the order of a few centimeters per year. The strength of the geomagnetic dipole component is currently decreasing at a rate that, if continued, would completely eliminate it in 1,000 years. In fact, the secular variation of the geomagnetic field observed by archaeomagnetism and paleomagnetism demonstrates significant changes over the time scale of 103 and 104 years. On the same time scale, but less frequently, the field experiences a complete reorganization known as a polarity transition
Page 15or geomagnetic reversal. All of these changes, which are rapid in a geological sense, reflect the dynamic features of the main geodynamo mechanism.
At periods of less than a few years to a decade, contributions from internal field sources originating in the core overlap the spectrum of transient external field sources in the magnetosphere and ionosphere. These external magnetic field sources fluctuate over characteristic times ranging from a few seconds to a few years or longer and have characteristic dimensions at the Earth's surface of 102 to 104 kilometers (km). These fields are due to energetic magnetic disturbances (sometimes exceeding thousands of nanoteslas) from natural electric current systems in the ionosphere and magnetosphere. They are triggered and/or driven by the interaction of the Earth's magnetic field with plasmas and radiation emitted by the sun. A network of recording magnetometers on the Earth's surface can be used in conjunction with orbiting spacecraft to monitor the temporal and spatial morphologies of these magnetic fields to provide important constraints on diagnosing fundamental physical processes in the solar-terrestrial environment.
Because these external fields are transient in space and time, they serve as natural low-frequency “radar” signals that diffuse into the Earth's interior. Some of this energy is scattered by various geological features at depth and arrives back at the Earth's surface. Analysis of these transient magnetic field variations (and their associated electric fields) at the Earth's surface offers a powerful means for “imaging” the conductivity structure of the Earth's interior and for understanding global and regional processes in the lithosphere, asthenosphere, and deep mantle.
Surveys of the “static” magnetic field components from ships, aircraft, and low-orbit satellites are used to understand the geological, tectonic, and thermal state of the Earth's crust. Magnetic surveys on land are used to characterize terrane boundaries, orogenic belts, and sedimentary basins, and thus the genesis and evolution of continents. Magnetic surveys in the marine environment place constraints on conceptual models
Page 16of seafloor spreading processes, transform faulting, and the evolution of hot spots. It is increasingly apparent, however, that “static” magnetization is not static in the conventional sense. Rather, this magnetization varies in space and time as the primary magnetic signatures are overprinted by fundamental thermal and petrological processes or are modified by tectonic processes that translate, rotate, or deform the crust.
The Magnetic Anomaly Map of North America, published by the Geological Society of America in 1987, was a milestone in the understanding of the static magnetic field of the continent. Yet, this digital data base has critical limitations caused by inadequacy of data in some locations, disparate survey specifications, and uneven treatment of regional fields. A more accurate, second-generation digital data base of the low-altitude magnetic field of the continent could be developed by employing a combination of satellite and high-altitude aircraft surveys to “stitch together” the numerous low-altitude surveys that were used to construct the map.
One of many significant contributions of Magsat, a dedicated magnetic field satellite flown by NASA in 1979-1980, was the measurement of long-wavelength magnetic anomalies which suggest that the petromagnetic character of the lithosphere has been modified through geological time; this finding has profound implications for the evolution of the oceanic and continental crust. However, at the level of precision needed for such refined studies (5 nanoteslas or less), static-field survey data can be significantly contaminated by external fields from transient ionospheric sources. In the future, more accurate surveys—using an Earth-based, global monitoring network—must compensate for these external fields on a more systematic and comprehensive basis than is presently done.
An upgraded global monitoring network is not only essential for correcting magnetic survey data; it would also be used to define the conductivity structure of the Earth's lithosphere and upper mantle and to study fundamental plasma processes in the magnetosphere. Ground-based
Page 17data are critical for discriminating between temporal and spatial effects in satellite observations of processes in the magnetosphere.
There is a strong dependency between space-based and ground-based observational facilities. These facilities, which require major funding commitments and involve long lead times for implementation, have long-term value to many federal agencies and to the scientific community at large.
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 and to apply this understanding to a variety of scientific problems and to technical and societal needs. This characterization should be undertaken using satellites, aircraft, ships, and surface measurements (for example, observatories and regional arrays), to provide a better understanding of dynamic processes in the Earth's interior and its geospace environment, from the inner core to the outer boundaries of the magnetosphere.