Page 155APPENDIX B OPERATIONAL WORKING GROUP REPORTS
B1. OPERATIONAL PLATFORMS
Appendix B1 was largely developed by the following workshop group: James Heirtzler (Group Leader), J. Arkani-Hamed, J. Behrendt, A. Chave, C. Cox, W. Dewhurst, B. Donovan, M. Engebretson, K. Gebhardt, I. Gough, A.W. Green, W. Hanna, R. Hansen, D. Herzog, T. Hildenbrand, P. Hood, V. Labson, L. Law, J. MacQueen, R. McPherron, P. Mulligan, B. Narod, L. Newitt, R. Pawlowski, J. Quinn, C. Raymond, F. Rich, J. Slavin, P. Tarits, P. Taylor, P. Wannamaker, R. Wold, L. Zanetti.
Stationary Platforms on the Surface and SeafloorIntroduction
The Earth's magnetic field varies on time scales of years to fractions of seconds; the sources of these variations are both internal and external to the Earth. These temporal variations are not uniform about the Earth's surface but have local and regional differences determined by their sources and by the physical structure of the Earth and the space surrounding it.
These temporal magnetic field variations may be used to study the following:
resistivity structure of the Earth's crust, mantle, and core-mantle boundary (CMB);
secular-variation sources at the CMB;
main field origins;
morphology of ionospheric and magnetospheric current systems and plasma distributions;
sources of pulsations (ultra-low frequency [ULF] waves);
atmospheric-ionospheric coupling effects on atmospheric dynamics and weather patterns through monitoring of solar quiet (Sq) variation;
ocean dynamics (barotropic flow, internal waves, and so on); and
magnetic activity indices and the nowcasting and forecasting of magnetospheric and ionospheric disturbance.
To use these temporal variations effectively in studies like those listed above requires a hierarchy of local, regional, and global magnetometer arrays (both vector and scalar). Depending on the purpose, array spacings may vary from kilometers to hundreds of kilometers. Because more than half of the Earth is covered by oceans, magnetometer arrays must also be placed on the ocean bottom.
Just as there is a wide range of array separations in the spatial domain, there is also a wide range of frequencies (or periods) in the time domain. The study of magnetospheric pulsations (ULF waves) is concerned with periods from about 1 second to a few thousand seconds. For magnetotelluric and induction studies, the range in periods is from seconds to hours, and sometimes to days. Secular-variation and CMB studies utilize periods of years, tens of years, and hundreds of years. In these long-term studies, the baseline drifts of the instruments become major sources of error; thus, independent, “absolute” instruments must be used to precisely determine instrument “baselines” to 1 or 2 nanoteslas.
Similarly, a wide range of amplitude resolution is required: from 1.0 picotesla (1.0 milligamma) in the case of ULF waves to 1.0 nanotesla (1.0 gamma) for secular-variation studies.
The specifications cited are well within the reach of current technology; instruments with these resolutions and accuracy can provide key data for studying the important problems listed above.
The technology is available today for implementation of a global network of geomagnetic observatories and magnetometer arrays to address critical problems in the solid Earth and space physics.
Page 157Issues and Approaches Current Status—Surface Platforms
Surface platforms are of mainly two types: magnetic observatories and variation stations. Magnetic observatories are dedicated to recording long-term (years) change in all three components of the magnetic field (as well as shorter-term changes on scales of hours and minutes). Magnetic observatories are primarily distinguished from variation stations by the use of independent absolute instruments at the observatories to accurately determine baselines in order to establish long-term trends (secular variation). Data from magnetic observatories will typically be 1-minute values or hourly mean values with a resolution of about 1 nanotesla (sometimes 0.1 nanotesla). Currently the data are sent to World Data Centers (WDCs); prior to this transmittal, the data are usually not available for 1 ½ to 5 years, and sometimes not until 10 years. There are about 180 observatories worldwide that send data in digital form to the WDCs. They are mostly in the Northern Hemisphere, with a dense network in Europe. There are very few in the Southern Hemisphere; with the exception of a few island observatories, there are none in the ocean areas.
Variation stations record only variations in the vector components of the Earth's magnetic field and do not attempt to determine baselines. Although some arrays of variation stations are operated on a semipermanent basis in Canada, Alaska, and northern Europe, most are operated on a “campaign” basis. Variation stations are used in studies of ULF and magnetospheric or ionospheric current systems and for induction and magnetotelluric studies of crustal and upper-mantle resistivity structure (such as in the EMSLAB project).
Repeat stations are temporary magnetic observatories with “absolute” instruments that are set up for a few days at repeat sites at intervals of 2 to 10 years (usually 5 years). Data from these repeat stations are used to augment data from permanent observations and satellite surveys in making magnetic charts and spherical harmonic models of the Earth's magnetic field.
Page 158Current Status—Seafloor Platforms
Vector magnetic variations have been measured on the ocean bottom with amplitude resolution up to 0.1 nanotesla and time resolution up to about 60 seconds. The data are used in magnetotelluric and induction studies of crustal and mantle resistivity. The platforms have been operated in “campaign” modes for periods of weeks and months (EMSLAB, Barotropic Electromagnetic and Pressure Experiment (BEMPEX) and upcoming studies across passive margins and rift zones). No permanent platforms have been established, and no absolute measurements have been made to establish secular variation of the vector components of the Earth's magnetic field, although there is a nascent Japanese program.Current Status—Instruments
More than half of the classical observatory magnetometers of a few decades ago have been replaced by digital, electronic magnetometers. The electronic magnetometers themselves are also being improved. Recent developments have resulted in the availability of high-resolution, very stable, self-biasing, ring core fluxgate vector magnetometers with typical characteristics:
noise: 0.02 nanotesla/hertz @ 0.1 hertz;
temperature drift: 0.1 nanotesla/°C;
baseline drift: 5 nanoteslas per year;
power: 1-2 watts; and
output: analog and digital.
These instruments can provide analog and digital data with resolution of better than 0.1 nanotesla and accuracy of 1 or 2 nanoteslas at 1.0-second intervals. Lower-power instruments have been developed for special applications.
