Observational programs have led to important advances in geomagnetic research and its application to other geophysical disciplines. More than 200 years ago, comprehensive surveys by ship were begun; more than 150 years ago, permanent magnetic observatories were established around the world; approximately 50 years ago, magnetic surveys by aircraft were begun. In the 1970s, initial surveys by satellites were undertaken; today, measurements made directly on the ocean floor are becoming available.
The study of geomagnetic phenomena continues to grow. It is increasingly apparent that the time has arrived to optimize the use of geomagnetic facilities through shared use and the introduction of advanced technology.
Observational facilities are discussed below in four categories:
1. land and ocean floor measurements;
2. marine and aircraft measurements;
3. satellite measurements; and
4. prehistorical reconstructions, historical data, and laboratory measurements.
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Page 51 4 OBSERVATIONAL NEEDS AND FACILITIES Observational programs have led to important advances in geomagnetic research and its application to other geophysical disciplines. More than 200 years ago, comprehensive surveys by ship were begun; more than 150 years ago, permanent magnetic observatories were established around the world; approximately 50 years ago, magnetic surveys by aircraft were begun. In the 1970s, initial surveys by satellites were undertaken; today, measurements made directly on the ocean floor are becoming available. The study of geomagnetic phenomena continues to grow. It is increasingly apparent that the time has arrived to optimize the use of geomagnetic facilities through shared use and the introduction of advanced technology. Observational facilities are discussed below in four categories: 1. land and ocean floor measurements; 2. marine and aircraft measurements; 3. satellite measurements; and 4. prehistorical reconstructions, historical data, and laboratory measurements.
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Page 52 Land and Ocean Floor Measurements Approximately 180 magnetic observatories send digital data to World Data Centers ( Figure 4-1) in support of the geomagnetic studies described in this report. At present, 13 observatories are operated by the United States. Although their actual number is significant, they are unevenly distributed; most are located in the Northern Hemisphere, particularly in Europe. There are no observatories on the ocean floor; consequently, vast areas of the globe—the ocean basins—are not sampled. In addition to standard observatories, there are temporary or semi-permanent variometer stations, which do not record absolute values, as well as repeat stations for recording long-term baseline fluctuations (typically measured at intervals of a year or longer). In the past, arrays of many tens of simultaneously recording variometers have been temporarily deployed to detect electrical conductivity anomalies in the crust and upper mantle or to record activity in the ionosphere and magnetosphere, but at present there are no arrays of high-frequency instruments available to the scientific community in the United States. The uneven distribution of magnetic observatories, which is the result of historical circumstances, provides incomplete knowledge of the field behavior and biases many global studies, including the model of the IGRF and the derivation of magnetic activity indices such as the auroral electrojet magnetic activity index (AE) and the disturbance storm time equatorial magnetospheric activity index Dst.1 The IGRF is a spherical harmonic description of the magnetic potential to degree and order 10. 1 The much-used Kp index (planetary K-index) as traditionally constructed, while purportedly a “global” index, is an indicator of magnetic activity at midlatitudes and does not rely on a regional or global distribution of observatories as do the AE and Dst indices, respectively; thus, the Kp index is not “biased” in the same way that the high-latitude AE index and the low-latitude Dst index are.
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Page 53 FIGURE 4-1 Global distribution of magnetic observatories in operation in 1993 (courtesy NOAA-National Geophysical Data Center).
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Page 54 To fit an observational data set to this degree and order with observatories on the Earth's surface requires a magnetic observatory for each 2,000 × 2,000-km square on the surface—whether land or ocean. For magnetospheric and solid-Earth studies at equatorial and high latitudes, magnetic variations from magnetospheric and ionospheric sources have smaller spatial and temporal scales, requiring observatories to be more closely spaced—for example, 3° in latitude (about 330 km) and 2 hours in local time. In some cases, significant temporal variations of global interest occur over time scales as small as a few tens of seconds, and certain classes of pulsations have characteristic periods of a few seconds. Ideally, it would be useful to have a global network continuously sampling at data rates of a few hertz or more. However, present needs of most of the scientific community can be met with data continuously acquired at rates of 1 sample per second, with allowances for data rates of 10 samples per second for specific coordinated campaigns. An excellent beginning for real-time recording from land magnetic observatories has been the INTERMAGNET project, which uses four satellites to relay digital magnetic data from about 25 observatories to four ground Geomagnetic Information Nodes (GINs). Development of the global network envisaged here would require the installation of additional observatories. Some of these new observatories could be established at sites selected for other purposes: at STEP sites, at the Crustal Dynamics (very-long-baseline interferometry [VLBI], Satellite Laser Ranging [SLR], or GPS) geodetic Fiducial Laboratories for an International Natural Science Network (FLINN) geodetic stations of the United States or other countries, or at Incorporated Research Institutions for Seismology (IRIS) or other seismic network stations. Ocean bottom installations can be located on transoceanic communication cables that have recently become available to the scientific community. In addition to the observatory network, there is a critical need for a portable network facility to measure magnetic variations for induction studies and for studying the ionosphere and magnetosphere. For regional studies of sources in the magnetosphere, sites must be spaced on the order of the height of the ionosphere or greater (from approximately 100 to 1,000 km or more). For solid-Earth studies, sites must be spaced from
