Geomagnetism applications

Cover. Global magnetic observatory locations depositing records in World Data Center-A in 1994.

Free on application to U.S. Geological Survey, Information Services Box 25286, Federal Center Denver, CO 80225

1. Diagram showing present model of magnetospheric field distorted by arrival of disturbed solar wind.....................
2. Graphs showing H-component field variations for a geomagnetic storm...................................................................
3. Graphs showing c0111parison of pipe-to-soil current on a pipeline system aild geomagnetic field chailges ...............
4. Map showing areas of ignoous rock aild auroral zone locations .................................................................................
5. Electron density proftles at middle latitudes during daytime quiet aild disturbed geomagnetic conditions ...............
6. Graphs showing correspondence of daily failge of X-field component in subauroral, auroral, aild polar cap zones to radiowave propagation quality index for paths from western Germaily to various locations during a geomagnetic storm perioo............................................................................................................................................
7. Graphs showing occurrence of Global Positioning System problems aild large hourly failge of geomagnetic field variation............................................................................................................................................................... 9
8. Diagram showing surface magnetic field response from buried magnetic material exhibiting dipole magnetization............................................................................................................................................................... 10
9. Map showing tracks of U.S. Navy Project Magnet flights for which data were deposited at National Oceailic aild Atmospheric Administration National Goophysical Data Center......................................................................... 11
10. Schematic representation of detection of seamounts using aeromagnetic techniques................................................ 12
11. Field contour chart, State of Minnesota....................................................................................................................... 13
12. Schematic of ship-towed magnetometers responding to oceail-bottom magnetic stri~tions from sea-floor spreading of magma with frozen-in field alignments.................................................................................................. 14
13. Graph showing one-dimensional inversion of magnetotelluric data to show conductivity (or resistivity) as function of depth.......................................................................................................................................................... 14

CONTENTS

A'bstract ... .. ... .. ............... .. .. ........ ....... .. ........................... .. .. ............... .. ... .. .. ... .. ..... ....... ..... 1 Introouction ..................................................................................................................... 1 Physics of the Earth's Space Environment..................................................................... 1 Satellite Damage ............................................................................................................. 2 lnductioo in Long Pipelines ............................................................................................ 4

Global Positioning System ... ~························································································· 8

Conductivity Structure of the Earth ................................................................................ 8 Surface Area Traverses ........................................................................................... 8 Aeromagnetic Surveys............................................................................................ 9 Ship-Towed Magnet0111eters ................................................................................... 10 Magnetotelluric Sounding of the Earth's Crust...................................................... 13 Cooductivity of the Earth's Upper Mailtle.............................................................. 15 Paloomagnetic and Archaoomagnetic Studies ................................................................ 16 Magnetic Charts ..................~ ........................................................................................... 17 Navigation by Magnetic Charts ...................................................................................... 20 Goomagnetism and Weather ........................................................................................... 21 Goomagnetism and Life Forms....................................................................................... 22 Magnetic Observatories .................................................................................................. 24 Tropospheric and Ionospheric Field Observations ......................................................... 27 Magnetospheric Measurements....................................................................................... 28 Solar-Terrestrial Disturbance Predictioos ....................................................................... 29 Summary aild Conclusions.............................................................................................. 30 References Cited............................................................................................................. 30

FIGURES

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14. Schematic section showing 2D electrical resistivity model of Juan de Fuca plate subduction region near

VailCOUVer lslaJld, c3nada ........................................................................................................................................... 15

15. Upper ma11tle conductivity profile obtained from a11alysis of quiet-day geomagnetic records at North America11 observatories................................................................................................................................................................ 16
16. Diagrams showing reversal patterns for the Reykja11es Ridge south of Icela11d ......................................................... 16
17. Maps showing horizontal intensity of geomagnetic field............................................................................................ 18
18. Graphs showing annual total rainfall at three locations in South Africa compared with double sunspot cycle ......... 20
19. Map showing global a11nual rainfall difference between sunspot maximum a11d sunspot minimum, 1860-1917 ..... 21
20. Graph showing Ethesian winds occurrence, Athens, Greece, 1891-1961.................................................................. 21
21. Map showing polar view of averaged surface pressure cha11ges 3 days following 14 geomagnetic storm sudden commencements ........................................................................................................ ~................................................. 22
22. Diagram showing variation of height difference of pressure surfaces before and after a solar magnetic sector boundary passage......................................................................................................................................................... 23
23. Graph showing average response of vorticity area index for magnetic sector boundary passage .............................. 23
24. Diagram showing magnetic fields of the human body compared to geomagnetic field levels and to sensitivity of SQUID magnetometer................................................................................................................................................. 24
25. Graphs showing comparison of group psychopathological syndrome expression index a11d geomagnetic index...... 25
26. Map showing global magnetic observatory locations depositing records in World Data Center-A in 1994 ............. 26
27. Map showing locations of USGS staildard magnetic observatories and sites occupied repeatedly for mapping purp<>ses .............................. ............................. .................................. ........................................................... 26
28. Map showing locations of INTERMAGNET observatories operating in 1993 and reception range of four geostationary satellites servicing these observatories.................................................................................................. 27