Total field instruments such as the D.C. polarized proton magnetometer, the Overhauser proton magnetometer, and optically pumped
Page 159magnetometers can provide absolute scalar data at sample intervals from 0.1 second to 5 seconds with precision from 1.0 picotesla to 0.1 nanotesla and accuracy from 0.01 nanotesla to 1.0 nanotesla. Current Status—Data Collection and Distribution
The reporting of results from magnetic observatories to World Data Centers is a major problem. In a recent experiment to see how promptly annual means were sent to WDC-A, for example, it was learned that many observatories were 3 years late. Under the newly established INTERMAGNET program, geomagnetic data from participating geomagnetic observatories are transmitted in near real time via geostationary satellites or computer links to collection and distribution centers know as Geomagnetic Information Nodes (GINs). Four satellites are used (GOES-E, GOES-W, Geostationary Meteorological Satellite [GMS] and Meteorological Satellite [METEOSAT]) which cover most of the globe from 70°N to 70°S latitude. Data consist of 1 minute samples of each of the three components of the magnetic field from a fluxgate magnetometer and of the total field from a proton magnetometer (all with resolution of 0.1 nanotesla). In some cases, 1-hour and 3-hour geomagnetic disturbance indices, similar to the K index, are generated at each site and transmitted via satellite. The data, which include the best available baseline values, are transmitted at 12-minute or 1-hour intervals.
About 25 observatories in North America, Europe, Africa, the Pacific, and Antarctica are now participating in INTERMAGNET. GINs are now operating in Golden, Colorado, United States; Edinburgh, Scotland, United Kingdom; Paris, France; and Ottawa, Canada. Since no GIN can see all of the INTERMAGNET satellites, it is planned to make inter-GIN transfers each 24 hours via computer net so that each GIN will have a complete global geomagnetic data set that is not more than 24 hours old. Users will receive global data sets on-line via telephone-computer link, by E-Mail, or later by CD-ROMs that will be updated at yearly intervals.
Page 160Future Initiatives—Surface Platforms
The existing network of magnetic observatories should be expanded to provide better global coverage, particularly in the Southern Hemisphere. This should be considered because the technology exists today to establish relatively inexpensive, unmanned observatories reporting via satellite or telephone modem. However, the requirement still remains for periodic visits to take absolute measurements for maintaining good baseline control.
Capabilities of available instruments make it possible to have observatory data with good baseline control, resolution of 0.1 nanotesla, and 1.0-second sampling in near real time from a global network. Although the data rates of satellites now used by INTERMAGNET limit time resolution to 1.0 minute, 1.0-second data are available at INTERMAGNET observatories for collection by auxiliary systems or may be transmitted using higher data rate satellite systems.
The same low-power, high-resolution vector magnetometers used at observatories may be coupled with equally low-power data collection systems to produce sets of relatively inexpensive and highly portable array magnetometers. These array magnetometer systems may be used in regional arrays to discover and map anomalous electrical resistivity structures in the continental crust and upper mantle. By augmenting these array magnetometer systems with electric field sensors, MT soundings can later be made over regional structures mapped by the array magnetometer studies. These array systems have the same operating characteristics as those earlier proposed by the Consortium on Array Magnetometers (CAM).
As suggested in the CAM proposal, these same array systems can be used for studies of ULF waves, field-line resonances, and the morphology of magnetospheric source fields and current systems.
Consideration should be given to establishing a publicly funded set of 25 array instruments (such as proposed by CAM) to be maintained by a university member or by a public agency (such as USGS), and available for use by both the solid-Earth and space physics communities. Cooperative use of array instruments should result in the magnetospheric
Page 161community's receiving help from the induction community in understanding the significant effects of Earth structure on the characteristics of ULF waves (particularly on polarizations). Future Initiatives—Seafloor Platforms
To effectively study both the solid Earth and the space above it, magnetic observations must be made over the entire Earth, including the two-thirds covered by the oceans. The technology exists today to put low-power vector and scalar magnetometers with data acquisition systems at permanent sites on the seafloor. These sites could be autonomous and interrogated remotely by surface vessels. Or they could be coupled to surplus transoceanic cables to transmit their data continuously to land terminals. It is technically feasible to take independent absolute measurements on the seafloor using an Earth-seeking gyro for vertical and directional references and using fluxgate and proton magnetometers in a declination inclination magnetometer (DIM) system. The DIM system would be lowered from a surface ship or a submersible, or even left in place and periodically actuated remotely from the surface; for cable sites, the emplaced DIM could be actuated through the cable. The possibilities for these seafloor systems, as well as for obtaining absolute measurements on the seafloor, were seriously covered in papers and discussions at a session during the 1991 IUGG meeting in Vienna and should be further developed.
In a recent study, the Earth was divided into 128 equal area elements of 2,000 × 2,000 kilometers (km). Of these, 71 contained some land, leaving 57 “ocean” elements. Some 32 of the ocean elements contained islands, leaving 25 elements as potential candidates for ocean bottom magnetic observatories. Approximately one-half of the land elements now have observatories. Although 2,000-km spacing will satisfy main field modeling, it does not quite meet the needs of secular-variation source studies. The need for 700-to 1,400-km spacing could be satisfied by temporary ocean bottom instruments around a permanent “anchor” observatory.
Page 162Future Initiatives—Global Geomagnetic Indices
Planetary indices of geomagnetic disturbance such as the planetary K-index (Kp), disturbance storm time (Dst), and auroral electrojet magnetic activity index (AE) are currently available to users with delays of weeks (Kp) to years (AE). Although activity indices from a few selected sites are now available through INTERMAGNET and NOAA's Space Environmental Services Center (SESC) in Boulder, Colorado, in near real time, true planetary activity indices are not yet available with a promptness approaching “near real time.” Users whose activities are adversely affected by geomagnetic disturbances urgently need real-time notice of the disturbance state of the magnetosphere.Recommendations
Selected geomagnetic observatories using suitable low-noise magnetometers should begin to collect three-component data at 1-second intervals for use by the magnetospheric and induction communities.
Long-period, two-component horizontal electric field measurements (similar to those made at Tucson, Arizona, in the 1920s) should be made at a worldwide network of selected geomagnetic observatories. These data, in conjunction with long-period geomagnetic data, can be used to study deep-Earth resistivity structure.
Where islands are not available, geomagnetic observatories utilizing three-component magnetometers and total field absolute magnetometers should be established on the ocean bottom. Methods should be developed for taking absolute measurements on the ocean floor. This effort should be coordinated with the IRIS Ocean Seismic Network; in general, efforts should be made to collocate geomagnetic observatories with planned seismic observatories. A task force should be formed to design and
Page 163estimate the cost of the ocean bottom observatory and absolute measurement system.