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Page 55 1 to 10 km, depending on the expected depth to target, over areas approximately 100 km to 500 km on a side. Marine and Aircraft Measurements Following the development of the proton precession magnetometer in the late 1950s and the use of magnetic anomalies to study seafloor spreading, a vigorous program was undertaken to measure the total field with ship-towed magnetometers. The number of marine measurements increased by a factor of 10 from the late 1950s to the late 1960s. By the late 1980s, however, measurements dropped to the level of the 1950s in spite of the utility of these data to a number of geophysics disciplines. Vast areas of the ocean basins are still grossly undersampled. These data are essential not only in reconstructing patterns of plate motion but also in developing main field reference models (for example, the IGRF), because they are often acquired in regions where no other data exist. With the advent of the satellite-aided GPS, the accuracy and usefulness of such data are greatly increased. The U.S. Navy and the Defense Mapping Agency have classified data bases that cover continent-sized areas of the oceans. These data are of excellent quality for science and, if released in usable form, would be of great benefit to marine geophysics. Similarly, industry-acquired magnetic data are proprietary. In some areas of the world, no other magnetic data exist, aside from either military or industry sources. These data would be helpful in constructing regional-scale magnetic maps for research purposes or in updating the IGRF to develop more accurate models in the areas covered by the data. They could be filtered or averaged to maintain their scientific usefulness while protecting the proprietary interests of the source. The needs of the scientific community will not be met by simple addon activities (for example, “ships of opportunity”) or by gaining access to classified or proprietary data; new surveys are necessary. Project Magnet aircraft operated all over the world and provided important data for the construction of magnetic charts during the 1959-1989 period, but it may be phased out of operation. If this occurs, an enormous hiatus in critical
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Page 56 activity will need to be filled by other agencies or countries. Historically, Canadian, German, and Soviet (now Russian) groups have conducted long track-line surveys. In the United States, P-3 aircraft are operated occasionally for surveys by the U.S. Navy. At the present time, opportunities exist for using Russian aircraft at a greatly reduced cost. Continent-scale magnetic anomaly maps now exist for North America (excluding Mexico), the Commonwealth of Independent States, Europe, Australia, China, the Arctic, and much of Africa and South America. Many of these maps, including the U.S. and North American maps, are of variable quality, due largely to the variable quality of the data used to construct them and to difficulties in leveling the data. The existing digital data set for the United States is of limited use for addressing many scientific problems. Additional surveys are required to stitch together existing data, fill in gaps in coverage, and replace substandard data. Satellite Measurements The value of satellite measurements of the Earth's magnetic and plasma fields has been evident since the first measurements by Sputnik 3 in 1958. Subsequent pioneering missions include the Magsat mission that made a major advance in the characterization of the internal magnetic field; the Defense Meteorological Satellite Program (DMSP) that has allowed the auroral plasma environment to be monitored routinely; the GOES/SMS (Synchronous Meteorological Satellite) program that has provided routine energetic particle and magnetic field data at synchronous orbit; and the ISEE-3 (International Sun-Earth Explorer) spacecraft that has provided nearly continuous solar wind monitoring at the L-1 Lagrangian point. These missions were extremely successful in advancing the understanding of the geomagnetic environment from the outer reaches of the magnetosphere to the interior of the Earth. However, much remains to be done. Characterization of the spatial morphology of the Earth's magnetic field at one moment in time is not sufficient to under
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Page 57 stand the internal dynamo. It is equally important to characterize temporal variations over months, years, and decades. Measurement of electric currents associated with the auroral plasma environment is a key element in characterizing the state of the magnetosphere, because Joule dissipation provides almost twice as much energy deposition into the upper atmosphere as particle precipitation does. Commerce, the military, and society increasingly rely on a network of sophisticated communication and other service satellites in geosynchronous orbit, but these systems are vulnerable to the flux of particles and fields. In many cases, the impact of solar disturbances on these satellite systems can be mitigated with only a few minutes' or even a few seconds' notice of an impending hazard. This warning is sufficient to allow vulnerable systems to be shut down and backup systems to be brought on line. Thus, monitoring the plasma environment of the magnetosphere at a variety of longitudes is essential. In fact, the ISEE-3 measurements at the L-1 point proved that the state of the magnetosphere could be forecast with as much as an hour's warning. Such forecasts, if made on a continuous basis, would be of great benefit to operations that are affected by geomagnetic activity. Nevertheless, at this time there are no real-time data available from L-1, nor are there any firm plans to provide these data. Prehistorical Reconstructions, Historical Data, and Laboratory Measurements Descriptions of the main geomagnetic field must take into account its long-term history. The behavior 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. Much would be gained by examining and evaluating the yet-unstudied historical records of maritime nations such as the Netherlands, Spain, and Portugal. The longer-term behavior of the field, on time scales of 103 to 105 years, is determined by a combination of records from archaeomagnetism, lavas, and lake and marine sediments. This is an extremely important time scale to aid in an understanding of the fundamental mechanism of the core dynamo.
