U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3029

November 2018

Figure 1. Honolulu Magnetic Observatory in 1915 

(Photograph from Carnegie Institution for Science, 

Department of Terrestrial Magnetism).

Honolulu Magnetic Observatory

Tucked in a grove of thorny mesquite trees, on 

an ancient coral reef on the south side of the Hawaiian 

island of Oahu, west of Pearl Harbor, a small unmanned 

observatory quietly records the Earth’s time-varying 

magnetic field. The Honolulu Magnetic Observatory is 1 

of 14 that the U.S. Geological Survey (USGS) Geomagnetism Program operates at various locations across the 

United States and its territories.

Data from these observatories, Honolulu, and those operated 

by institutions in foreign countries, record a variety of magnetic 

signals related to a wide diversity of physical phenomena in the 

Earth’s interior and its surrounding outer-space environment. USGS 

magnetic observatory operations are an integral part of a U.S. 

National Space Weather Strategy for monitoring and assessing 

natural hazards that potentially threaten important technological 

systems (National Science and Technology Council, 2015).

In support of a national project for geomagnetic field mapping 

and for investigating the complex time dependence of the geomagnetic 

field, in 1899, the U.S. Congress appropriated funds to the Coast 

and Geodetic Survey for the establishment of magnetic observatories 

(Pritchett, 1899). Plans were made for installing several observatories 

at a geographically widely distributed set of sites across the country 

(Bauer and Fleming, 1903), including on what was then the newly 

acquired territory of Hawaii.

The Honolulu Magnetic Observatory was built in 1901, and 

routine data-acquisition operations started in 1902 (Hazard, 1909); 

an early photograph of the observatory is shown in figure 1. Since 

the Honolulu Magnetic Observatory was first installed, it has been 

moved twice, once in 1947 and once in 1960, but each location was 

within a 10-kilometer distance of the original site (Coast and Geodetic 

Survey, 1963). In 1965, geomagnetic monitoring operations and the 

administration of the Honolulu Magnetic Observatory were transferred 

to the short-lived Environmental Science Services Administration. 

Soon after the creation of the National Oceanic and Atmospheric 

Administration in 1970, the Geomagnetism Program was transferred to 

the USGS; a 2013 photograph of the observatory is shown in figure 2.

From 1902 to 1984, the recording of geomagnetic time variation 

at the Honolulu Magnetic Observatory was accomplished through 

measurements using automatic photographic systems; hourly 

magnetic-field values were estimated by hand-scaled measurements 

of paper magnetograms. In 1983, 1-minute resolution data from the 

Honolulu Magnetic Observatory began to be routinely acquired using 

digital-electronic magnetometer systems, and in 2004, 1-second 

resolution data began to be routinely acquired. From 1902 to present, 

auxiliary calibration measurements (typically once per week) have 

been made at the Honolulu Magnetic Observatory site so that 

accurate definitive data can be produced.

Today, the Honolulu Magnetic Observatory is an integral part 

of the USGS geomagnetic observatory network that is supported by 

the U.S. Government for a variety of pure and applied geomagnetic 

science applications (Love and Finn, 2011). The Honolulu Magnetic 

Observatory is 1 of more than 120 observatories operated around the 

world and whose operations are coordinated and promoted through History the INTERMAGNET Consortium (Love and Chulliat, 2013).

Magnetic Signals

Data from the Honolulu Magnetic Observatory provide us with 

a history of geomagnetic field variation; in figure 3A an hourly and 

annual-mean time series of the horizontal intensity of the field from 

1902 to 2016 is shown. Over this long duration of time, the horizontal 

intensity at the Honolulu Magnetic Observatory has exhibited a 

general decrease—part of a global diminishment in geomagnetic 

field intensity caused by dynamic geophysical processes in the Earth’s 

deep interior. More rapid variation, caused by transient change in 

the Earth’s surrounding outer-space environment, especially during 

transient geomagnetic storms, are caused by occasional disturbances 

in the solar wind plasma—electrically charged particles that are 

emitted from the Sun.

In figure 3B a 1-minute resolution time series of variation 

in horizontal geomagnetic intensity at the Honolulu Magnetic 

Observatory during the great magnetic storm of March 1989 is 

shown. During this storm, geomagnetic field variation generated 

electric fields in the Earth’s electrically conducting interior that 

caused the collapse of the entire high-voltage transmission system 

Figure 2. Honolulu Magnetic Observatory in 2013

(Google Earth, LDEO-Columbia, National Science Foundation and National 

Oceanic and Atmospheric Administration.)
of Québec, Canada, plunging the province into an electricity 

blackout (Allen and others, 1989). The March 1989 and Halloween 

(October 2003) storms caused operational stress in high-voltage 

electricity systems across the United States as well.