TABLES

1. Percentage of days with cases of disease or death from myocardial infarction in Sverdlovsk, Russia, as a function of active a11d quiet geomagnetic conditions .................................................................................................. 24
2. Some features of principal satellites used for geomagnetic field mapping ................. .......................... ...................... 28
3. Daily forecast of geomagnetic storm conditions a11d subsequently observed conditions, 1989-1991....................... 29 By Wallace H. Campbell1

ABSTRACT

The societal uses of knowledge acquired from geomagnetism studies include research on space environment and satellite damage, pipeline management, electric power grid failure, communication interference, global position determination, mineral resource detection, Earth formation and structure, navigation, weather, and magnetoreception in organisms. Continuing observation of the geomagnetic field, togeth~r with careful archiving of these records and development of mechanisms for disseminating these data, guarantees the enduring utility of geomagnetism studies in today' s world.

INTRODUCTION

The observation of the Earth's magnetic field is a modern multiuse technology. Results obtained from such studies are used in many ways. Each period range of natural geomagnetic field fluctuations is applicable to specific areas of study. For example, consider these:

1. From 0.25 seconds to 1 minute: Earth crust explo-

ration, detection of hidden conductivity anomalies, electric power transformer failures, hydromagnetic wave propagation, and revelation of magnetospheric processes.

2. From 1 minute to 24 hours: structure of magneto-

spheric deformation and currents, thermospheric heating and winds, ionospheric currents and tides, and conductivity characteristics of the Earth mantle and continental coastlines. Geomagnetic storms in this time scale affect a multitude of manmade systems such as satellites, communication systems, electric power grids, and long pipelines.

3. From 1 day to 1 year: fluid motions within the

Earth's core and at the core-mantle boundary, solar activity and solar sector changes, tropospheric weather changes, and magnetospheric deformation. Our main-field magnetic navigation charts are obtained from data in this period range.

4. From 1 year to 100 years: changes in the Earth's

outer core dynamo-field moment, solar-cycle variability, and climatic variation in solar-weather relationships.

5. From 100 years to 3,000 years: evidence of the

Earth's polar wandering, non-dipole outer-core drift patterns, and historic climatic changes from archaeomagnetic and ·1ava-flow magnetic samples.

6. From 3,000 years to 150 million years: main-field

reversals and dipole field disappearances, paleomagnetospheres, and continental drift.

The topic of geomagnetism applications is so broad that it is difficult to organize a description. I introduce the subject with a discussion of the uses of geomagnetism in understanding our space environment so that the reader can appreciate

ruptions. Next, I discuss the spectacular effects of geomagnetic storms on satellites, pipelines, electric power grids, communication systems, and geographic position determination. Perhaps not as sensational, but of immediate application, are studies utilizing geomagnetism for surveying the Earth's composition, both for mineral discovery in the crust and for revelation of the Earth's structure. Magnetic charts and navigation represent the more ordinary uses of geomagnetism that do not create headlines in national magazines but form the preponderance of applications to human activity. Interesting possible future directions for geomagnetism applications are the study of global weather and living organisms. I conclude this discussion of geomagnetism applications by providing with some information about observation of the geomagnetic fields and predictions of disturbance.

PHYSICS OF THE EARTH'S SPACE ENVIRONMENT

The Earth has a dipolelike magnetic field called the "main field" that originates from electric currents within the Earth's liquid outer core. These currents probably are driven by a gravitational growth of the inner core and are organized by the spin of the Earth. The dipolelike field defines geomagnetic coordinates at the Earth's surface whose poles are tilted by about 11° with respect to the geographic coordinate system. We have learned all this from the years of carefully maintained records of our Earth's main field and its changes.

The geomagnetic field of the Earth extends its control over charged particle motions far into space, a region that has been named the "magnetosphere." Despite its name, this magnetosphere is not spherical in shape. Although clearly dipolar in fotm out to several Earth radii (Re), it assumes a more elongated teardrop appearance near its outer boundary. The outer shape of the magnetosphere is fashioned by a constant arrival of plasma of ionized particles and associated magnetic fields from the Sun called the "solar wind." The flfst information on this space behavior came from geomagnetic observatory records interpreted early in this century before the advent of satellites.