Geomagnetic observatory coverage of ocean areas should be planned to take advantage of ocean islands, where possible, since costs of installation and periodic absolute measurements would be much less than on the ocean bottom. Collocation with existing or planned seismic or geodetic observatories should be sought where feasible.
The INTERMAGNET observatory network should be expanded in the auroral zones (Eastern Canada and Siberia) and in lower midlatitude regions to provide a sufficient array of stations for calculation of real-time AE and Dst indices.
The planned deployment of six U.S.- and four U.K.-unmanned Automatic Geophysical Observatories (AGOs) in the Antarctic should be encouraged. Recordings with modern low-noise instruments should be maintained at manned sites for both magnetospheric research and induction studies.
A publicly funded set of approximately 25 three-component array magnetometers and data acquisition systems (with provision for electric field registration) should be obtained. These systems would be similar to those previously proposed by the CAM. The systems would be time-shared by the induction and magnetospheric communities and would be maintained by one of the universities involved or by USGS.
An adequate supply of state-of-the-art wideband MT systems should be developed and maintained for academic use.
Selected geomagnetic observatories with low-noise magnetometers should compute dB/dt at 1-second intervals for electric power companies on a campaign basis. Should these data prove useful, consideration should be given to computing a dB/dt index on a permanent basis and transmitting it in real time to electrical power companies to warn of potential damage to electrical power systems from geomagnetic storms.
A chain of geomagnetic observatories reporting in near real time through INTERMAGNET should be established in the equatorial
Page 164zone to provide corrections for the U.S. Navy POGS satellite and possible future magnetic mapping satellites.
INTERMAGNET and Solar-Terrestrial Energy Program (STEP) should cooperate closely in establishing complementary global geomagnetic data bases.
Transoceanic cables that are no longer required should be made available for passive monitoring of electric field potentials.
Airborne/Shipborne FacilitiesEnhanced Capabilities
Recent technological advances in airborne/shipborne platforms greatly enhance understanding of the nature and evolution of the lithosphere. For example, the development of optically pumped magnetometers offer an order-of-magnitude improvement in sensitivity over the commonly utilized proton precession magnetometer developed in the middle 1950s. Electronic navigation to meet national map accuracy standards complements these improved magnetometers. Aircraft and ships appear to be available to address the scientific needs outlined in this appendix, except for a long-range aircraft to traverse remote oceanic areas. With the availability of platforms and with the many advances in airborne/shipborne acquisition systems, accurate magnetic data can be routinely collected for a wide range of applications, from the detection of hydrocarbon seepage at a scale of 1:5,000 to fundamental structures related to the formation and evolution of continents at a scale of 1:5,000,000.
Page 165Data Acquisition Ships of Opportunity
The problem of major data gaps or poor data resolution should be addressed by capitalizing on ships of opportunity operated by NOAA, University-National Oceanographic Laboratory System (UNOLS), USGS, NSF, or private industry. The (federal) NOAA fleet is encouraged to routinely acquire gravity and magnetic data. The utilization of willing vessels of opportunity within the merchant and geophysical industries fleet would also help fill gaps. While systematic surveys provide more specific information for analysis, single tracklines available from various vessels over time can provide valuable information. In addition, geographic areas of mutual interest among private industry, government, and academia should be identified and agreements negotiated (1) to share mobilization and demobilization costs; (2) to share data acquisition and processing costs; and (3) to foster a greater synergism through the open exchange of nonproprietary Earth sciences information. Achievement of these objectives will require the identification of key resource people in the respective organizations.National Airborne Survey
The existing digital magnetic data set of the United States and its continental shelves cannot adequately address many scientific and societal problems. Effects due to datum shifts and nonuniform observation levels result in errors that severely limit the usefulness of the present national data set. Detailed analysis suggests that these data are adequate for a narrow range of anomaly wavelengths (40 to 500 km). Because exploitable mineral and ground water resources occur at depths from 1 meter to a few thousand meters, their delineation requires magnetic anomaly information at wavelengths considerably less than 40 km. Anomaly wavelengths greater than 500 km are important to any regional geological study and to verify and complement satellite field interpretations. Clearly, it is
Page 166time for a national commitment to a new magnetic data set of higher quality at all wavelengths.
The compilation of such a magnetic data set requires both the reprocessing of old data and the collection of new data. In limited areas (about one-third of the United States) data were collected with survey specifications appropriate for the geological setting and with calibrations established with long-distance flight-line data. However, two-thirds of the United States contains data collected at an inappropriate survey spacing or altitude.
Canada, Finland, Russia, Zimbabwe, Liberia, and Australia are among the countries that have established cost-effective national airborne geophysics programs. For example, Canada has collected more than 9 million line kilometers of magnetic data at a spacing of 0.8 km, has published more than 9,500 aeromagnetic anomaly maps, and has distributed 30,000 aeromagnetic anomaly maps per year (the most requested item of the Geological Survey of Canada). The Canadian aeromagnetic program has been a very successful endeavor that has led directly or indirectly to the discovery of many ore deposits. The overall cost of the aeromagnetic program has been recovered, many times over, by the general economic benefits to the country that result from such discoveries and from taxes that are subsequently paid to the provincial and federal governments.
In Finland, magnetic and electromagnetic data have been collected at a flight-line spacing of 0.4 km for the entire country. Because the resulting data set proved to be a valuable national resource, the airborne geophysics program was expanded to refly the country at a flight-line spacing of 0.2 km. A highly successful high-resolution aeromagnetic survey program of the State of Minnesota was recently completed using a line spacing of 0.4 km over most of the state. In comparison, the average flight-line spacing in the United States is about 4 km, which is inadequate for most geophysical analyses.
The U.S. national data base clearly needs to be upgraded to modern international standards. The operational platforms and technology exist to carry out this task. Steps should be taken:
to provide a plan and budget to merge existing data using advanced techniques in the compilation of the second-generation magnetic anomaly map of the United States;
to assess the quality of existing data over the United States extending to a few hundred kilometers offshore; and
to provide a cost-benefit study for the replacement of all substandard existing data with new data.
Using a suitable aircraft (for example, a B-57), it should be possible to cover the continental United States at an altitude of about 20 km and comparable line spacing in a cost-effective way. Data thus acquired would bridge the gap between low-level and satellite data, define regional magnetic fields, and provide coverage of the continental shelf extending out to a few hundred kilometers offshore. Vector data should also be available for main field modeling. If successful, the project could be expanded to other parts of the world. The survey could be expected to have important interfaces with various geomagnetic disciplines.