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Page 58 The behavior of the geomagnetic field on time scales of 106 years or more is determined principally from paleomagnetic measurements. Although there is a first-order record of the field for the past 200 million years from oceanic crust, many details of that record remain to be determined from the paleomagnetic study of rocks exposed on land or recovered from the seafloor. Measurements of the field beyond 200 million years must rely on continental records of good quality, which are rare. The interpretation of archaeomagnetic and paleomagnetic data requires substantial laboratory facilities. Current efforts are distributed through many universities and some government agencies. Studies of rock physics and petrology that can be applied to magnetic surveys require specialized facilities dedicated to the mineralogical, petrological, and geochemical aspects of magnetic petrology. Since no single investigator or institution now supports the necessary range of analytical and experimental facilities needed for such studies, an increased level of collaboration will be required among individual investigators measuring both physical and chemical properties. In an analogous way, laboratory studies of electrical conductivity provide the means for inferring the nature of physical and chemical processes in the Earth from estimates of the conductivity from field observations. Such laboratory experiments require a thorough understanding of the ways in which the oxygen and sulfur fugacities, pressure, and other aspects of the physical-chemical environment affect conductivity. In addition, one needs to consider the stability and interconnectedness (pore geometry) of conducting fluids (aqueous and partial melts), and grain-boundary phases (such as graphite and sulfides). In order to relate studies on small laboratory scale samples to understanding crustal- and upper-mantle-scale bulk conductivities in the Earth, it is necessary to model the bulk electrical response of composite materials and networks.
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Page 59 Recommendations A modern, well-distributed system of observational facilities, including land and ocean floor observatories as well as satellite, aircraft, and ship-based facilities, is essential for the collection of geomagnetic data to address the problems outlined in this report. Collection of prehistorical data is also essential for understanding the long-term behavior of the geomagnetic field. The order of the following recommendations does not imply a priority ranking. A coordinated effort should be undertaken, perhaps on a phased basis, to complete the ground-based global magnetic observatory network at the density required for this geomagnetic initiative. The installation of approximately 20 island stations and 25 ocean-bottom stations would be a major step forward in developing this network in support of ongoing work to update the IGRF and for magnetospheric and solid-Earth induction studies. The community of solid-Earth and space scientists should work with the relevant government agencies to develop detailed implementation plans for the full network and to address funding and management issues. They should also address issues such as location of sites, data resolution (for example, 12 bit versus 16 bit; 0.05- versus 0.5-nanotesla instrument noise levels), acquisition rates, and accessibility to the data by the scientific community in quasi-real-time. Expanding global real-time networks such as INTERMAGNET should be considered as a means of implementing this global network. The needs of the scientific community for high-quality magnetic data can be met with the release of classified and propriety data and with a coordinated program to collect new data. Special efforts should be made to communicate to the DOD and to industry the usefulness and mutual benefits of making classified and proprietary data available to the scientific community. The geomagnetic community should take advantage of “ships of opportunity” and long-range aircraft for surveying selected continental areas and vast areas of the ocean basins, particularly
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Page 60 in the southern oceans, antarctica, and the Arctic. A high-altitude (20 km) survey over the United States should be undertaken for baseline control and for regional charting. Data from a high-altitude survey would provide a consistent data set free from intense local anomalies derived from upper crustal rocks, which is needed for leveling and “stitching together” individual low-altitude magnetic surveys to upgrade the magnetic anomaly map of North America. A program of satellite missions and measurements is recommended to provide data for both magnetospheric and solid-Earth studies. These should include a main field mission over at least two solar cycles to provide data on both the Earth's core and the fields induced in the Earth by magnetospheric and ionospheric currents; plasma and magnetic measurements at three equi-spaced longitudes in geosynchronous orbit to provide information on the present state of the magnetosphere; and a high-resolution mission at low altitude, such as the proposed mission of the Applications and Research Involving Space Technologies Observing the Earth's Field from Low Earth Orbiting Satellite—called ARISTOTELES 2 (see Appendix B in this report) for lithospheric studies. Reconstructions of the long-term variations in the Earth's magnetic field should be made using improved archaeomagnetic and paleomagnetic measurements in order to improve the understanding of some of the fundamental time constants of the geodynamo. 2 As of the writing of the final draft of this report in late 1993, ARISTOTELES appears to have been canceled, but GAMES (Gravity and Magnetic Earth Surveyor), a similar satellite, is being considered by NASA.