The Sun is the source of geomagnetic field disturbances. When the phrase "solar-terrestrial activity" is used, the intent is to restrict the subject to those increases of energetic particles and electromagnetic fields that originate at the Sun, travel to the Earth's magnetosphere, and have drastic effects on the Earth's upper atmosphere and geomagnetic field. The activity is on time scales that are short in the human perception of events. The Sun is said to be "active" when the magnitude of such changes is distinguishably large with respect to the average behavior over tens of years.

Uniquely active regions of the solar surface are responsible for the coronal mass ejections of energetic particles and fields. Although relatively random in occurrence, these events that encounter the magnetosphere are organized by a solar activity cycle of about 11 years and by the solar-surface rotation, which has about a 27-day period). Upon intercepting the Earth's magnetosphere, the solar blast of particles

cometlike structure, compressing the day-side geomagnetic field to an 11-Re position and even past 6 Re during great stotms. The downwind magnetospheric tail extends far past the Moon's 60-Re orbit Surface magnetic observatories respond to this distortion of the magnetosphere.

An interplanetary magnetic field travels in the solar wind, defined at the Sun and frozen-in with the solar-ejected particles. When this field that arrives at the day-side magnetosphere is southward in direction, a major geomagnetic stotm ensues in which currents of charged particles follow complicated routes through the magnetosphere (fig. 1). This stotm is a period of Earth surface field disturbance, recorded at geomagnetic observatories throughout the world.

Individual interrelated magnetospheric and ionospheric processes that occur during the period of a geomagnetic stotm are called "substotmS" (Rostocker, 1993). Studies of substorms connect the severe changes in the magnetospheric tail region, partial ring currents encircling the Earth, and strong currents that precipitate particles along Earth field lines into the high-latitude auroral region. Auroras occur as this precipitation excites oxygen and nitrogen of the upper atmosphere to glow at the prescribed wavelengths that are allowed by their atomic and molecular structure. In these aurorally active regions the upper atmosphere is heated and the ionospheric conductivity is so enhanced that intense "electrojet" currents flow.

Magnetic observatories throughout the world provide continuous detailed monitoring of the individual processes that transpire in the space about the Earth, a region of major national economic interest. Magnetic data from selected observatories are presently grouped to fotm global indices of the activity. Auroral electrojet (AE) indices are used to describe the critical particle precipitation regions at high latitudes. Kp (and ap and Ap derived from Kp) indices follow the general global activity levels. Dst indices, from selected middle- and low-latitude stations, represent the averaged distant magnetospheric changes that defme a geomagnetic stotm (fig. 2). Let us next consider some of the ways that geomagnetic disturbance indicators are useful in quantifying the impact of geomagnetic storms on our present society.

SATELLITE DAMAGE

Onboard computers in space systems orbiting the Earth have lifetimes that are mostly determined by the accumulated radiation damage to their circuitry from energetic particles in the magnetosphere. Solar-cell arrays powering the satellites also lose a few percent of their efficiency each year of solar-terrestrial environment exposure. Stotm-time high-latitude field-aligned currents heat the thetmosphere causing it to expand upward and move toward the Equator. these thermospheric winds and density increases modify the drag on satellites at their typical equatorial and polar orbiting

velocity can cause transitory tracking loss and eventually shortens the satellite lifetime.

Synchronous-orbit satellites at about 6.6 Re commonly experience malfunctions from stotm-time solar particle events. During some stotmS the magnetospheric compression by the solar wind forces the magnetospheric boundary inward past the geostationary satellite position; such transitions have been found to correlate with numerous satellite operation anomalies (Allen and others, 1989). The offset of the geomagnetic and geographic poles and equators (determined from our accurate global magnetic field charting) affects the geographic longitude distribution of particles that produce satellite damage; geostationary longitude placement of spacecraft becomes a compromise location based in part on geomagnetic considerations determined from global field modelling by the U.S. Geological Survey (USGS) and other national organizations.

Allen and Wilkinson (1992) summarized the space effects of the stotms of October 19-21, 1989:

LAYER BOUNDARY OF PLASMAS PH ERE OVAL INDUCTION IN LONG PIPELINES INDUCTION IN ELECTRIC POWER GRIDS

Power pools serving the entire northeastern United States also came perilously close to a comparable calamity with similar cascading system failures during the same geomagnetic storm that affected Hydro-Quebec. In addition, the stonn destroyed transformers at the Salem Nuclear Plant of the Public Service Electric and Gas Company, at a replacement cost of about $12,000,000. During this loss of power output, the PSEGC replacement energy cost was approximately $400,000.

