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Page 155 APPENDIX 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 Seafloor Introduction 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);
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Page 156 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.
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Page 157 Issues 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.
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Page 158 Current 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
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Page 159 magnetometers 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.
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Page 160 Future 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
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Page 161 community'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.
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Page 162 Future 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
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Page 163 estimate 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
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Page 164 zone 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 Facilities Enhanced 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.
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Page 165 Data 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
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Page 179 be 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.
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Page 180 Continuous 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
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Page 181 Regatta 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
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Page 182 ing 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).
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Page 183 Planned 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
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Page 184 events, 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.
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Page 185 Other 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
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Page 186 magnetometers 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
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Page 187 necessary 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.
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Page 188 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.
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Page 189 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.
Representative terms from entire chapter: