Cover. Global magnetic observatory locations depositing records in 

World Data Center-A in 1994. 
Geomagnetism Applications 

By Wallace H. Campbell 

U.S. GEOLOGICAL SURVEY CIRCULAR 1109 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

Free on application to U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names iii this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Campbell, Wallace H. (Wallace Hall), 1926-

Geomagnetism applications I by Wallace H. Campbell. 

p. em. - (U.S. Geological Survey cirrular ; 1109) 

Includes bibliographical references (p. ) 

1. Geomagnetism-Industrial applications. I. Title. II. Series. 

QC815.2.C36 1995 

538'. 7-dc20 94-35641 

CIP 
CONTENTS 

A'bstract . .. .. ... .. ............... .. .. ........ ....... .. ........................... .. .. ............... .. ... .. .. ... .. ..... ....... ..... 1 

Introouction ..................................................................................................................... 1 

Physics of the Earth's Space Environment..................................................................... 1 

Satellite Damage ............................................................................................................. 2 

lnductioo in Long Pipelines ............................................................................................ 4 

Inductioo in Electric Power Grids................................................................................... 5 

Communication Systems................................................................................................. 7 

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 

1. Diagram showing present model of magnetospheric field distorted by arrival of disturbed solar wind..................... 3 

2. Graphs showing H-component field variations for a geomagnetic storm................................................................... 4 

3. Graphs showing c0111parison of pipe-to-soil current on a pipeline system aild geomagnetic field chailges ............... 5 

4. Map showing areas of ignoous rock aild auroral zone locations ................................................................................. 6 

5. Electron density proftles at middle latitudes during daytime quiet aild disturbed geomagnetic conditions ............... 7 

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............................................................................................................................................ 8 

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 

m 
IV CONTENTS 

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

V ailCOUVer 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 
GEOMAGNETISM APPLICATIONS 

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 exploration, 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 magnetospheric 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 

1U.S. Geological Survey, MS 968, Box 25046, Denver, Colorado 

80225. 

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 

the geomagnetic field structure and sources of the major disruptions. 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 
2 GEOMAGNETISM APPliCATIONS 

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 

and fields blows the Earth's dipole field into an elongated 

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 

altitudes near 500 km. The attending decrease in orbital 

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: 
MAGNETOPAUSE 

TROUGH ut 1a: • 

PLASMAS PH ERE 

MIDNIGHT SOUTH 

BOUNDARY OF 

OVAL 

LAYER 

4-. 

(C'(//": 

C'~S'...y~~4< ~~~«'"" ~,)'-

MAGNETOPAUSE 

CURRENTS 

LOOP CURRENT 

Figure 1. Present model of magnetospheric field distorted by the arrival of disturbed solar wind. Courtesy of World Data Center-A. 

Cl) 

~ 

~ 

~ ~ 1:'!1 

V) 
4 GEOMAGNETISM APPUCATIONS 

GEOMAGNETIC STORM 

KAKIOKA-H f.::::::7"""-'----+-----------,---=:l I 250 

gammas 

00 06 12 18 00 06 12 18 24 

UNIVERSAL TIME A 

DEC 19, 1980 DEC 20, 1980 

50 

(/) 

<( 0 :2: 

:2: 

<( -50 (!) 

~ -100 ui 

Cl 

::> -150 1-

:J 

a... -200 :2: 

<( 

-250 

00 06 12 18 00 06 12 18 24 

B DURATION, IN HOURS 

Figure 2. H-component field variations for a geomagnetic storm 

of December 19, 1980, for the period December 19-20, 1980. 

A, Variation at Dst observatories. B, Computed Dst hourly index. 

During the particle events and magnetic storms of this period in October 

both GOES-5 and -6 experienced SEUs [single particle upsets], polar 

orbiters experienced multiple switching off of microwave transmitter 

units-operators quit 'counting' anomalies and just left the system 

turned off until conditions improved. TDRS-1 had 50 RAM [random 

access memory] hits on the 19/20th in the radiation susceptible memory 

chips. Even the hardened lDRS-2 and -3 experienced SEUs. Polar orbiting UOSAT-2 had many SEUs, particularly in the expanded South Atlantic Anomaly region. Significant power panel degradation occurred on 

GOES-5, -6, and -7. Commercial geostationary satellites had power 

panel degradation, pitch glitches, and SEUs (137 events reported). 

Bright red aurora were seen at unusually low latitude over Japan during 

these events. On October 20-21, Ap* = 162 [Ap* is the maximum 24-hr 

running mean of the ap index] ..• on board ATLANTIS [space shuttle] 

the astronauts reported irritating 'flashes' in their eyes as energetic protons penetrated the optic nerves. Although they are repated to have 

'retreated' to the innermost shielded part of the Shuttle, the eye flashes 

did not subside until the proton event ended. 

Satellites have been completely disabled during periods 

of high solar-terrestrial disturbance activity. As an example 

of major storm-time problems for geostationary satellites, 

Wrenn (1994) of the U.K. Defense Research Agency 

reported, "On January 20, 1994, the ANIK El and E2 communications spacecraft suffered serious failures of their 

momentum wheel control systems. It is likely that the satellites were subjected to reduced lifetimes and to bulk revenue 

losses running into tens of millions of dollars." 

Presently, it is thought that failure of spacecraft circuitry is caused by disturbance-time internal dielectric 

charging of satellite components and the resulting subsequent discharge. Satellite shielding (a major weight-cost 

consideration) is limited by projected lifetime considerations 

that rely on the accurate interpretation of past solar-terrestrial disturbance behavior extrapolated from geomagnetic 

storm infOtmation provided by our observatories. 

Adequate storm warning systems allow the implementation of protective procedures or a delay of command 

transmission to more favorable times for some spacecraft. 

Space shuttle programs have plans to abort extravehicular 

activities or even the flight itself during major storms to 

protect astronauts from expected exposures by storm-time 

particles. The supersonic airplane, Concorde, has arrangements to be brought to lower altitudes in extreme, 

storm-time, exposure-threatening conditions. Real-time 

geomagnetic field information from selected observatories 

is an important part of the alarm system used at space-environment forecasting centers. 

INDUCTION IN LONG PIPELINES 

The Alaska oil pipeline is essentially a 769-mile 

(1.28x103 km)-long, surface-grounded conductor extending 

from about 69° geomagnetic latitude at the Arctic Ocean to 

about 62° geomagnetic latitude at the North Pacific Ocean. 

Those geomagnetic latitudes overlap the auroral zone of 

intense field-aligned and ionospheric-electrojet currents. 

The resistance per unit length of the pipe is about 4.8xt0-6 

ohms per meter. Approximately half of the pipe length is 

buried. The central third of the pipeline parallels the geomagnetic latitude directions favored by the auroral electrojet 

current. Because of the high pipe conductivity with respect 

to the ground (to which it is electrically connected by zinc 

grounding cables), the strong fluctuating ionospheric currents (in the preferred geomagnetic east-west direction) at 

geomagnetic storm times induce currents to flow in the pipeline. When the pipeline was originally placed in the ground, 

small cuts in the electrically insulating surface coating 

occurred. An induced fluctuating current travels between the 

pipe and ground; when directed appropriately, that current 

causes pipe corrosion. The amount of corrosion is dependent 

on the frequency and amplitude of the storm-time source 

current, the exposed area of the pipe, the material in which 

the pipe is embedded, the frequency dependence of the corrosion process, and the frequency dependence of the local 

Earth induction (Campbell, 1986). 

For high-latitude pipelines such as those in Alaska and 

Canada, the effect of corrosion from the induced geomagnetic storm current maximizes with the 5-30-minute period 

field fluctuations. Figure 3 shows the correspondence of pipe 

to soil currents and geomagnetic activity for a high-latitude 

pipeline. During large storms induced currents in the 
INDUCTION IN ELECTRIC POWER GRIDS 5 

30 300 CJ) 

<( 

...J 

CJ) 

w 

I0 

z 

<( a: z :J 20 200 0 ~ :::c X: a: a: w Q D... 

CJ) <(' 

(!) Q 

z <( 

£5 GEOMAGNETIC z <( 10 ACTIVITY 100 <( 

w u a: ~-

0 

0 

z 

<( 

w 

:2: 

0 0 

0 5 10 15 20 25 30 

DATE 

Figure 3. Comparison of pipe-to-soil current (vertical line segments) on the Leismer Gas pipeline system northeast of Edmonton, Canada, and geomagnetic field changes. DRX is the daily range of the X component. Modified from Shapka (1993). 

pipelines can be as high as as 1,000 amperes. Estimates of 

corrosion effects over the lifetime of the Alaska pipe were 

obtained by comparison of presently measured pipeline currents with the USGS College and Barrow observatory magnetic field records and an extrapolation of the field's historic 

behavior. It is believed that the high-latitude pipelines are 

corroding faster than originally anticipated because induced 

currents were not adequately taken into consideration in the 

original design. Only recently did some pipeline companies 

begin measuring these induced currents to institute corrective procedures. 

At all latitudes, pipeline corrosion is also caused by the 

steady currents generated either from the difference in contact potential at grounded points in the pipeline or from the 

induction of currents from nearby manmade systems. Most 

pipelines are protected from this harmful current by application of an overriding current sent onto the pipe to make all 

exposed areas negative (a cathode) with respect to ground. 

Corrosion engineers have regularly scheduled surveys of 

their pipeline potential to adjust their cathodic protection 

devices. Geomagnetic storms give rise to induced currents in 

all pipelines throughout the world. If the corrosion engineer 

surveys are made during geomagnetic storms, improper 

cathodic protection voltage levels may actually cause an 

increase in corrosion. 

Near dip equator locations, strong daytime electrojet 

ionospheric current fields and enhancement of low-latitude 

geomagnetic storm fields induce strong currents to flow in 

the pipelines that parallel the electrojet The great intensity 

of these equatorial source currents exacetbates corrosion 

compared to higher latitudes, and greater attention must be 

paid to cathodic protection devices. The equatorial regions 

of intense induced currents are determined by geomagnetic 

surveys. 

Geomagnetic data from USGS observatories help make 

it possible for the National Oceanic and Aunospheric 

Administration (NOAA) Space Environment Forecast Center to warn U.S. pipeline management companies of the 

present and expected activity levels that are of importance to 

their corrosion protection methods. 

INDUCTION IN 

ELECTRIC POWER GRIDS 

During severe magnetic storms (KP-7), damaging currents can be induced in power grids, particularly at higher 

latitudes. The effects vary from simple tripping of circuit 

breakers resulting in temporary electric power blackouts of 

cities to destruction of expensive power station transfonner 

banks and major economic loss from extensive private and 

industrial power disruption. The problem occurs when 

stonn-induced currents appear in three-phase transformers 

that are electrically connected by long transmission lines. 

Destructive localized heating occurs in the windings, capacitor banks become overloaded and trip out, protective relays 

fail, and power transmission is degraded or lost completely. 

As an example, on August 4, 1972, a large geomagnetic 

stonn (Kp=90) caused the failure of a 230-kV power 

transformer at the British Columbia Hydro and Power 

Authority of Canada. During the great magnetic storm 

(Kp=9+) on March 13, 1989, major field fluctuations centered near southeastern Canada and the northeastern United 

States caused a 9-hour blackout of the 21,000-MW Hydro 

Quebec electric power system. A vivid description of the 

failure was provided by Blais and Metsa (1993) of 

Hydro-Quebec. 
6 GEOMAGNETISM APPUCATIONS 

Extreme on 

rch 13, 198~ 

Flgure 4. Areas of igneous rocks (shaded area) and auroral zone (diagonal-lined area). Interconnected power systems are vulnerable in the overlapping regions during geomagnetic 

storms. Modified from Kappenman (1992). 

Telluric currents induced by the storm created harmonic voltages and currents of considerable intensity on the La Grande network. Voltage asymmetry on the 735-kV network reached 15 percent. Within less than a minute, 

the seven La Grande network static var compensators on line tripped one 

after the other ... With the loss of the last static var compensator, voltage 

dropped so drastically on the La Grande network (0.2 p.u.) that all five lines 

to Montreal tripped through loss of synchronism (virtual fault), and the 

entire network separated. The loss of 9,450 MW of generation provoked a 

very rapid drop in frequency at load-centre substations. Automatic underfrequency load-shedding controls functioned properly, but they are not 

designed for recovery from a generation loss equivalent to about half system 

load. The rest of the grid collapsed piece by piece in 25 seconds. 

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 
COMMUNICATION SYSTEMS 7 

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 

the signal plane of polarization related to severe changes in 

total electron content along the propagation path. Storm-time 

signal phase and amplitude radiowave scintillations disrupt 

both satellite (at 109 Hz) and surface high-frequency (HF) 

communications. The scintillations arise from scattering by 

ionization irregularities in the altitudes above 200 km when 

the ionosphere is dense and unstable during geomagnetic 

storms. 

400 

100 L-.......... ~-----~~------------L--------~----------~ 

sx1011 1011 2x1011 5x1011 1012 

ELECTRON DENSITY, IN NUMBERS PER CUBIC METER 

Figure S. Electron density proftles at middle latitudes during daytime quiet and disturbed geomagnetic conditions. Modified from 

Norton (1969). 