Power-grid vulnerability is dependent on the nearness to a region of maximum auroral electrojet currents, the interconnection pattern of the power-grid system, and the regional geology of high-resistance igneous rock. Figure 4 illustrates the regions of concern for North America. The rapidly fluctuating stonn-time induced currents are thought to enter and exit power systems through the grounded connection of transformers, causing a high level of half-cycle saturation and dramatically increasing reactive power consumption and localized destructive heating; intolerable system voltage depression, unusual transmission line flow, and relay malfunction follow.

Present protection strategies for power companies to deal with geomagnetically induced currents (GIC's) involve both system redesign, based on the history of geomagnetic field at the critical locations over a solar cycle, and adequate warning of impending storm onsets and recovery times. Improvements in geomagnetic storm forecasting mostly depend on expansion of the global geomagnetic information real-time network (IN'IERMAGNET, described in the section on "Magnetic Observatories"), as well as careful reanalysis of past records.

COMMUNICATION SYSTEMS

Modern communication systems rely primarily on satellite transponders, radiowave links, oceanic and land-based cables, telephone line connections, microwave transponders, and fiber-optic cables. Even though the last two apparently are not susceptible to geomagnetic storm induction problems, the universal interconnection of transmission facilities brings problems to all systems. First realization of major communication problems began with unmanageable induced voltages on telegraph lines during a severe geomagnetic storm period of August 28 to September 4, 1859. As an example of troubles in more recent years, 80 percent of all long-distant telephones in Minneapolis were silenced by the great (Kp=90) magnetic storm of March 24, 1940. At the same time Bell System's transatlantic cable experience significant difficulties when an estimated 2,650 volts appeared across the line. The major (Kp=90) geomagnetic storm of February 10, 1958, induced 2,700 volts on the Bell System cable from Newfoundland to Scotland and fluctuated voice communications from "squawks to whispers" (Lanzerotti and Medford, 1989). The magnetic storm of August 4, 1972 shut down a "Long-Haul" coaxial communications cable between the States of Illinois and Iowa. Storm-time induced current fluctuations on communication cables that do not fail nevertheless cause problems in business digital data and facsimile transmissions.

At high latitudes, satellite transionospheric radiowave signals during storms suffer from refraction and rotation of

total electron content along the propagation path. Storm-time signal phase and amplitude radiowave scintillations disrupt both satellite (at 10 Hz) and surface high-frequency (HF) communications. The scintillations arise from scattering by ionization irregularities in the altitudes above 200 km when

storms.

Geomagnetic storm conditions upset the expected pattern of received signals at those radiowave transmission frequencies that depend on the ionosphere as a reflecting medium. This problem is particularly severe at auroral and polar latitudes where the ionospheric conductivity is greatly enhanced during storm conditions. In the past, these regions were not near major population areas, but demographic changes have increased national dependence on high-latitude communications. At lower latitudes the storms are responsible for phase changes in the very low frequency (VLF) navigation systems, fadeouts of short-wave communication links, and major modification of usable radio frequencies. Global ionospheric models used for prediction of the propagation conditions all need geomagnetic disturbance indices for critical adjustments.

Both the F-region and total electron content (TEC) of the ionosphere are adversely affected during geomagnetic storms, but the relationship are not easily predicted. At times there is an increase in ionization in the morning hours and a severe decrease in the-evening (fig. 5). Small storms can sometimes cause major disruption of the total electron content. Figure 6 illustrates the degradation of high-frequency

transmissions along six representative transmission paths during a severe magnetic storm period in March 1990.

Some transmitters have flexibility in the selection of the broadcast frequencies. For these, the usable frequency is predicted from Ap index values computed from the geomagnetic records and disseminated by the forecasting centers. In addition to government and industrial broadcasters, more than 1 million amateur radio operators in the world make use of geomagnetic storm "nowcasts" and forecasts.

GLOBAL POSITIONING SYSTEM

e1 • I • I

II IIIII I

Figure 7. Occurrence of Global Positioning System (GPS) problems (solid circles) and large hourly range of geomagnetic field variation (vertical lines) reported in 1987 by R.L. Coles (Geophysics Division, Geological Survey of Canada, Ottawa).

The ftrst type of magnetic field contamination source, ftxed magnetization, requires a permanent adjustment fQr an unchanged plane configuration, but the effect must be regularly reevaluated because physical shock to the aircraft (such as from a hard landing) can change the magnetization to a new local-field arrangement Periodic recalibration airswings over the observatory are important. The second type of contamination, induced magnetization, can be evaluated for the calibration flight paths, but the induction effect changes for each survey region in which the Earth's main field is significantly different The third type of contamination should be evaluated in a manner similar to that for the second with the addition of calibrations for the usual aeromagnetic cruising speeds. The fourth type of contamination, fields from in-flight static charge buildup and dissipation, is difficult to compensate because charge variations follow weather conditions; the range of contamination can be discovered through a comparison of magnetometer readings from overflights of the ftxed observatory location in a variety of weather situations.