An appropriate task group should define detailed requirements and specifications for this program. This task group should define survey specifications, select an appropriate platform, and decide what type of instrumentation should be used. In performing its task, the group should consider costs involved against scientific gains to both the lithospheric and main field scientific communities.High-latitude Airborne Surveys
High-latitude aeromagnetic surveys, associated with other airborne surveys, are a central element of the geomagnetic initiative. The Arctic, southern oceans, and Antarctica have special needs. Remote offshore areas such as the Bering Straits and Chukchi Sea should be surveyed by long-range aircraft.
Long-range aircraft, such as the U.S. P3 or Russian Ilyushin 18, are needed to fly low-level surveys in Antarctica over adjacent oceanic crust
Page 168and the ocean continent boundary. Line spacing will generally be wider than ordinarily acceptable for continental surveys. Short-range, ski-equipped aircraft are necessary to fly closely spaced lines and are needed over ice-covered continental areas of Antarctica. Planes currently being used are Twin Otters (in the United States and United Kingdom) or Dornier 228s (Germany) with fuel delivered to the field camps by LC130s (a ski-equipped C130). Aeromagnetic surveys should be conducted in combination with radar ice sounding (2-to-3-km penetration) and airborne gravity as is presently being done by the CASERTZ (Corridor Aerogeophysics South East Ross Transect Zone) program. Rock Properties
Rock magnetic studies are commonly conducted independently of the required mineralogical, geochemical, and petrological background for linking magnetic property data to interpretation of magnetic anomalies. No single investigator has—or is likely to acquire—the broad expertise and wide range of analytical and experimental facilities that are necessary to produce all the measurements necessary to construct detailed models of lithospheric magnetization. For the study of the geology and geophysics of rock magnetism relevant to lithospheric magnetic anomalies, existing laboratories need realistic support, which includes technical personnel.
New facilities are also required, and, in particular, instruments are needed to measure magnetic properties at elevated pressures and temperatures. Ready access is needed to instruments capable of imaging and analyzing magnetic minerals and associated silicates at micron and submicron scales, including conventional optical and electron instruments as well as such new developments as atomic force microscopes. Furthermore, it is essential that critical magnetic property data measured on given samples be complemented by analyses of other physical and chemical properties of the same samples. These studies may include, but are not limited to, mineralogy, mineral chemistry, geothermobarometry, geochemistry of major and trace elements, radiochronology, elastic parameters, thermal and electrical conductivities, and densities. These
Page 169supporting data would enable the magnetic properties to be placed in a lithospheric context and meaningfully related to magnetic anomalies. Minimal Survey Requirements Regional Low-Level Airborne Surveys
Surveys should be flown with high-sensitivity magnetometers and state-of-the-art survey aircraft compensation. Compensation figure of merit of the survey aircraft should be less that 3 nanoteslas. The diurnal variation should be removed using a combination of control lines and base station magnetometers. Over areas where magnetic rocks are close to the surface, a basic flight-line spacing of 1 km or less should be utilized with the control/traverse line ratio being about 1:10. The surveys should be carried out by qualified parties specializing in the aeromagnetic survey technique who would adhere to a set of survey specifications. The survey should be monitored at both the field survey and compilation stages. Draped, gradient, and vector surveys may require more stringent standards. It may be appropriate to include other geophysical sensors (such as very-low-frequency electromagnetic sensors) that would provide additional geophysical information at little additional cost.Ship Surveys
The optimum marine survey specification should be to acquire data along regularly spaced lines. However, it is recognized that survey standards will often be governed by the primary mission of the vessel and sponsoring organization. In the case of NOAA vessels, line spacing and sample density are usually governed by the requirements to acquire high-quality multibeam sonar soundings. Ideally the direction of survey lines should be based on the orientation of geological structure and, most importantly at low latitudes, by the orientation of the magnetic field. Gradiometer data should be routinely collected to avoid problems with diurnal corrections and external fields at high and low latitudes. For
Page 170consistent compilation, it is necessary to reduce the gradiometer data using standardized methods. Because there is a wide range of international groups acquiring data, this could become a major difficulty.
The routine collection of marine magnetic data has not evolved to provide information compatible with the resolution of swath bathymetry systems. As such, information on oceanic crustal structure and processes is lost by the inability to exploit the detailed information contained in the bathymetry in conjunction with analysis of the magnetic anomaly data. Although deep-tow magnetometer technology exists, such systems are expensive and must be towed slowly. A desirable compromise is the development of a mid-depth-tow magnetometer that can be towed at 7 to 10 knots yet provide higher-resolution measurements. Deep-tow measurements are still very useful for studying the ridge axis.Positioning
For all surveys, positioning systems such as the GPS or the USSR Global Navigation Satellite System (GLONASS), should provide point position (three dimensional for land surveys) to the national map accuracy standard. All map/chart products, whether over land or ocean, should be in compliance with the National Map Accuracy Standards and should be referenced to a recognized geodetic system.Technology Development Airborne System Improvements
To achieve accuracies of better than 1 nanotesla in high-resolution aeromagnetic surveys, several issues should be addressed. Aircraft magnetic compensation methods need improvement to fully utilize new high-sensitivity magnetometers. Considerable care is also required in the removal of temporal variations, including secondary fields due to currents induced in the Earth (for example, along coastlines). A viable approach to remove temporal variations may be the deployment of several vector
Page 171magnetometer base stations to define the transfer tensor. A study is required to determine the optimum spacing and number of such magnetometers. Precise vertical position control using some combination of barometric, radar, and/or laser altimeters together with satellite navigation is vital. Magnetometers
The main direction to be encouraged is the development of vector, gradiometer, and vector gradiometer systems, because more interpretational techniques can be applied to the resultant data. For instance, vector or gradiometer systems make possible the immediate recognition of three-dimensional anomalies from a single profile. Algorithms have already been developed to exploit these additional data to interpret the presence of three-dimensional structure from profile data. Further, gradient data can be used to improve interpolation from flight lines onto a regular grid and to relax survey specifications and reduce costs.
Combinations of vector and gradient data have not yet been explored from either an instrumentation or a computational point of view. Given that both have individually shown substantial advantages, the use of the combination should be explored as a long-range development.