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 

(HF) radio propagation quality for fixed-frequency station 

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. 
8 GEOMAGNETISM APPUCATIONS 

DRX ,-~--~--~----~--~----~--~--~ 

600 

400 

200 

RPQ 

6 

5 

4 

3 

2 

10 11 12 13 14 

MARCH 1989 (DAYS) 

15 16 

Figure 6. Correspondence of the daily range of the X-field component (DRX, in nanoteslas) in subauroral, auroral, and polar cap 

zones to the radiowave propagation quality index (RPQ) for paths 

from western Germany to the United States, Iran, Australia, Japan, 

Italy, and England during a geomagnetic storm period in March 

1989. Modified from Hruska and others (1990). 

GLOBAL POSITIONING SYSTEM 

The Global Positioning System (GPS) now used for 

navigation and surveying depends on propagation-time 

delays for spread-spectrum signals transmitted by 24 satellites. These satellites are in six separate, near-circular orbital 

planes at an altitude of about 20,000 km. Signals from at 

least four can be received at any given point near the Earth's 

surface. The signals, detected simultaneously by the user's 

uniquely designed GPS receiver, are automatically processed to determine global position. For authorized government users, accuracy is expected to be 8 m absolute position 

and to be 3 mm relative position between two locations. For 

u~tricted users of the system (mostly boat and ship navigation), the accuracy is expected to be 50 m absolute and 5 

mrelative. 

During a geomagnetic storm a disturbed ionosphere 

introduces into the propagation of the encoded signals irregular delays that are proportional to the total ionospheric electron content along the propagation path and inversely 

proportional to the square of the carrier signal frequency. 

Propagation delays due to geomagnetic storms can represent 

errors of 100m in position (Kleusberg, 1993). Storm notifications from warning centers, employing real-time data from 

geomagnetic observatories, alert GPS subscribers to 

positioning error periods during which GPS fixes are unreliable and corrective measures need be considered. 

Ionospheric modelers compute corrections from the available 

geomagnetic activity indices. During some geomagnetic 

storm conditions, rapid variations (scintillations) introduced 

by the ionosphere also do damage to the position reckoning. 

Figure 7 shows, for one month in 1987, the occurrence ofGPS 

problems at times of high geomagnetic field levels. 

CONDUCTIVITY STRUCTURE 

OF THE EARTH 

Two interrelated geomagnetic methods are used in the 

study of the Earth's electrical conductivity. In one method, 

magnetic irregularities caused by various crustal deposits 

are determined by an area survey. The embedded materials 

provide the field changes and can be best interpreted when 

the geomagnetic activity (a noise source) is as low as possible. For proper evaluation of surface surveys for crustal 

magnetic irregularities, the contribution from the Earth's 

main field must be removed. Typically, the International 

Geomagnetic Reference Field or Definitive Geomagnetic 

Reference Field (IGRF and DGRF, prepared with field 

models designed by USGS and other national organizations) representation of the main field in the study region is 

subtracted from the data set. 

In the other method, natural geomagnetic field fluctuations due to source and induced currents are interpreted to 

give a multidimensional crustal conductivity structure or a 

one-dimentional proftle of the upper mantle. A disturbed 

field is required to provide the frequencies for probing the 

Earth's crustal layers, and long-wavelength quiet-field variations are used to probe deeper, into the Earth's mantle. 

SURFACE AREA TRAVERSES 

Magnetic irregularities occur because of the variation in 

the percentage of magnetite (Fe304) or related minerals in 

the Earth. Magnetite is present in most rocks, although perhaps just very small amounts. Both permanent and induced 

magnetization are the principal sources of fields for these 

surveys. Surveys are normally performed in geomagnetically quiet times. Compensation for quiet-field variations are 

obtained by subtraction of the variations determined from a 

stationary, local baseline field magnetometer (commonly at 

a standard geomagnetic observatory). The profiling method 

depends on the changes in magnetic response resulting from 

the variations in amount of buried magnetite. Occasionally, 

horizontal gradient measurements are appropriate. Proper 

interpretation of the surveys depends on accurate collation of 

the magnetic information with surface geology, gravity 

records, and seismic proftles. 
CONDUCTnnTYSTRUCTUREOFTHEEARTH 9 

200 r- • • e1 • I • I I I • I 

IReported GPS problems 

·--

I- -

(/) 

<( - - _J 

(/) 

UJ 

1-

0 

z 

<( - - z 

?; 

u..i' 100 r- - (!) 

z 

<( 

a: 

> I- - _J 

a: 

::::l 

0 

:I: I- -

'-- -

-

II II l1l ll1 II IIIII I 

-

II~ ll1 II I II I II 

I 1 I I I 

145 150 155 160 165 170 175 

DAY 1987 

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). 

Some small-area traverses are simply interpreted 

from the change in field strength and direction as the sensor is moved along a given line. Figure 8 illustrates the 

different responses of a magnetometer for a simple magnetite alignment. In addition to surveys for mineral discovery 

and geological interest, buried archaeological formations 

have been revealed by magnetic surveys of this type. The 

detection is facilitated by the fact that manmade structures are typically of simple geometric design, consistently of one material differing from that of the 

immediate general area, and at shallow depth. The contrasting magnetic properties of archaeological interest are, 

as a rule, easily detected in the uniform overburden of 

soil, water, or rocks. 

A more precise electromagnetic technique for determining the location of anomalous structures in the Earth 

is called "geomagnetic depth sounding" (GDS). This 

method uses the three orthogonal magnetic field components and is the favored means for studies using a 

large-area array of temporary observing sites. Appropriate 

equations relate the vertical Z-component field to the horizontal (H) and depth (D), or X and Y, components for each 

source frequency. Conducting structures are interpreted 

using special contour plots of the region (Gough and 

Ingram, 1983). 

AEROMAGNETIC SURVEYS 

In an aircraft, a magnetometer is subject to contaminating magnetic fields from four sources within the plane 

(assuming that flight personnel take care with their apparel): 

(1) permanently magnetized aircraft parts, (2) parts whose 

magnetization is induced from the presence of the Earth's 

main field, (3) currents generated in electrically conducting 

materials (eddy currents) as the plane moves through the 

Earth's main field, and (4) fields from the changing static 

charge arising from the plane in flight. Adequate compensation for each of these contaminants is a serious concern of all 

aeromagnetic programs. Compensation methods involve 

placement of permaloy strips and current carrying coils near 

and about the geomagnetic field sensor head and (or) computer program adjustments to the output registration. 

To determine the compensation values for aircraft, prescribed airswings are made over a major magnetic observatory. The airborne magnetometer responses are evaluated 

with respect to the speed, yaw, pitch, and roll of the aircraft 

in the four cardinal directions. Calibration flights, as well as 

surveys, are made on geomagnetically quiet days, and to 

avoid the costly repeat of measurement along flight tracks 

flight planning depends on reports of concurrent and predicted activity from geomagnetic observatories and forecasting centers. 
10 GEOMAGNETISM APPUCATIONS 

+~ -----

VERTICAL 

~---

+ 

HORIZONTAL 

Figure 8. Surface magnetic field response from bmied magnetic 

material exhibiting dipole magnetization. 