The most extensive regional aeromagnetic surveys have been made by the U.S. Navy to support production of global magnetic charts of importance for navigation. Figure 9 illustrates the coverage of this global aeromagnetic data set now available from the magnetic field archives at the National Geophysical Data Center (NGDC/NOAA) from its Project Magnet program from 1950 through 1990. These surveys were made at high levels, usually between 15,000 and 25,000 ft (4.6-7.6 km) elevation. Over the years, navigational accuracies have increased from ±5 nautical miles (9.3 km) to ±100m. Three-component fluxgate magnetometers calibrated with an optically pumped metastable helium magnetometer gave Project Magnet field determination accuracies of approximately ±15 gammas. Project Magnet is continuing with improved system technology providing greater field accuracy.

High-resolution flights for detailed geomagnetic surveys typically are flown at altitudes between about 500 and 1,000 ft (152-305 m). Flight-line separations are estimated to be about twice the distance, B-A, from the geological magnetic basement (B) to the aircraft (A). The sample interval is, by rule, less than one-fourth of B-A. Flights are usually restricted to the quieter geomagnetic activity days using disturbance information from the nearest geomagnetic observatories and activity predictions from the space environment forecasting centers.

Three methods are used for adjustment of the flight-time measurements to the variations in geomagnetic activity. (1) Mapped fields are taken to be the difference between the observation and the field measured at a base station magnetometer. (2) With a checkerboard-grid flight pat-

into accord; these adjustments define linear alterations of the remaining data samples. (3) Best estimates of the quiet-day field variation (Sq) defmed for each data sample location and time are removed from the data. Sq can be obtained from a local magnetic observatory or, in absence of such a base station, from a program, WDCA-SQ1, developed by the USGS (Campbell and others, 1989) and available from World Data Center-A.

Figure 10 illustrates the aeromagnetic evidence of a major seamount Figure 11 depicts the magnetic contouring used in determination of the regional geology of the State of Minnesota. Hydrothermal alteration and metamorphism can modify crustal magnetization. Magnetic anomaly maps showing these changes are used in the detection of energy and mineral resources. Aeromagnetically detected anomalies over oil-bearing layers have been interpreted as due to magnetite that formed from chemical processes as a result of microseepage of the petroleum. Oil and gas production fields of the Alaskan Navarin Basin and the European North Sea Basin were discovered using aeromagnetic surveys. Improvements in aeromagnetic techniques over the years have increased mapping contour intervals from a "standard sensitivity" of 10 gammas, to a "medium sensitivity" of near 1 gamma, to a "high sensitivity" of about 0.1 gammas.

SlllP-TOWED MAGNETOMETERS

Unequivocal evidence of continental drift, tectonic motion, and sea-floor spreading was obtained from

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Figure 15. Upper mantle conductivity profile (dots) obtained from analysis of quiet-day geomagnetic records at North American observatories. Solid line segments indicate corresponding values of density obtained from seismic data. Modified froin Campbell and Anderssen (1983).

Figure 16. Reversal patterns for the Reykjanes Ridge south of Iceland Modified from Heritzler and others (1966) (modified from Tarling, 1971). A, Schematic of pattern. B, Field strength from single traverse. C, Polarity time sequence assuming spreading rate of 1 em/year.

PALEOMAGNETIC AND ARCHAEOMAGNETIC STUDIES

of formation or matched between similarly dated samples to ftx their tectonic movement. In some cases, established polar wandering curves must be used to determine the likely age of a group of rocks taken from a single, tectonically stable block. Global patterns of continental drift, established from

planet. Relationships have been established, dating back 800 million years, between paleomagnetic field intensity changes (obtained from deep-sea sediment cores), the Earth's orbital eccentricity, and climatic changes (Wollin

Archaeological materials that acquired a magnetic remanence at an earlier time when they were heated (such as bricks or clay pots) can be matched with the historical geomagnetic field intensity and direction to fix the sample age. Dating as far back as 5,000 years is possible. For the last century, about 1° in changed field direction represents 5 years of age. Sample remanence can also be used to reconstruct pot-shard and archaeological structure positions by establishing parallel magnetic vector orientations.

MAGNETIC CHARTS

Charting of the changing Earth's surface magnetic field has been a necessary and regular function of the major nations of the world since the early days of global exploration. Typically, contour lines of equal (isomagnetic) increments of field are plotted. Because mercator projections preserve azimuthal relationships, magnetic field declination is best represented on such maps. For consistency, other field components are similarly displayed. Figures 17A and B are examples of some USGS field charts for 1990 made for popular illustration purposes (more detailed technical charts are the most commonly used products).