Satellite PlatformsScientific Framework
The need for spacecraft measurements has been defined in the previous sections. In an effort to support these data requirements that pertain to diverse scientific interests and applications (civilian, industrial, and military), the series of present, impending, and planned missions and missions of opportunity has been considered. The specific scientific disciplines relate to the following:
the magnetosphere, ionosphere, and atmosphere;
lithospheric magnetic fields;
electromagnetic studies of the solid Earth and oceans; and
main field and core processes.
In addition, there has surfaced a widespread concern for the quality of collected data of any form that are directly affected by the instantaneous condition of the magnetosphere. Pragmatic concerns with regard to communications systems, power grid reliability, navigational ability, and satellite and general space survivability necessitate the continuous measurements of magnetic fields not only in the proximity of the Earth, but also—and perhaps even more importantly— throughout the magnetosphere and in the solar wind. The central theme is the monitoring—and ultimately prediction—of the magnetospheric condition through the modeling and understanding of dynamic processes.
Figure B1-1a is a DE-1 auroral image registering a major magnetic storm on 13 March 1989 (Allen, J., H. Sauer, L. Frank, and P. Reiff, Effects of the March 1989 solar activity, EOS, Trans. AGU, 70:1479, 1989). Superimposed are magnetic field disturbance data from the field-aligned current system that accompanies such events. These storms are not atypical, especially during the downside of solar maximum. The auroral oval during nonstorm times is commonly around 70° to 75° magnetic latitude (MLAT). Typical magnetic storms can expand the oval to 50° to 60° MLAT; this has been observed consistently in the Upper Atmosphere Research Satellite (UARS) magnetic field data. One example on October 28, 1991, showed field-aligned current disturbances at 45° MLAT (35° geographic latitude!).
It is not just the location of these effects that is significant but the size of the polar cap (the area of open magnetic field lines within the oval), which reflects the increased magnitude of energy stored in the magnetosphere, most of which is dissipated in the ionosphere. This very stressed configuration of the magnetosphere is contained by the entire current system, and the ionospheric auroral oval region can be considered as the focus of that system. The storm processes release this energy and depend not only on solar wind factors (pressure and IMF originating from flare regions as well as other solar and interplanetary processes) but also
Page 173on the inertia, inductance, stored energy, and general internal magnetospheric processes and structure.
Figure B1-1b is an artist's conception of the field-aligned current (FAC) system: The DE-1/HILAT image establishes the correlation between the auroral luminosity and location and the FAC intensity and location. This artist's FAC figure goes further: it sketches the three-dimensional configuration of the FAC system and also shows the correlation of the FACs and the horizontal ionospheric electrojet Hall current. The statistical pattern of low-altitude field-aligned currents is the “focus” of the entire magnetospheric system and its dynamics.
The accompanying ionospheric currents (electrojet Hall currents) increase comparably in amplitude and move in concert with the field-aligned currents and the aurora. The two large-scale Hall electroject currents flow generally in the longitudinal direction over 6 to 10 hours of local time and their dynamic motion, for example in latitude, has the largest inductive effect on the Earth's surface (including conducting artifacts such as transmission lines and pipelines). This Hall current is generally perpendicular to the ionospheric electric field and, and except for motion, time variations, and the end points, is not dissipative. The horizontal ionospheric Pedersen currents that connect the FACs are parallel to the local electric field and are the primary dissipation currents that heat and create the ionosphere in darkness; this energy input is generally larger (up to a factor of 10 for intense storms) than the particle precipitation energy input. The tens of millions of amperes of horizontal ionospheric currents from this March 13, 1989, event overloaded the entire power grid of Quebec and a partial section of Scandinavia's grid, disrupting power for more than 9 hours; similar events disrupt general communication, particularly via satellite.Specific Scientific and Societal Issues
The following section presents a discussion of the role of satellite platforms in addressing specific scientific issues itemized in the working group reports in this appendix.
Page 176Scientific Issue A1: Magnetosphere, Ionosphere, and Atmosphere
The solar wind couples to the Earth's magnetic field through the processes of magnetic reconnection and viscous interaction. Energy mass and momentum extracted from the solar wind through these processes generates a global convection system that transports magnetic field and particles from the dayside to create a large cometlike tail on the nightside. This convection system, however, is unstable. After merging on the front side, newly opened field lines are transported by the solar wind to the nightside, and temporarily stored in the tail. About an hour later they reconnect forming closed field lines. This process allows the tail-like field lines to collapse earthward accelerating particles into the inner magnetosphere where either they precipitate into the atmosphere causing aurora or they are injected into the Van Allen radiation belts enhancing the ring current. This sequence of events—corresponding to a complete circuit through this global convection system—is called a magnetospheric substorm. A magnetic storm is produced when a number of substorms occur in rapid succession, building up an intense ring current.
Separation of charges in the moving magnetospheric plasma creates electric fields that are projected onto the ionosphere along highly conducting field lines. These fields drive electrical currents in the ionosphere which—in regions of electric field in conductivity gradients—couple to the magnetosphere through field-aligned currents. Almost all magnetic activity is caused by variations in the intensity of these magnetospheric field-aligned and ionospheric currents.
To explain how these electrical currents affect the Earth's outer magnetic field a quantitative description of the processes that produced them must be obtained. This is accomplished by simultaneously measuring the magnetic fields and particles in the various regions. Upstream monitors in the solar wind provide a description of the solar wind input to the magnetosphere. Magnetometers at geosynchronous orbit detect the changes in locations of the boundary of the magnetosphere, the ring current, and the tail current. Magnetometers on polar-orbiting spacecraft obtain snapshots of the location and strength of the field-aligned currents connecting the magnetosphere to the ionosphere.
Page 177Networks of ground stations make it possible to provide synoptic maps of the ionosphere currents that connect to the field-aligned currents.
Models of solar wind coupling, substorms, and storms are based on data from this broad collection of spacecraft and ground observatories. These models can be used in magnetospheric research to help operational platforms mitigate the adverse effects of space weather or to subtract magnetospheric “noise” from data used in studies of the solid Earth.Scientific Issue A2: Lithospheric Fields
The first crustal magnetic anomaly maps from Magsat revealed previously unknown, long-wavelength anomalies (400 to 4,000 km). It is not known whether the source region of these anomalies extends below the lower crust to the mantle. The lack of Magsat anomaly resolution has hindered the interpretation of these features. Data from ARISTOTELES improves the crustal anomaly field resolution because the satellite will, at times, be in a low (250 km) circular orbit. In addition, gravity data will be recorded simultaneously. The gravity data will aid in the magnetic interpretation by constraining the density of the source rock. One of the working group recommendations is to promote new satellite magnetic missions with orbits as low as possible in order to define source regions in the lithosphere more precisely.
Derivation of a reliable magnetic anomaly map is the most important tasks. Nonlithospheric “noise” in the map—if unrecognized—may be attributed to magnetization of the lithosphere and may lead to incorrect conclusions regarding the magnetic properties of the lithosphere. Therefore, it is essential to evaluate the noise level at each data-processing stage. A regional magnetic anomaly is usually derived through the following processes:
Measurements. Aside from possible instrument drift and errors, the uncertainty of the location of measurement, especially in the sea, could introduce significant errors to the data.
External field component. The magnetic data are usually collected during the external field quiet periods, but this is not
Page 178always possible, especially in the polar regions where the quasi-steady external field is significantly disturbed even during very quiet periods.
IGRF removal. The IGRF models used to remove the core field component may not be accurate on a regional scale, though more recent models derived from Magsat data are probably reliable.
Gridding. This is a nontrivial task, since the sample density along tracks is usually much higher than the track separation.
Patching neighboring surveys. The regional magnetic anomaly maps are derived by patching many survey data.
Continental-scale magnetic anomaly maps have been derived by stitching together smaller-scale surveys recorded over a wide time interval. While important, these composite maps have suffered from an insufficiently determined zero field reference level. ARISTOTELES data, being measured at a constant altitude, would provide a refined core-field reference surface to which those composite data could be related.Scientific Issue A3: Electromagnetic Induction
The electrical conductivity of the crust and upper mantle is probed by the response of these layers to low-frequency electromagnetic waves, magnetic impulses, and electrical currents in the magnetosphere and ionosphere. The magnetic signatures observed with arrays of magnetometers on the Earth's surface are a summation of signals generated in the ionosphere and magnetosphere and of the induced signals generated in the conducting layers below the Earth's surface.
In order to separate the source signal from the induced signal unambiguously, the source signal must be specified. By increasing the accuracy of the source signal specification, the accuracy of the induced signal will be increased. While models of the ionospheric currents such as the solar quiet and auroral electrojet currents may be useful, the best approach is to specify the source signal based on data from satellites that are in or near the source currents. This requires continuous monitoring of the magnetic field. For complete monitoring of the sources, there must
Page 179be a geosynchronous satellite on the dayside and another one on the nightside. In addition, a low-Earth orbit satellite at high latitudes and another at low latitudes are required. These satellites need to be in operation continuously—both to measure the very low frequency waves and magnetic currents in the source region and to be available at all times that electrical induction measurements might be made. Scientific Issue A4: Main Field and Core Processes
The Earth's geomagnetic main field, which is produced by dynamo action in the Earth's core, varies slowly (but erratically) with time in such a manner that is predictable for only very short time intervals (on the order of 5 years or less). Based on fundamental physics, dynamo theories to date are inadequate to make any meaningful predictions. Yet nearly all geomagnetic disciplines depend on the ability to accurately isolate the main field from magnetic field observations in order to properly study the spatial and temporal residual or anomalous magnetic field as well as the core itself. Therefore, phenomenological models of the Earth's main magnetic field and its secular variation must be relied upon for this purpose. As input, this requires data from satellites that continuously monitor the Earth's magnetic field on a global scale, in the altitude range of 300 km to 1,000 km. ARISTOTELES in the near term and DMSP/POGS and the NOAA polar satellites upgraded to Magsat quality in the long term will provide the necessary observational platforms for main field and secular field analyses. Other lower-altitude satellites between 150-km to 300-km altitude are needed to resolve the intermediate-wavelength crustal anomalies.Magnetic Field Satellite Requirements
To address the scientific and societal issues described above, five types of measurements are required.
Page 180Continuous Measurement of the Geomagnetic Field (Magsat-Quality Instrumentation)
The geomagnetic field should be measured continuously with sufficient sensitivity, attitude knowledge of the vector components, and knowledge of the noise to record the field and its secular change over two solar cycles (22 years). This can be accomplished by adding instruments to, or modifying instruments on, existing satellite series such as DMSP, POGS, or the NOAA polar weather satellites. Continuous time series would not only record secular variations but, in addition, the elusive short-term (1-year) main field changes or “jerks.”Low-Altitude Surveys
There have been numerous attempts to study the lithospheric magnetic anomaly field with Magsat data (about 500 km altitude). The wavelength of these anomalies (up to a few thousand kilometers) and their amplitude (from a few nanoteslas to a few tens of nanoteslas) make them difficult to identify with precision in Magsat data. Their amplitude will be increased by several times (depending upon their wavelength) if measurements are taken at an altitude of only 200 km. These anomalies are largely static in nature, so a global survey with a single satellite (with fine spatial resolution), rather than a survey with multiple spacecraft, is required. Multiple spacecraft, however, would assist in a better understanding of the external field transients that would benefit magnetospheric workers, and, indirectly, solid-Earth workers.Sun-Earth Lagrangian Point Measurements
The magnetospheric research community, including civilian and military agencies, has a need for real-time data specifying the state of incoming solar wind and the state of the interplanetary magnetic field. This information is used in short-term forecasts of the ionosphere and magnetosphere. This need is satisfied by a magnetic field spacecraft at the Sun-Earth first Lagrangian point. The Russian program had included the
Page 181Regatta spacecraft, stabilized by solar sails, to fulfill this need, but construction was stopped due to financial difficulties. Geosynchronous Measurements
The geosynchronous orbit is well populated with communication, weather, and surveillance spacecraft which occupy fixed locations in the geomagnetic field. If properly equipped with magnetometers, these spacecraft would provide unique opportunities to monitor the effects of the major current systems that create the outer portions of the Earth's magnetic field and sources of hydromagnetic waves propagating through the magnetosphere. It is necessary that there be at least three spacecraft spaced equally in local time. There are presently two NOAA/GOES spacecraft that provide two of the three vehicles needed. Appropriate processing of these data provides important proxies for solar wind conditions when not directly observed by an upstream monitor.