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 pattern, adjustments are applied to bring the crosstrack values 

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 
180° W 140° W 100° W 60° W 20° W 20° E 60° E 100° E 140° E 180° E 

60° N 

30° N 

oo 

30 5~ 

60° s 

~s -

Figure 9. Tracks of U.S. Navy Project Magnet flights for which data were deposited at National Oceanic and Atmospheric Administration National Geophysical Data Center. 

Courtesy of R. Bubmann (World Data Center- A, 1994). 

I 

! 

~ 

~ 

~ 

--
12 GEOMAGNETISM APPUCATIONS 

Figure 10. Schematic representation of the detection of seamounts using aeromagnetic techniques. From U.S. Naval Ocean Research and Development Activity report (1993). 

ocean-bottom magnetic records. In both directions perpendicular to mid-oceanic ridges, ship-towed magnetometers 

have detected symmetric patterns of magnetization that 

were acquired during cooling of hot magma coming from 

spreading-region mid-oceanic ridges just tens of meters 

wide. Since the first discovery of the patterns, national 

geomagnetic programs for ship-towed magnetic surveys 

(fig. 12) have had as their objective the global establishment of the chronologies for sea-floor geomagnetic polarity 

changes so that the structure and evolution of the sea floor 

can be understood. 

The magnetometers are usually towed at about 

500-1,000 ft (152-305 m) from the vessel. Ocean surface 

measurements are made at about 50 ft (15 m) below the 

surface for stabilization. Ocean-bottom magnetometers 

have been towed at dephts of about 3,000 ft (914 m). On 

occasion, gradient determinations are made with separated sensors towed in line. Compensation for the permanent and induced magnetization of the ship and for eddy 

currents is similar to that for aircraft, although greatly different in magnitude, and relies on standard geomagnetic 

observatory calibration. 
CONDUCTnnTYSTRUCTUREOFTHEEARTH 13 

Figure 11. Field contour chart for the State of Minnesota compiled from aeromagnetic data stored at the National 

Oceanic and Atmospheric Administration National Geophysical Data Center. Courtesy of R. Buhmann (World 

Data Center-A, 1994). 

Both proton (total-field) magnetometers and the combination of fluxgate (vector) and proton (for calibrating 

the fluxgate) magnetometers are used in ship-towed systems. Ship-towed recordings have been valuable in augmenting the data base for global magnetic field modeling. 

As knowledge of the geology of continental margins has 

become increasingly important for oil exploration, 

shipborne magnetic surveys of the coastal regions have 

gained further significance. 

MAGNETOTELLURIC SOUNDING OF 

THE EARTH'S CRUST 

Remote sensing of the Earth's subsurface crustal structure is an important part of the commercial development of 

mineral and energy resources. One discovery technique 

involves determination of the ratio of orthogonal surface 

electric and magnetic fields to resolve the electric conductivity (siemens per meter) 0r resistivity (ohm-meters) proftle of 

the Earth beneath the measurements. 
14 GEOMAGNETISM APPUCATIONS 

Figure 12. Schematic of ship-towed magnetometers responding 

to ocean-bottom magnetic striations from sea-floor spreading of 

magma with frozen-in field alignments. From U.S. Naval Ocean 

Research and Development Activity report (1993). 

To provide some understanding of the method, let us 

consider the following. The currents that are induced to flow 

in the conducting Earth decrease with the oscillation period 

and with the depth into the conductor. The parameter, a. 

called the "skin depth," defines the distance within a uniformly conducting Earth at which the diminishing field of 

oscillating period, T seconds, reaches lie (about 37 percent) 

of its surface amplitude (where e=2.71828 ... ). The attenuation of source signal with depth into the Earth is similar to 

the radiowave fading experienced by an auto driver listening 

to his radio on entering a tunnel. In addition to a 

period-amplitude relationship, there is a phase rotation of the 

electromagnetic wave as it penetrates the conducting 

medium. 

Measurements at the Earth's surface give the sum of the 

induced and source fields. The ratio of the phase and amplitude of the surface electric and magnetic fields provides the 

specific information on the interior conducting Earth structure. In operation, this method of determining electric conductivity has been named ''magnetotelluric" (MT) sounding 

for the combination of magnetic and telluric fields that are 

used. The equations that allow us to convert (transfer) the 

field measurements into conductivity are called "transfer 

functions." 

To a first rough approximation in the region of the measurement, the apparent conductivity, cr, in siemens per meter, 

is 

where T is the field fluctuation period in seconds, By is the 

magnetic field in gammas, and Ex is the electric field in millivolts per kilometer (the subscripts x andy refer to orthogonal directions). In a given conductivity half-space, longer 

(/) 

cr: 

w 

fw 

~ 

0 

..J 

Q 

~ 

::i 

f-

"-

w 

0 

N 

RESISTIVITY, p, IN OHM-METERS 

1 10 100 1000 

o .--,-,,,ITr--,-,rn~--.-,,TITn 

200 

400 

600 

I. / 

0.1 

.. ...... -·-

I 

I 

• \ 

• I 

I 

f 

I 

I 

I • 

0.01 

CONDUCTIVITY, cr, IN SIEMENS PER METER 

0.001 

Figure 13. One-dimensional inversion of magnetotelluric data to 

show conductivity (or resistivity) as function of depth. Dashed line 

is smoothed form computation (dots). Solid line represents conductivity model structure. Modified from Schmucker (1970). 

period field fluctuations penetrate to greater depths. The 

apparent conductivity thus determined is assumed to be valid 

to a penetration depth of Z kilometers approximately given 

by 

Present analysis techniques are considerably more 

complex than these equations indicate. The relationship of 

both the amplitude and phase of the electric and magnetic 

fields enters the more exact computations. With observations at a multitude of frequencies, a depth profile of the 

apparent conductivity for the region is obtained. This prome is taken to be a smoothed representation of the true 

conductivity. A model structure of the crust is then 

adopted with an appropriate depth scale to fit the MT profile (fig. 13). Processing the data in this way is called 

"MT inversion" (Oldenburg, 1990). 

The interrelationship of the magnetic arid electric (telluric) field (MT) vectors is substantially more intricate for 

the realistic anisotropic and laterally inhomogeneous Earth 

in which conductivity must be represented as a function of 

directions other than depth. The problem resolves into modeling minimum-structure arrangements of conducting 

masses of defined boundaries in two or three dimensions 

(called 2D or 3D models). Figure 14 illustrates the 2D model 

of the Juan de Fuca plate subduction zone of southwestern 

Canada obtained from a combination ofMT and GDS survey 

techniques. 
CONDUCTnnTYSTRUCTUREOFTHEEARTH 15 

PACIFIC OCEAN---~~VANCOUVER ISLAND~ o( > ~MAINLAND 

SEAWATER or-=---

If) 

a: 

w 

20 

tu 40 

2 

0 

--' 

>2 

z 

~- 60 

"-

w 

0 

80 

200 

-

-

D 

D 60 ohm-meters 

D 200 ohm-meters 

D 5,000 ohm-meters 

150 

GEORGIA BRITISH 

COAST 12 10 14 STRAIT COLUMBIA 

100 50 0 50 100 150 200 

DISTANCE, IN KILOMETERS 

Figure 14. 2D electrical resistivity model of Juan de Fuca plate subduction region near Vancouver Island, Canada. Black horizontal bars 

beneath sites 10, 12, and 14 on Vancouver Island indicate the seismic reflector location. Modified from Kurtz and others (1990). 