Regular changes of the field that occur over a period of years are called "secular changes." These changes can result from change in the current magnitude or area of the principal current loop within the Earth, shift in the alignment of the dipole axis (the dipole North Pole is moving about 18 km northward and 5 km westward each year), and westward drifting of the non-dipole part of the main field. The present westward drift of the geomagnetic pole location (nearly 0.04° per year) is consistent with paleomagnetic evidence that the Earth's dipole and spin axes may coincide about every 10 years. The motion of the dipole about the spin axis is called ''precessional drift." All these changes require constant recharting of the Earth's field at 5-year intervals with information of the expected linear corrections to be made between charting epochs.

If the dipole field components are removed from a model, the remaining field (non-dipole field) shows contours that drift westward at a rate of about 0.1 o each year, much faster than the dipole field. This drift would cause anomalies to circle the globe in about 3,000 years were it not for the fact that the non-dipole anomaly forms may be noticeable altered in less than 100 years. On average, the non-dipole components are changing by about 50 gammas per year. There is some evidence that the anomalies may be severely modified in form as they pass the Pacific Ocean region. Accurate registtation of the Earth's field has only been possible for the last 100 years, so our understanding of the drift of these regional features is quite limited. The drifts are used by paleomagneticians to determine the processes within the liquid outer core that generate the Earth's main field A correlation coefficient of 0.9 has been established between the change in westward drift and the change in Earth rotation speed several years earlier (1.0 represents perfect correlation, and 0 represents no correlation). The correlation has been ascribed to differences in the rotation speed of the Earth's mantle, outer core, and inner core. There is some evidence that the time variations of the larger Gauss coefficients, representing the spherical harmonic analysis of

slope in 1970 (compared to other years), which has been called the "geomagnetic jerk." The explanation, in terms of Earth core and mantle processes, has been exciting considerable research interest.

The global IGRF and DGRF (International and Definitive Geomagnetic Reference Fields) models are usually tabulated to spherical harmonic analysis internal polynomial degree and order 12 (although analyses are carried to higher levels) because, at about that level, the higher polynomials begin to reflect Earth crustal anomalies. Those anomalies of small size, less than about 100 km, generally have been correlated with geological surface features. The field effects of these anomalies disappear rapidly with altitude and make very little, if any, contribution to the space environment The reference field models are subtracted from area surveys to provide the geologist with crustal conductivity charts.

Buried in the geomagnetic observatory records are annual and semiannual changes. These systematic changes can be caused by the seasonal deformation of the magnetosphere by the particles and fields arriving from the Sun and by the equinoctial-month preference of solar-terrestrial disturbance activity interactions. The separation of internal and external parts in the spherical harmonic analysis of the field helps remove most of these distortions from the internal representation; however, a part of the disturbances induces currents within the conducting Earth that are not so simply removed. Adjustments to the spherical harmonic coefficients are made using empirical relationships to a global activity index, Dst, determined from records of selected standard, land-based observatories. The induction contributions to the main-field analysis would be responsible for an 11-year solar-cycle fluctuation in the field model coefficients.

The major problem for the main-field analysis, the poor distribution of surface observations, was originally thought to be solved by a new reliance on satellite measurements of

/ ·Declination in degrees -Change in minutes per year

z g

fA

NAVIGATION BY MAGNETIC CHARTS

necessary .Part of modem man's activities.

GEOMAGNETISM AND WEATHER

Year-to-year changes in growing season length, tree-ring separation, temperature, rainfall, thunderstorms, storm tracks, and winds show clear year-to-year fluctuations relative to sunspot (or double-sunspot) cycles (figs. 18-20). Northern Hemisphere annual mean temperatures are lowest near sunspot maximum and highest near sunspot minimum. Tropopause height over the western Pacific region varies, in phase, with the sunspot cycle. At equato-

of solar maximum. Droughts in the western United States follow a 22-year sunspot cycle. Courtillot and others (1982) showed a relationship between the secular variation of the geomagnetic field and the global temperature from 1860 to 1980.

There is a well-known co-variation of sunspot (solar) activity and geomagnetic disturbances. From our present knowledge we would expect that the thermospheric heating by electric currents associated with geomagnetic storms would cause global modification of atmospheric pressure, the persistent patterns of which control seasonal weather conditions. Presently, an active research topic is the full understanding of the exact geomagnetic mechanisms by which weather is modified.

