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Ongoing Geomagnetic field 1-second value measurement by JMA 

MINAMOTO Yasuhiro 

Kakioka Geomagnetic Observatory 

Japan Meteorological Agency 

Abstract

Geomagnetic field one-second data exchanges have become an important issue in 

the INTERMAGNET community and other concerned. The Japan Meteorological 

Agency (JMA) is one of the pioneers of the one-second measurement of the 

geomagnetic field on a routine basis. We have observed and provided one-second data 

values of the geomagnetic field for about 25 years at Kakioka and for more than 10 

years at Memambetsu and Kanoya. This report presents the ongoing one-second value 

measurements at the JMA’s observatories, in particular specifications of the 

magnetometers with a wide frequency range and their calibration to share the 

information on our one-second measurements with the observatory community. 

The main instruments for one-second measurements are three-axis highly sensitive 

fluxgate magnetometers. The fluxgate magnetometers’ conditions are monitored by 

tiltmeters and thermometers. One-second total force measurements are being made only 

at Kakioka, because the baseline of the total force calculated from the H and Z 

components of the fluxgate magnetometer data is so stable. 

The fluxgate magnetometers for the one-second measurement are routinely 

calibrated by using an external DC/AC voltage source that generates magnetic fields 

inside the sensor unit. The magnetometers are also calibrated by applying artificial 

magnetic fields with a set of large square Helmholtz coils where each side is 3 meters 

long and errors in axis alignment are less than 6 minutes of arc. 

Introduction 

The NTERMAGNET program has been collecting 1-minute data of three vector 

components of geomagnetic fields measured at magnetic observatories worldwide. 

Those data are distributed in near real time and are used by academic and industrial 

communities such as space weather, navigation, and solid Earth studies. Scientific 

research and other applications are increasingly in need of geomagnetic field data, with 

more frequent sampling and higher resolution (Korepanov, 2008). Geomagnetic field 

one-second data exchanges have become an important issue within the 

INTERMAGNET program. 

One-second data of the geomagnetic field have been observed and provided for 

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about 25 years at Kakioka (36o

13’56”N, 140o11’11”E), for more than 10 years at 

Memambetsu (43o

54’36”N, 144o11’19”E) and Kanoya (31o25’27”N, 130o52’48”E) by 

the Japan Meteorological Agency (JMA). Kakioka started measuring four 

components of the geomagnetic field, declination (D), horizontal force (H), vertical 

component (Z), and total force (F) every three seconds by using the four optical 

pumping magnetometers in 1970’s and first published 3-second values in our 1983 

yearbook (Kakioka Magnetic Observatory 1984). One-second measurements of the 

geomagnetic field at Kakioka started in 1984 (Kakioka Magnetic Observatory 1985). 

A fluxgate magnetometer was installed for the one-second measurements at Kakioka in 

1990 (Tsunomura et.al., 1994). Similar one-second value measurements started at the 

two branch observatories, Kanoya in 1996 (Kakioka Magnetic Observatory, 1997) and 

Memambetsu in 1997 (Kakioka Magnetic Observatory, 1998). 

These one-second data are available via the World Data Center for Geomagnetism, 

Kyoto (http://swdcwww.kugi.kyoto-u.ac.jp/). The data including the one-second values 

and their products of the three observatories are also provided as a yearbook on a 

CD-ROM entitled “Report of The Kakioka Magnetic Observatory” annually. This 

CD-ROM is available to the public from a data providing project of the Japan 

Meteorological Business Center (http://www.jmbsc.or.jp/). 

This report presents the ongoing one-second measurements at the JMA’s 

observatories. The specifications of the magnetometers with a large frequency range 

are presented along with the calibration results of the magnetometer. Technical details 

of our magnetometer systems are given in Owada et.al. (1998) and references are 

therein.