For maximum effectiveness, the Lagrangian and geosynchronous monitors require real-time data archiving and distribution.“Imaging” the Three-dimensional Ionospheric Current Systems
Spacecraft in low-Earth, polar orbits will pass though the major field-aligned current systems between the magnetosphere and ionosphere within about 15 minutes per hemisphere. These ionospheric-magnetospheric current systems are of keen interest to investigators studying the geospace environment but are “noise” to those trying to measure the Earth's main field and the contribution from static magnetization in the lithosphere. These current systems change rapidly in three dimensions and are found, with different characteristics, at all latitudes. Three or more spacecraft equipped with magnetometers, located at different local time orbits, would show the first-order features of these currents as the solar wind fluctuates. This in turn would help in understanding these complex processes and would provide valuable input to models specifying the state of the magnetosphere and ionosphere (see Figure B1-1). NOAA and DOD have a series of environmental monitor
Page 182ing spacecraft in low-Earth polar orbits. If appropriate magnetometers were placed on these platforms, they would cost-effectively satisfy this operational need. Present, Planned, and Suggested Magnetic Field Satellite Missions
The operating and planned satellites relevant to this report are reviewed below. Readers should be aware that the status of operating and planned satellites may have changed appreciably between the time of the workshop and publication of this report.Presently Operating
POGS Series. Polar Orbiting Geomagnetic Satellite, launched by the U.S. Navy in April 1990, (polar orbit 89.98° inclination, 730 km altitude) is providing scalar data that will be distributed to the scientific community through the NOAA/National Geophysical Data Center (NGDC) World Data Center. The field magnitude is accurate to about 10 nanoteslas. Its lifetime is expected to be 3 years; follow-on missions are planned. The purpose of the mission is to provide scalar data for the 1995 epoch DOD world magnetic model of the main field and its secular variation.
UARS. Upper Atmosphere Research Satellite launched by NASA in September 1991 (57° inclination, 600 km altitude). The magnetometer is part of the Particle Environment Monitor that monitors energy input, particles, and current. It was designed with 2 nanotesla resolution and tens of nanoteslas absolute accuracy per axis. UARS is a very stable platform facilitating baseline removal and allowing the remote measurement of ionospheric currents.
NOAA/GOES Series. These spacecraft have operated continuously from 1974 to present; all carry magnetometers. IMP-J, launched in 1972, still provides solar wind and outer magnetosphere measurements and monitors, in elliptical orbits at 35 Re (Earth radii).
Page 183Planned Satellites
The following satellite missions are in various planning and funding stages, with various possibilities of being realized.
ISTP. At the time this report was written, the NASA International Solar Terrestrial Program spacecraft were scheduled as followed: Geotail, launch July 1992, equatorial, 100 Re; Polar, launch scheduled for 1993, polar, 4 Re; Wind, launch also scheduled for 1993, first Lagrangian point (L-1 point).
DMSP/POGS. NASA, the Air Force Geophysical Laboratory, and the U.S. Naval Oceanographic Office have been studying the possibility of placing high-resolution magnetometers on the U.S. Air Force's Defense Meteorological Satellite Program satellites. Two of these satellites are always kept in orbit for meteorological observations and, if precision magnetometers for intercalibration were part of the routine instrumentation package, long-term magnetic field measurements would be assured. A magnetometer now carried within the body of the satellite for orientation purposes shows as much as 8,000 nanoteslas of spacecraft-generated noise. Future satellites of this series are identified by “Blocks” with the following characteristics:
Block 5: Six satellites in the 1994-2005 time period, vector magnetometer with some attitude information, 5-m boom.
Block 6: Satellites in the 2005-2015 time period, full vector plus scalar capability, 8-m boom, attitude transfer to 1 arc minute, with upgrade to 10 arc seconds possible.
FREJA. A Swedish auroral scientific satellite with fields, plasma, waves, and imaging data. The platform spins at 10 rpm, it has an orbit with 600 × 1,700-km altitude, a 63° inclination, a 2-year lifetime, and a planned October 1992 launch. The magnetic field experiment is boom-mounted and computer-based with 2 nanotesla resolution, attitude to a fraction of a degree, and high sampling rate with band-passed channels for wave measurements. The microprocessor, among other duties, will process the data with fast Fourier transform algorithms, detect storm
Page 184events, and store full orbit and burst data in local memory. The goal is tens of nanotesla absolute accuracy in the vector measurements.
Tethers. NASA and the Italian Space Agency plan to start deploying magnetometers on long tethers from the Space Shuttle and from Delta rockets. Most of these instruments will gather data for a few hours or days but, for one Delta experiment in 1994, a tethered magnetometer will be deployed 46 km upward from an altitude of 200 km, then the tether will be cut and the instrument package will assume an orbit at about 500 km altitude. There are expected to be four tethered deployments in the next 2 years.
ARISTOTELES. This NASA/European Space Agency (ESA) spacecraft will carry a French gravity gradiometer as well as scalar and vector magnetometers. It will be launched into an orbit initially at 780 km altitude. After 2 months it will descend to near 200 km and stay there for 6 months. It will then rise to an altitude of 700 km, where it is planned to stay for 3 years. The earliest launch date is 1997, but a 1998 launch date is more likely. The scientific community has expressed the need for a pre-1999 launch to avoid the next solar maximum during 2000 to 2003.
MFE/Magnolia. This was to have been a joint NASA/CNES mission using two sets of fluxgates and total field instruments, launched from a French Ariane rocket. Recently NASA decided to give ARISTOTELES a priority ahead of MFE/Magnolia.
NOAA/TIROS. NOAA operates polar TIROS weather satellites that might provide high-altitude (800 km) platforms for magnetic field measurements.
Oersted. Oersted is a satellite mapping mission proposed by representatives of various agencies in Denmark. It would have a triaxial vector magnetometer and an Overhauser scalar magnetometer for absolute total field measurements. It would also have a high-energy particle detector array and star imagers for accurate attitude determination. It would operate at 600 to 800 km altitude. Several planning meetings have been held with international (including NASA) representation. Exact launch date and orbital configuration are uncertain.
Page 185Other Suggested Missions
Multiple Magsat-Quality Mission. All fields of investigation would benefit from having several Magsat-quality magnetometers in orbit at one time. One approach to this would be to have a Magsat-quality magnetometer and its follow-on overlap by several months.
APAFO. The Advanced Particles and Fields Observatory was selected by NASA and ESA as a U.S./Earth Observing System (EOS) investigation to be flown on the second European Polar Platform. It was a boom-mounted package with vector and scalar magnetometers, collocated star trackers, and particle instruments. At this time APAFO is manifested by ESA but not funded by NASA.