Some specific connections between the geomagnetic storm and weather have been established. Figure 21 shows

changes, 3 days after a geomagnetic storm in winter months. Solar sector boundary crossings (registered as interplanetary magnetic By field changes at the magnetospheric boundary) are associated with large increases in geomagnetic activity. Figure 22 shows the systematic decrease in Northern Hemisphere high-latitude atmospheric pressure surfaces about 4 days following a sector boundary crossing. A low-pressure

GEOMAGNETISM AND LIFE FORMS

superconducting at the significantly higher temperature of liquid nitrogen. High-temperature superconducting quantum interference device (SQUID) magnetometers are now being used in medical research to map the magnetic fields associated with human high-order mental functions. Specific response areas of the brain have been identified for cognitive functions, epileptic seizures, Alzheimer's anomalies, and so forth. Brain-wave frequencies span the range of geomagnetic micropulsations and geomagnetic storm oscillations; the geomagnetic fields are considerably more intense than brain waves (fig. 24). The question of whether or not human brain processes respond to external field stimulation has not yet been definitively answered.

There are reports of geomagnetic disturbance effects on man. In a 4-year study, Becker and others (1961) observed a positive correlation between the monthly sums of geomagnetic K indices at Fredricksburg and the monthly admissions to two mental hospitals in Syracuse, New Yolk. They found that the probability of obtaining such a relationship by chance was 1 in 1,000. Nikolaev and others (1976) described an extremely interesting study of the psychopathic behavior of inmates at a Moscow mental hospital. From April 1975 through January 1976 each hospital worker contacting patients rated the degree (0 to 5) to which an inmate evidenced his or her psychosis. All numbers were averaged daily for each patient, and a hospital daily average "disturbed condition" index, S, was obtained. The daily values of S were compared to the daily geomagnetic Ap index, to the polar region geomagnetic field strength, and to polar sector structure indices. Figure 25 shows the increase in disturbed mental behavior of the patients during disturbed geomagnetic field conditions.

Novikova and Ryvkin (1977) of the Sverdlovsk Medical Institute reported on a 1961-1966 study of the deaths due to myocardial infarction in Sverdlovsk, Russia. Table 1 shows the results of about 3,000 cases of infarction and about 1,000 deaths. On magnetically active days in all years both morbidity and mortality were higher than on magnetically quiet days, with a probability of 0.005 that such a situation was random. For the same study group, Gnevyshev and others (1977) found that the largest number of sudden deaths from cardiovascular disease occurred within the fli'St 24 hours of a geomagnetic storm.

As we look at some of the special effects that have been connected to geomagnetic storms we should keep in mind two cautions. (1) Although associations and correlations between phenomena are important for the first steps in revealing the basic understanding of the physics of any relationship, correlation does not mean dependence. Many of the related phenomena may exist on separate branches of

ary variation; for example, two phenomena, each exhibiting only a seasonal change for different reasons, could show a statistical correlation. (2) Random associations of the averaged behavior of geophysical and biological phenomena can occur. Correlations should not be taken as defmitive relationships but rather as interesting possible subjects for further independent investigation.

MAGNETIC OBSERVATORIES

Magnetic observatories throughout the world (fig. 26) operate primarily on national funding to provide services considered to be of public importance. The present-day objective for magnetic observatories is the digital determination of the vector magnetic field to about 0.1-0.01 gammas resolution and about 2 gammas absolute value at a sampling rate of about one value every minute (and for special rapid variation studies one value every 0.1 seconds). This means that stations transmitting to a storage and retrieval center can fli'St compress data considerably, at least to 10--10-5 of

automated for digital recovery at almost any convenient time, as seldom as once a year. At selected sites valuable for space-weather analysis, immediate data transmission via telephone lines or satellites is used. Although geostationary satellites can schedule interrogations of the remote station throughout the day, polar orbiting satellites permit data to be retrieved on about two passes per day at low latitudes and about once every 2 hours at polar latitudes.

The USGS operates 13 magnetic observatories: 7 in the contiguous United States, 3 in Alaska, and 1 each in Hawaii, Puerto Rico, and Guam (fig. 27). Data from these stations are returned instantaneously to the Golden, Colorado, central collection point by satellite and ground-based electronic submission, the USGS obtains copies of other world geomagnetic records from WDC-A archives for global mapping and research purposes.

In recent years a consortium of national geomagnetic observatory leaders, led by the USGS, have arranged a cooperative satellite data recovery system, from about 40 observatories, called INTERMAGNET (international real-time magnetic observatory network). Figure 28 shows the 1994 distribution of the contributing stations and participating satellites. One-minute digital data from INlERMAGNET observatories are broadcast, at 12-minute and 1-hour intervals, to special geostationary satellites within the observatory transmission signal reception window. At selected locations called GIN's (geomagnetic infonnation nodes), these data are collected from the satellite transmissions and distributed to participating agencies. Immediate preliminary appraisal of Selected INlERMAGNET recordings provides an important part of the NOAA Space Environment Forecasting Center evaluation of the present and expected space weather affecting satellites, communication, and electric power transmission. After detailed data cleaning and quality checking, computer readable flies of archived INTERMA-GNET data are collated and made available to the public.