There are always measurement errors, so the data cannot be fit exactly. Often more than one model will satisfy the 

MT responses. Special techniques have been developed to 

evaluate the misfit between the data and the model. Usually, 

the true distribution of conducting materials is not obtainable, nor is the true error of the computations. Therefore, 

there is always considerable reliance on a comparison with 

seismic evidence of the Earth's substructure; major changes 

at specific depths must be collocated. Inverse modeling 

accuracy also depends on the detailed laboratory measurement of geologic material conductivities. 

Natural micropulsation field variations, which have a 

wide range of frequencies and great horiwntal uniformity (at 

least at latitudes away from the auroral zones), are a convenient source field in the application of magnetotelluric prospecting techniques. Although crustal modeling concerns the 

ftrSt 10 km below the Earth's surface, reliable conductivity 

determinations have been extended to about 200 km. The 

MT method is used principally for mineral discovery, geothermal prospecting, and general evaluation of the crustal 

structure and geology. The technique is particularly valuable 

where sedimentary overburden might conceal important 

substructure. The New Madrid, Missouri, seismic zone and 

San Andreas, California, fault zones have been delineated by 

MTmethods. 

Special continuous MT monitoring is occasionally 

employed in active regions where crustal conductivity properties can change. Such measurements have proved useful in 

active volCanic and seismic fault zones and have the potential of providing warning precursors of major surface 

disruption. MT techniques have also been used to determine, 

from the Earth's surface, the bum-front location of firedamaged coal mines. 

CONDUCTIVITY OF THE 

EARTH'S UPPER MANTLE 

For application to conductivity modeling, the size of 

geomagnetic current sources must be large with respect to 

skin depth in the probed region. The source must also be well 

behaved; meaning it must change only slowly and smoothly, 

in linearly prescribed ways. The quiet-daily variation, Sq, 

currents obtained from field records of standard magnetic 

observatories meet this requirement as a source for determining the Earth's upper mantle conductivity. 

The method of analysis uses the separated external 

and internal spherical harmonic analysis (SHA) field representation coefficients. The special mathematical properties established for the SHA allow each harmonic to be 

analyzed independently for the induction response. The 

method (Campbell, 1987) involves computation of the 

ratio of the internal to the external components expressed 

in terms of phase and amplitude and the establishment of a 

transfer function for producing the conductivity profile 

from the spherical harmonic coefficients. Then for each 

SHA coefficient the depth (d, in kilometerS) to a layer of 

conductivity (a, in siemens per meter) that could be 

responsible for the induced field measurements is 

obtained. 
16 GEOMAGNETISM APPUCATIONS 

0.50 

a: 

w 

f4.5 w 

iC :2 

w 0.35 4.4 i= f- z w w 

:2 4.3 u 

~ u 0.30 iii I 4.2 ::::> 

s u 

~ 0.25 4.1 a: 

w 

~- a.. 

4.0 (J) 

> :2 

i= 0.20 <( 

3.9 a: u (!) ::::> 

0 0.15 3.8 ~ z 

u 

0 3.7 ~ Ui 0.10 z 3.6 w 

0 

0.05 3.5 

3.4 

3.3 100 200 300 400 500 600 700 

DEPTH, IN KILOMETERS 

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). 

Because the profiles that are obtained are simply the 

best fit of conductivities (a) that can produce the same fields, 

the values are called "substitute" conductors at "equivalent" 

depths. The reality of the computed values depends on independent corroborating seismic and laboratory evidence. In 

figure 15, the upper mantle density, obtained from seismic 

data for the same depth range, is plotted together with the 

substitute electrical conductivity. The similarity in pattern 

changes with depth supports the assumption of a true representation of the upper mantle conductivity profile determined from the magnetic fields. 

Measurement of electrical conductivity by geomagnetic procedures, established independent of seismic evidence, provides valuable insight into upper mantle 

composition. Conductivity values can differentiate 

between materials determined as possible compositions 

for the Earth's structure by high-temperature and -pressure laboratory modeling. 

PALEOMAGNETIC AND 

ARCHAEOMAGNETIC STUDIES 

Ocean-bottom mapping of mid-ocean ridges, which 

represent the spreading zones of continental drift, shows 

recurring field-direction patterns that extend perpendicular 

B c 0 

NORMAL 2 

J: 

1- (/) (.9 a: z 4 <l: UJ UJ a: ~ >- 1- u.. (/) 0: 0 0 

...J :5 6 (/) 

UJ 0 z 

u:: CL 0 

:::; 

...J 

8 ~ 

REVERSED 

10 

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. 

to the spreading axis. Molten mantle materials at temperatures above the Curie point (the temperature at which magnetization is lost) extrude into the ridge and harden, locking 

in a record of the main field at the time of cooling. Gradually, 

at about 1-3 em/year, this upwelling zone lays down a "magnetic tape recording" of ancient magnetic field directions 

that clearly shows reversal patterns back many millions of 

years (fig. 16). For regions where the present spreading rate 

is established, a consistency assumption provides a preliminary dating of reversals. The present field direction is called 

''normal," the opposite direction "reversed." 

Paleomagnetic reversals of field over time form an 

irregular pattern, much like tree growth rings, that can be 

matched to samples of unknown date to establish their time 
MAGNETIC CHARTS 17 

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 

the paleomagnetic evidence, give record of the formation our 

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 

and others, 1977). 

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 104 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 

the modeled main field, showed a relatively sharp change in 

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 
18 GEOMAGNETISM APPUCATIONS 
40° •• 

30° I 

, 

.... ·..: ....... . 

, 

I 

I 

I 

...,.. 

, 

, 

, 

.I ' 

I ' 

/ ·Declination in degrees 

-Change in minutes per year 

110° 

40° 

30° 

. . . . . . -... ·'\:.'• ·, I 

~ 

'-~ ... - '8·e.v 

' ' 

100° goo 80° 

Figure 17. Horizontal intensity of geomagnetic field contoured for global presentation (facing page) and continental United States (above, this page). Courtesy of N. 

Peddie (USGS, 1994). 

!: 

E; 

z 

d 

(') 

g 

~ 

fA 

...... 

\0 
20 GEOMAGNETISM APPUCATIONS 

the Earth's field from space; however, such measurements 

have introduced their own unique problems. Internal to the 

satellite orbit are ionospheric dynamo and auroral electrojet 

currents as well as high-latitude electrojet and field-aligned 

currents; the satellite data determined internal (main) field is 

therefore contaminated by sources that are avoided at surface 

observatories. Satellite lifetimes are short with respect to 

surface observatory locations, thus curtailing most secular-change determinations. 

Special corrections to the satellite data must be made 

because one of the requirements for the application of the 

spherical harmonic analysis is that no currents passed across 

the surface of analysis. For satellite altitudes of 500 km or 

more, at higher latitudes strong currents flow along the 

Earth's field lines through the analysis region. Presently the 

ground and space data are best combined for the magnetic 

field analysis epochs. 

Most countries provide maps of their respective areas at 

various levels of detail. Global and U.S. magnetic fields are 

determined by the USGS every 5 years. These charts are 

available to the public, at nominal cost, from the USGS in 

Denver. 

NAVIGATION BY MAGNETIC CHARTS 

Originally, the compass was a necessary part of maritime navigation. A ship's latitude could usually be determined at night from the elevation of stars in the region of 

North or South Pole zenith; however, ftxing the ship's longitude depended on a daytime determination of the difference 

between the time of highest Sun elevation and the ship clock 

time, set when leaving home port. Between the night and 

daytime latitude-longitude determinations, and in overcast 

conditions, the ship navigated with the compass. The prime 

years of global oceanic exploration fostered designs of 

improved ship clocks for better longitude fixes. The 

east-west distortions of charts from those times give evidence of those early errors in longitude. 

Using maritime charts of the global field, the magnetic declination angle (difference between geographic 

north and the compass direction) became bOth a method 

for guiding the ship's course and for verifying a longitude 

determination. Global magnetic charting became a major 

government-supported science. Ships equipped with 

advanced technology now use the GPS satellite positioning, yet most of the world's ocean-going vessels still navigate with the magnetic compass and charts. All major 

countries of the world provide magnetic charts as navigational aids for their maritime fleets. Although our flight 

maps also contain magnetic declination information, the 

information is rarely used by large aircraft except as a traditional means for identifying airport runway names; small 

aircraft, however, use compass readings and magnetic 

chart information for inflight course alignment. 

6 

a: 

UJ 

N 

>- a: 

<( 

a: 

1-

ii5 

a: 

~ 

...J 

...J 

<( 

LL. 

z 

<( 

a: 

...J 

<( 

::> 

z 

z 

<( 

a: 

UJ 

co 

~ 

E 

E 

0 

0 

N 

~ 0 1-

0 

a.. 

(/) 

z 

~ -100 

1910 1920 1930 1940 

YEAR 

1950 1960 

2 

3 

1970 

Figure 18. Smoothed annual total rainfall at three locations in 

South Africa compared with double sunspot cycle. Locations: 

(1) Rustenburg (lat 26° S., long 27° E.); (2) Bethal (lat 2~ S., 

long 30° E.); and (3) Dundee (lat 28° S., long 300 E.). Modified 

from King (1975) (reported in Herman and Goldberg, 1978). 

The North and South Pole positions and the 

non-dipole field anomalies move steadily westward at differing rates. These changes require constant recharting of 

the declination (along with contours of the anticipated 

annual rate of change) every 5 years. Knowledge of the 

local magnetic declination is in constant use throughout 

the world. Almost all topographic maps of small areas 

contain, along with the date of map production, a graphic 

indication of the magnetic declination (and annual 

change) for the area Seasoned backcountry travelers use 

a map and compass to establish their routes. Surveyors 

still make note of compass alignments; some older property lines, originally run with only compass headings, 

have had to be recreated from historic magnetic information. The use of magnetic field charts continues to be a 

necessary .Part of modem man's activities. 
GEOMAGNETISM AND WEATIIER 21 

Figure 19. Global annual rainfall difference (in centimeters) between sunspot maximum and sunspot 

minimum, 1860-1917. Diagonal-lined area is greater rainfall at maximum. Modified from Claton (1923) 

(reported in Herman and Goldberg, 1978). 

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 equatorial latitudes, average annual rainfall is greater during years 

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 

the recurrent high-latitude, Northern Hemisphere pressure 

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) 

180 

170 ---Days of Ethesian winds 160 - - - - - Number of sunspots 150 

140 

130 (/) 

1-

120 0 

a.. 

(/) 

110 z 

::J 

80 - 100 (/) 

LL 

90 0 

a: 

80 w 

III 

~ 

::J 

~ 

70 ::J 

z z 

z 60 - 60 

<( 

a: 50 w 

a.. 50 - (/) 40 

>- <( 30 0 

40 - 20 

10 

32 - I I 

1890 1900 1910 1920 1930 1940 1950 1960 

YEAR 

Figure 20. Ethesian winds occurrence, Athens, Greece, 

1891-1961. Modified from Carapiperis (1962) (reported in Herman 

and Goldberg, 1978). 

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 
22 GEOMAGNETISM APPLICATIONS 

Figure 21. Polar view of averaged surface pressure changes 3 days following 14 geomagnetic storm sudden commencements in the Northern Hemisphere for winter months of 1950-1970. Contours are differences in millibars, and 

regions of pressure increase are diagonal-lined. Modified from Sidorenkov (1974) (reported in Herman and Goldberg, 

1978) 

vortex index was determined from the high-latitude Northem Hemisphere area enclosed within the 300-millibar atmosphere pressure contour. Figme 23 shows that the vortex 

index regularly goes to a minimum value about 1 day after 

the sector boundary crossing. Global magnetic observatories 

are a necessary part of the scientific exploration of the 

weather-field relationship. 

GEOMAGNETISM AND LIFE FORMS 

In 1974 RP. Blakemore of the University of New 

Hampshire discovered freshwater pond magnetotactic bacteria that aimed their movements by sensing the magnetic 

North Pole direction (Frankel and Blakemore, 1980). During 

their growth, these bacteria biologically synthesized as many 

as 20 highly purified cuboidal magnetite crystals, each 

approximately 50 nanometers in size, arrayed in a dipolar 

sbing aligned along the long axis of their bodies. Other magnetotactic bacteria, as well as magnetotactic green alga, were 

subsequently discovered. The importance of the alga was 

that, although bacteria do not have a cell nucleus and other 

cell features, algae and higher organisms do. 

The search for similar biological examples of crystalline magnetite subsequently led to discoveries of magnetite 

in the abdomens of honey bees and in the brains of homing 

pigeons, tuna, blue marlins, green turtles, dolphins, whales, 

and others. Honey bees dance to describe locations of their 

feeding sources; the dances are sensitive to the local magnetic field direction, as well as to the Sun's direction (Gold 

and others, 1978). There are also indications that the navigation of birds involves a sensing of geomagnetic field direction; homing-pigeon rally organizers, wary of disturbance 

fields, depend on forecasts of no geomagnetic disturbances 

to schedule their programs. In 1980, T.P. Quinn of the University of Washington showed that a 90° directional shift in 

horizontal component of the Earth's magnetic field caused 

approximately 90° changes in the mean direction of movement of sockeye salmon fry at night Baker and others (1983) 

of the University of Manchester discovered magnetite in the 

ethmoid cavities. of. humans. 