PEGASUS. The Pegasus series of satellites consists of those that can be launched from a high flying B-52 aircraft. Such a procedure may allow low-cost launches of small geomagnetic field satellites in the future. Such a possibility is being considered by several groups in the United States.Recommendations for Near-Term Missions
1. ARISTOTELES. The Magsat mission ended more than 10 years ago. There has been no subsequent solid-Earth (core, lithosphere, crustal) magnetic satellite mission. Therefore, the ESA/NASA ARISTOTELES mission is appropriate because, at various time intervals, this will be a low-altitude (200 km) circular-orbit satellite. Furthermore, the simultaneous acquisition of gravity data will allow synergistic potential field interpretations. The high-altitude second phase of the mission, in which the orbit will progress through all local times every 8 months, will provide valuable data on the main field, secular variation, and external fields. The projected lifetime of the high-altitude phase is 3 years.
2. ISTP, Geosynchronous, L-1 monitors. The NASA International Solar Terrestrial Program spacecraft are instrumented with
Page 186magnetometers and are scheduled for launch as follows: Geotail, July 1992; Polar, Wind, 1993. These will be located up to 250 of Re equatorial, 4 Re polar and L-1, respectively. The geosynchronous satellites must continue to be equipped with magnetometers, and the L-1 monitor efforts should go forward.
3. U.S. Environmental Platforms; Encourage Interagency Cooperation. The DMSP and NOAA Polar orbiting satellite systems provide ideal platforms for continuous measurements of the low-Earth magnetic fields. Interagency cooperation would ensure the optimal applications of these operational environmental satellites to provide routine monitoring of main field and secular-variation field, in addition to ionospheric fields, during both solar quiet and disturbed times. These satellite programs should be upgraded to yield magnetic data of Magsat-quality, or better.
4. Support International Missions. Encourage the continued U.S. support and participation in international missions that provide valuable opportunities to significantly enhance the amount of data available to support this initiative. Among the proposed experiments in the international community, Oersted, APAFO, and the European Polar Platform offer particularly exciting missions. Efforts should be undertaken to identify flight opportunities on operational platforms throughout the international community.Recommendations for Long-Term Missions
1. Multiple Magsat-Quality Missions to Accomplish Goals. The most desirable and beneficial to all fields of investigators interested in the geomagnetic field and its disturbances would be multiple spacecraft on the order of Magsat. Different altitudes and multiple, simultaneous platforms would help both modeling efforts as well as the longevity of the overall mission.
2. Significant Upgrade to DMSP/POGS and NOAA Polar. On each satellite, Magsat-quality or better is desired for DMSP Block 6 and 7, and NOAA's next-generation polar satellites. It is
Page 187necessary to have both vector and scalar instruments on these satellites to intercalibrate. Attitude accuracy is necessary.
3. Magnetospheric Magnetic Field Missions. The mapping of geomagnetic field lines from the Earth's surface to the outer regions of the magnetosphere is not adequate to accurately transfer magnetic disturbance vectors from the magnetosphere to the ionosphere. To solve this problem, a survey of the magnetic field between low-Earth orbit and magnetospheric altitudes is needed. These missions also provide the activity state of the magnetosphere as well as a measure of external fields.Conclusions
The tradition of magnetic field measurements from the original scientific recordings in the 1600s by Sir William Gilbert has continuously improved and progressed. The progression from solely ground observatory data to combinations with satellite observations culminated in the NASA Magsat mission of 1979-1980, resulting in the extremely accurate IGRF-80 magnetic field model. With the termination of the Magsat mission, this progression is in an apparent hiatus. As a result, the following goals and recommendations are set forth, giving the measurement of core and crustal fields first priority. This is evidenced in the goals as well as in the support of the ARISTOTELES mission.
The list of missions given above has been prioritized in two separate ways that will successfully meet the scientific goals set forth by the proposed initiative. The first proposed mission has been limited to urgent needs, presently available missions, and missions of opportunity. The latter two reflect common interests in magnetic field measurements bridging the various scientific and pragmatic disciplines and concerns. Efforts must be made to avoid possible duplication among agency programs. A synergistic interagency effort is a clear approach, in particular with regard to the DOD and NOAA operational polar platforms.
Finally, the long-term recommendations represent the continuance of high-quality magnetic field measurements satisfying the needs of all of the disciplines involved and the maintenance of the tradition of these important data. All considerations for satellite monitoring of the main field thus far have centered on continuous monitoring with Magsat-quality data. That means vector data with 10 to 20 arc second, or better than 5-nanotesla accuracy, together with absolute scalar data with 1- to 2-nanotesla accuracy. The scalar data are used to calibrate the vector data in-flight. Such are the ideal measurements. In practice, however, there are two ways to compromise these measurements and still acquire very useful data. The first compromise is to monitor the field only at periodic intervals, say at 10-year intervals, rather than continuously. In order for such a compromise to provide data for a really adequate study of long-term core and mantle phenomena, expansion of the surface observatory network as described in a previous section is required.
A second compromise is to acquire less accurate, hence less expensive, data. The least expensive route is to acquire data only from a fluxgate magnetometer. In the case where little or no attitude information is acquired, the data will be analyzed as field magnitude data only. Such are the data from the currently active POGS spacecraft of the U.S. Navy. In the case where attitude information of a few arc minutes is available, the vector data are very useful in mapping fields from field-aligned currents and ionospheric currents, though not for main field modeling. Such are the data from the DE-2 spacecraft which operated from late 1991 through 1992, and such are the data planned for the DMSP Block 5 series. The major difficulty with such data is the lack of absolute measurement. While low-drift-rate fluxgate instruments are indeed available, all are subject to some drift. On a spacecraft, such instruments are subjected to extremes in vibration, radiation, and temperature changes; it is impossible to know what drifts, if any, have occurred. Nevertheless, the DE-2 data were successfully used in one of the better candidate models used in the most recent update of the IGRF. However, three of those who submitted candidate models chose not to use these data.
Another compromise would be to fly only an absolute scalar magnetometer with no attitude determination. This is more expensive than flying only a fluxgate, because the instrument cost is greater. However, the measurements are not subject to drift and hence are better for main field modeling. A major shortcoming is that the measurements can make no contribution to the study of fields from field-aligned or ionospheric currents.