TROPOSPHERIC AND IONOSPHERIC FIELD OBSERVATIONS

About 60 successful rockets have been launched through the high-conductivity layers of the abllosphere for the purpose of measuring ionospheric currents. Most of these flights were designed to study the equatorial electrojet; the others were about equally divided between studies of the auroral electrojet, Sq currents, and the main fteld. Difficulties in obtaining platform stability, accurate orientation determinations, and exact trajectory tracking infonnation have restricted instrumentation to total-field measurements. On occasion, a fluxgate or optically pumped magnetometer

favorite for rocket measurements of ionospheric fields because of its relative simplicity, low weight and cost, insensitivity to temperature and voltage fluctuations, and relative

MAGNETOSPHERIC MEASUREMENTS

magnetometers aboard. A tethered staellite series, SEDS, is

Satellite observations that encompass the full Earth have the great advantage of a global coverage unmatched by surface observatories; however, several drawbacks limit the prospect for total reliance on satellite measurements for charting of the Earth's magnetic field. Field values are dependent on position accuracy of the fast-moving space platform, and satellite fluxgate instruments are 100 times less sensitive than the modern surface observatory system. Because of the satellite position above the current-carrying ionosphere, the external and internal parts of Earth's field, computed from spherical harmonic analysis, place the ionospheric currents interior to the satellite and thereby degrade a main-field determination. The best world-field models are obtained from a combination of the satellite and surface station records.

SOLAR-TERRESTRIAL DISTURBANCE PREDICTIONS

As the applications of geomagnetism to human activity increase, there is a corresponding growth in the global appetite for reliable predictions. Sufficient archived geomagnetic information is stored at the World Data Centers to allow accurate long-term statistical predictions (such as the number of daily disturbances of more than 100 gammas during a 40-year period in a specific region of the world). Solar-cycle predictions of the year and level of next year's average geomagnetic activity also can be provided with reasonable certainty. Forecasters do well in their appraisal of the average activity level for the next rotation of the Sun (about 27 days) and for the possible arrival of activity seen on the solar limb. All these predictions are important for protective designs of manmade systems sensitive to geomagnetic disturbances.

Problems arise with prediction of onset time, activity level, and disturbance duration of specific events. The lognormal form of the Dst index allows some estimate of storm recovery levels from recordings of the initial rise to maximum disturbance level. From the tracking of solar activity and coronal mass ejections, some storm prediction success has been accorded to the space-environment forecasters in their predictions of disturbed conditions for the next hour, day, and week. Table 3 lists the results of daily forecasts of storm conditions (daily Ap index greater than 50) for the period 1989 through 1991.

The 98.9 percent accuracy of the no-storm prediction is related to the high occurrence of quiet days and to the fact that easily observed quiet-Sun conditions always guarantee a low in geomagnetic variations. Predictions of suitable days for observatory baseline measurements, aeromagnetic flights, pipeline cathodic protection measurements, and so forth are highly reliable. Of the 30 storm days, 77 percent were not predicted. As long as temporary movement to a protective operational mode is not costly, many storm-sensitive systems are already benefiting from even the poor storm-prediction accuracy.

Although disturbance forecasting is a developing science, equally important to users is detailed ''nowcasting," an accurate apparisal of the present space conditions. Relying on these data, space programs, communication systems, power-distribution facilities, and so forth take protective actions that save millions of dollars of public and private funds. A vital part of the nowcasting capability is the input from the USGS and INTERMAGNET geomagnetic observatory programs.

The disturbance forecasting capability will be greatly improved by the placement of special satellites downstream in the solar wind. There is a position, called the Lagrange point LJ, about 1.5x1Q6 km from the Earth (at 235 Re it is about 1/100 of the distance to the Sun), where a satellite can circle the Sun in 1 year and appear to be relatively ftxed in

wind, was launched in November 1994 for daily ~piing of particles of fields. The second satellite at this location, called ACE for advanced composition explorer, is to be launched in 1997 and will provide continuous real-time data transmission of full solar wind particle and field information. Knowledge of incoming wind composition, velocities, and field directions are necessary for determining the reaction of the magnetosphere to solar-terrestrial disturbances.

The physics of the disturbed field and particle conditions from the Sun to the Earth's surface is still not fully revealed. Each year and new solar cycle brings us closer to a full understanding of solar-terrestrial processes and to a greater application of geomagnetism to societal needs. The system of national solar-terrestrial disturbance forecasting centers has made a major contribution to this endeavor. Forecasting improvement techniques presently rely on enlarging

in near real time. For the future, we can expect that our knowledge of the details of surface geomagnetic field responses will increase with advancement of our understanding of magnetospheric substorms, continuing the vital role of USGS standard magnetic observatories in disturbance forecasting.

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