Maugh (1982) reviewed the findings of magnetite in 

organisms and reported that in manunals, "the investigators 

found that the magnetic particles appeared to be surrounded 

by nervous tissue, suggesting the possibility of interaction 

between the particle and th~ brain." Kirschvink and others 

(1985) recently edited a collection of articles on magnetoreception in organisms. 

For many years the special superconducting properties 

of metals had to be studied in liquid helium at temperatures 

near absolute zero degrees Kelvin (-273°C). Recently, 

experimenters have developed materials that become 
GEOMAGNETISM AND UFE FORMS 23 

10m 

24 

20 

16 

11.4 

8.8 

5.3 

2.9 

1.4 

KILOMETERS 

-4 0 +4 

DAYS FROM SECTOR BOUNDARY 

30 

50 

100 

200 

300 

500 

700 

850 

MILLIBARS 

Figure 22. Variation of height difference of pressure surfaces between lat 4()0 -50° N. and lat 6()0 -70° N .. , before and 

after a solar magnetic sector boundary passage on day 0, based 

on superposed epoch analysis of 54 crossings during winters 

of 1964--1970. Approximate heights (in kilometers) of pressure surfaces (in millibars) are indicated. Modified from Svalgaard (1973) (reported in Herman and Goldberg, 1978). 

-6 

, ... I , __ ,..-, 

500,000 km2 

-4 -2 

' ' ' ' ~ 

0 2 

DAYS FROM SECTOR BOUNDARY 

4 6 

Figure 23. Average response of vorticity area index for magnetic 

sector boundary passage on day 0. Dashed curve represents 28 

boundaries, 1967-1970. Solid curve represents 26 boundaries, 

1964--1966. Modified from Wilcox and others (1973) (reported in 

Herman and Goldberg, 1978). 

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. 
24 GEOMAGNETISM APPUCATIONS 

1Q-4 

- EARTH MAIN FIELD 

1Q-5 

1Q-6 

- URBAN NOISE 

1Q-7 - GEOMAGNETIC STORMS 

1Q-8 - GEOMAGNETIC PULSATIONS (/) 

<( 

...J 

(/) 

w 

1- - LUNG PARTICLES ~ 1Q-9 

cl 

...J 

w 

u::: 

u - HUMAN HEART i= 1Q-10 w 

z - HUMAN SKELETAL MUSCLE (!) 

<( 

~ - FETAL HEART 

1Q-11 

- HUMAN EYE (STEADY) 

1Q-12 - HUMAN BRAIN (ALPHA) 

1Q-13 - HUMAN BRAIN (RESPONSE) 

1Q-14 

- SQUID NOISE 

1Q-15 

Figure 24. Magnetic fields of the human 

body compared to geomagnetic field levels 

and to sensitivity of SQUID magnetometer. 

Figure redrawn from Williamson and others 

(1977). 

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 

Table 1. Percentage of days with cases of disease or death from 

myocardial infarction in Sverdlovsk, Russia, as a function of 

active and quiet geomagnetic conditions. 

[Data from Novikova and Ryvkin (1977)] 

Morbidi~ Mortali~ 

Year Active Quiet Active Quiet 

1961 78.8 70.4 24.5 18:8 

1962 73.5 65.3 35.0 25.7 

1963 77.6 73.2 24.9 20.6 

1964 62.0 57.0 36.4 21.7 

1965 79.1 75.0 27.9 25.3 

1966 90.8 75.4 59.5 40.9 

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 

the development tree or may simply have a similar secondary 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 10-3-10-5 of 

their original time scale. For stations whose main use is other 

than warning and forecasting, the recording system can be 

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 
MAGNETIC OBSERVATORIES 25 

" 

3 

cn2 

0 

" 

3 

cn2 60 

20 

Q. 

<( 

Q. 

<( 

60 

40 

20 

0 

Q. 

<( 

Figure 25. Comparison of group psychopathological syndrome expression index, S, at Moscow mental hospital and geomagnetic index, 

Ap, for 9 months of 1975 and 1976. Modified from Nikolaev and others (1976). 

communication systems. Data from five of these stations are 

used in the preparation of the planetary geomagnetic activity 

indices for worldwide applications. On-line recordings of six 

of the USGS observatories are provided to the NOAA Space 

Environment Forecasting Center for integration into its 

warning and prediction services. 

For field modeling and charting, additional geomagnetic measurement sites ("repeat stations") are occupied on 

a 3-5-year repeated schedule (fig. 27). Special efforts are 

made at these locations to obtain the best quiet-condition 

absolute field levels. 

World Data Centers (WDC's), started during the International Geophysical Year (1958), are now the prime 

depositories for most national geophysical records in the 

form of charts, microfilm copies of charts, tables of values 

read from the charts, or computer files of digital recordings. 

The principal centers for archiving of the geomagnetic 

records are in Boulder, Colorado (WDC-A), Moscow, Russia (WDC-B), and Kyoto, Japan (WDC-C). Geomagnetic 

records are distributed, free of charge, to organizations that 

deposit data at the WDC' s and, for a small fee, to other 

individuals or organizations. Besides archiving and distributing geomagnetic records, these centers perform a great 

number of other services of importance to national and 

research requirements, such as producing indices, converting geomagnetic data to application formats for easy digestion, and collating related solar-terrestrial phenomena. 

Geomagnetic data from U.S .. observatories are carefully 

edited, cleaned of extraneous noises and quality checked at 

the USGS before deposition at WDC-A. In return for this 
26 GEOMAGNETISM APPUCATIONS 

Figure 26. Global magnetic observatory locations depositing records in World Data Center-A in 1994. Courtesy of S. McLean and M. 

Davis (World Data Center-A, 1994). 

Figure 27. Locations of USGS standard magnetic observatories (solid circles) and sites occupied repeatedly for mapping purposes (solid 

squares). 
TROPOSPHERIC AND IONOSPHERIC FIELD OBSERVATIONS 27 

Figure 28. Locations of INTERMAGNET observatories operating in 1993 and reception range (footprints) of four geostationary satellites servicing these observatories. Courtesy of A. Green (USGS). 

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 INTERMAGNET 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 

is used; however, the proton magnetometer has been a 

favorite for rocket measurements of ionospheric fields 

because of its relative simplicity, low weight and cost, insensitivity to temperature and voltage fluctuations, and relative 
28 GEOMAGNETISM APPUCATIONS 

Table 2. Some features of principal satellites used for geomagnetic field mapping. 