The Honolulu Magnetic Observatory is one of only four 

observatories in the world which contribute data used to calculate 

the storm-time disturbance index, a fundamental measure of 

magnetic-storm intensity (Sugiura and Kamei, 1991). Data from 

the Honolulu Magnetic Observatory are used by USGS scientists 

for analysis of storm-time magnetic disturbance (Rigler, 2017), 

and to analyze the statistics of magnetic storms that have occurred 

Figure 3. Time series of horizontal 

intensity measured at the Honolulu 

Magnetic Observatory. (A) Hourly 

measurements (blue); annual means 

(green). Observatory relocation, in 1947 

and 1960, introduced discontinuities. 

Intense magnetic storms, usually seen 

as transient negative perturbations in the 

time series, were seen, for example, 

during the International Geophysical Year 

(IGY); the great Quebec (March 12–15, 1989) 

and Halloween (October 29–31, 2003) 

storms are also indicated (red). A 

long-term decrease in horizontal 

intensity at Honolulu is seen since the 

observatory commenced operation in 1902. 

(B) One-minute resolution measurements

recording the Quebec storm (12:00 March

12 to 12:00 March 15, 1989).

in the past, enabling forecasts of the probability of their occurrence 

in the future (Love and others, 2015). 

Detailed analysis of Honolulu data allow scientists to test 

physics-based theories and to develop hazard assessments useful 

for mitigating the deleterious effects of magnetic storms. Honolulu 

Magnetic Observatory data are used by the Space Weather Prediction 

Center of the National Oceanic and Atmospheric Administration, the 

U.S. Air Force, and the National Aeronautics and Space Administration; they are used for academic research into the nature of 

Earth’s interior (for example, Kelbert and others, 2009), and they 

are used by scientists in other countries and by the private sector.

29,000

28,500

28,000

27,500

27,000

100

–100

–200

–500

–400

–300

0

1920

12 18 24 30 36 42 48 54 60 66 72 78 84

1940 1960 1980 2000

A

B Horizontal intensity (H), in nano Tesla (nT) Horizontal intensity (H) variation, in nano Tesla (nT)

Year

Hours from 00:00 12 March, 1989 Universal Time (UT) 

Observatory moved

International

Geophysical

Year

Québec Halloween

Annual mean

Intense magnetic storm

and identifier

EXPLANATION

Allen, J., Frank, L., Sauer, H., and Reiff, P., 1989, Effects of the March 

1989 solar activity: American Geophysical Union, Eos Transactions, 

v. 70, no. 46, p. 1479, 1486–1488, accessed January 31, 2018, at

https://doi.org/10.1029/89EO00409.

Bauer, L.A., and Fleming, J.A., 1903, The magnetic observatories 

of the United States Coast and Geodetic Survey in operation 

on July 1, 1902: Terrestrial Magnetism and Atmospheric 

Electricity, v. 8, no. 1, 11–29, accessed January 31, 2018, at 

https://doi.org/10.1029/TE008i001p00011.

Coast and Geodetic Survey, 1963, Magnetograms and hourly values, 

Honolulu, Hawaii, 1960: Washington, D.C., Department of 

Commerce, p. 1–149.

Hazard, D.L., 1909, Results of observations made at the Coast and 

Geodetic Survey Magnetic Observatory near Honolulu, Hawaii, 

1902–1904, Department of Commerce and Labor, Washington, D.C., 

Government Printing Office, p. 1–130.

Kelbert, A., Schultz, A., and Egbert, G.D., 2009, Global electromagnetic 

induction constraints on transition-zone water content variations: 

Nature, v. 460, p. 1003–1006, accessed January 31, 2018, at

https://doi.org/10.1038/nature08257.

Love, J.J., and Chulliat, A., 2013, An international network of magnetic 

observatories: American Geophysical Union, Eos Transactions, 

v. 94, no. 42, p. 373–374, accessed January 31, 2018, at

https://doi.org/10.1002/2013EO420001.

Jeffrey J. Love and Carol A. Finn

U.S. Geological Survey, 

Geomagnetism Program

References

ISSN 2327-6916 (print) ISSN 2327-6932 (online) http://doi.org/10.3133/fs20183029

Authors

Love, J.J., and Finn, C.A., 2011, The USGS Geomagnetism 

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Love, J.J., Rigler, E.J., Pulkkinen, A., and Riley, P., 2015, On the 

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Letters, v. 42, no. 16, p. 6544–6553, accessed January 31, 2018, at 

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