[Data from R. Langel and J. Heirtzler (Goddard Space Flight Center, National Aeronautics and Space Administration)] 

Instrumentation 

Satellite (nanotestlas) Type Inclination 

COSMOS-4g Proton (22) Scaler 50° 

OG0-2 Rubidium ( 6) Scaler 87° 

OG0-4 Rubidium (6) Scaler 86° 

OG0-6 Rubidium (6) Scaler 82° 

MAGSAT Fluxgate (22) Vector g7o 

DE-2 Flux gate (1 00) Vector goo 

UARS Fluxgate (20) Vector 57° 

POGS Fluxgate (30) Scaler goo 

FREJA Fluxgate (30) Vector 63° 

independence of sensor orientation. Thus far, rocketbome 

field observations have been limited to magnetometer sensitivity levels of a few gammas. 

Penetration of the ionospheric current carrying layers is 

indicated by the difference between the measured field and 

that expected from an inverse-cube-law decrease of the 

Earth's field (determined from the global field models) with 

increasing height Successful flights reach well above the 

100-120 km current layer to about 250 km. TheE-region 

location of ionospheric dynamo currents and the auroral and 

equatorial electrojet currents has been verified by rocketborne magnetometers. The influence of these currents on 

surface observatories has been interpreted from the rocket 

measurements. This information has led to better use of surface observatories as ionospheric disturbance monitors. 

MAGNETOSPHERIC MEASUREMENTS 

Equatorial and polar satellites are generally restricted to 

far above 300 km for atmospheric drag reasons; 

600-2,000-km altitudes and 1.5-3.5-hour orbits are typical. 

Geostationary satellites are limited to about 6.6 Re distance 

in the equatorial plane. The geostationary GOES weather 

satellites have become monitoring platforms for sampling 

local high-energy particles, X-rays, and geomagnetic fields. 

On occasions of a major solar-terrestrial disturbance, the 

day-side magnetospheric boundary may be pressed in past 

this geostationary position, providing shock -boundary crossing measurements for the in situ magnetometer and important disturbance and forecasting information. Interpretation 

of the surface observatory records has been confrrmed by 

comparisons with satellite field measurements. As a result, 

standard surface observatories have become the continuous, 

and less expensive, magnetospheric process monitors. 

Polar-orbiting satellite magnetometers make significant 

contributions to global magnetic field mapping. The problems in space are the detection of a small ambient field in the 

presence of sp~raft noise and the separation of spatial 

from temporal field changes. The inclination of these satelAltitude 

(kilometers) Operation 

261-488 October-November 1g64. 

413-1,510 October 1g65-September 1g67. 

412-g08 July 1g67-January 1g6g. 

3g7-1,og8 June 1g6g-July 1g71. 

325-550 November 1g7g-May 1g80. 

30g-1,012 August 1g81-February 1g83. 

574-57g September 1gg1-Present. 

-700 January 1gg1-0ctober 1gg3_ 

5g5-1,75g October 1gg2-Present. 

lites (the angle between the Earth's spin axis and the perpendicular to the plane of the orbit) should be close to 90° to 

provide full coverage of the Earth. An inclination value 

greater than 90° is used to indicate a rotational path of the 

satellite opposite to the Earth's rotation direction. An error in 

the pointing direction of the field sensors of 5 arc seconds 

can introduce a 1.5-nT error in measurements. Trajectory 

errors are mostly due to limitations in tracking-station timekeeping and to changes in satellite acceleration with varying 

magnetospheric conditions. At a typical satellite velocity of 

about 7 km/second, a 0.1-second time accuracy translates to 

700-m location accuracy along the track and 4.2-nT difference in total field. 

Early satellite geomagnetic field measurements were of 

the total-field (scaler) type made with "absolute" field measuring proton or rubidium magnetometers. In recent years, 

satellites have used the three-axis (vector) fluxgate magnetometers calibrated with collocated absolute sensors to about 

2-5 gammas. The sensors are usually placed on a boom 

2-8m from the body of the spacecraft whose attitude in 

space is determined with a star sensor and (or) GPS. A mechanism is then needed to monitor the magnetometer location 

with respect to the body of the spacecraft. The main electronics needs to operate in the range of about -10°C to +50°C. 

Table 2 lists some of the features of the principal satellites 

that have, to date, provided significant global field mapping 

information. 

Several additional magnetic field missions are planned 

at this time. The POGS measurement are to be continued 

from 1996 to 2000 with improved sensor designs on Defense 

Meteorological Satellite Program (DMSP) spacecraft. In 

September 1995 the Danish OERS1ED satellite is to be 

launched for an approximately 14-month-long mission. 

OERS1ED will use scalar·proton and vector fluxgate magnetometers in an elliptical 400-800-km polar orbit that is to 

drift slowly from a nearly Sun synchronous position. The 

SAC-C satellite with a vector fluxgate magnetometer is 

planned for a 1998 launch into a polar orbit at about 

500--600 km altitude. A group of three small satellites 

(S1EDI) projected for lauch in 1997-97 will have 
SOLAR-TERRESTRIAL DIS1URBANCE PREDICTIONS 29 

Table 3. Daily forecast of geomagnetic storm conditions and 

subsequently observed conditions, 1989-1991. 

[Values courtesy ofK.A. Doggett (National Oceanic and Atmospheric 

Administration, Space Environmental Laboratory). Storm conditions 

are defined as daily Ap>50] 

Forecast 

No-storm 

Storm 

No-storm 

1,052 

12 

Observed 

Storm 

23 

7 

magnetometers aboard. A tethered staellite series, SEDS, is 

also expected to have magnetometers. 

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 

the Sun-Earth line. The first satellite, called WIND for solar 

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 

the collection of selected surface observatory and space data 

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. 
30 GEOMAGNETISM APPUCATIONS 

SUMMARY AND CONCLUSIONS 

Applications of geomagnetism expand as our society 

grows in its technological capabilities. As our country 

increases its uses of the space environment, so increases 

the need for more exact information on the physics of the 

magnetosphere. This physics is intimately tied to understanding of magnetic field changes as well as monitoring 

those changes that impact satellite operation. Large geomagnetic storms affect pipelines, power-transmission networks, communication systems, location fixes, and so 

forth. Magnetic mapping plays a major role in the discovery of mineral resources and specification of their 

emplacement Solid-earth geophysics relies on geomagnetism for revelation of our Earth's formation and 

change. Geomagnetism is beginning to play a role in 

atmospheric weather and climate modeling. Magnetic 

charts are continuing their usage in marine navigation. A 

developing science of magnetoreception in organisms 

may offer explanation of reported biological responses to 

geomagnetic fields. The national requirements for geomagnetic disturbance warning and forecasting make growing demands for timely and accurate geomagnetic 

information on a global scale. Our national U.S. Geological Survey geomagnetic observatory network, the 

National Oceanic and Atmospheric Administration Space 

Environment Forecasting Center, and the geomagnetic 

data archival and retrieval facilities of World Data Center-A work together as a multiuse technology of immense 

value to our Nation. 

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Published in the Central Region, Denver, Colorado 

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