Three axis highly sensitive fluxgate magnetometer 

A schematic diagram of the system components to measure one-second values of the 

geomagnetic field at Kakioka is shown in Figure 1. A three axis fluxgate 

magnetometer is the main instrument for the one-second measurement. The 

magnetometer receives a trigger from an external clock every second to start a 

measurement. Both digital and analogue outputs are available from this fluxgate 

magnetometer. Only the digital output is used to obtain the one-second values. It is 

important to note that the one-second data have not been corrected by absolute 

measurement. 

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tiltmeters thermometers

DC/AC external signals

External trigger

(clock) Analogue output

Digital output 1-sec value

calibration

Highly sensitive 

fluxgate 

magnetometer 

large Helmholtz coils

(only at Kakioka)

Fig.1 Procedure for obtaining one-second value 

The main specifications of the fluxgate magnetometer at Kakioka are shown in 

Table 1. The magnetometers at Memambetsu and Kanoya are equivalent to that at 

Kakioka. This magnetometer is highly sensitive and stable so that it is capable of 

providing 10Hz sampling data sampling as well. The resolution of 0.01nT is 

practically enough because we estimate natural noise levels at three observatories are 

about 0.03nT. 

Temperature coefficient <0.5nT/degree 

Stability <±0.1nT/day

Measuring range -600nT to +600nT 

Error in axis direction <6’ 

Output noise level <0.05nT 

digital output resolution 0.01nT 

Table1. Specifications of a three axis Fluxgate magnetometer installed at KAKIOKA. 

A timing chart of one-second sampling is shown in Figure 2. External triggers that are 

controlled by a GPS clock are used for time stamps. The three components, H, Z, and D 

are sequentially sampled 32 times each second. The 32 samples for each vector 

component are averaged to output a one-second value. It takes 6 milliseconds to 

sample one datum. As a result, the first 576 milliseconds during a second are used for 

sampling. Then, the last 200 milliseconds are used for serial data output. This sampling 

protocol is restricted by the original analogue/digital converter that was employed when 

192
the system was designed to produce digital output in 1990. The analogue output can be 

sampled at other sampling rates. Currently, the analogue output is highpass-filtered and 

is digitized into 0.1-second (10Hz) values. 

1000msec=1sec

800msec

H

1

Z

1

D

1

H

2

H Z D

32 32 2

output 

RS232c

External trigger 

(GPS)

6msec

3

576ms average for 1-sec value

00:00:00 00:00:01

Fig.2 Timing chart of one-second value sampling 

Two tiltmeters and a thermometer are installed on the inside of the outer cover of 

the sensor unit to monitor the condition of the magnetometer (Fig.1). The northward 

and eastward tilt angles and the temperature are measured every second. 

Figure 3 shows one-year segments of the daily mean temperature and tilts angles of 

the fluxgate magnetometer’s sensor unit at Kakioka. The magnetometer sensor is set 

5 meters below the ground level resulting in a temperature change less than 3 degrees a 

year. Since the temperature coefficient of this magnetometer is less than 0.5nT/degree 

(Table.1), a change due to the temperature in the magnetometer output is less than 

1.5nT throughout the year. 

The tilt varies rapidly in an irregular manner compared to the temperature. 

Tiltmeters’ data in July and September 2007, which are marked by ovals in Fig.3, show 

changes in the magnetometer level due to heavy rains. Those changes can be seen in 

one-second values measure by the fluxgate magnetometer as well. 

The temperature and tilt variations are used to correct the baseline of the variation 

measurement by the fluxgate magnetometer. Our current observatory is to apply the 

baseline to the One-minute values but not the one-second data. 

193
20

40

60

Feb- 07May- 07Aug- 07Nov- 07Feb- 08

0

4

8

12

80

16

20

seconds

sensor temperature

tilt N-S

tilt E-W

°C

Fig.3 Daily mean temperatures and tilts of the fluxgate magnetometer’s sensor at Kakioka from 

Feb.2007 to Jan. 2008 

Continuous total force measurement 

The total force has been observed every second at Kakioka since 1984. An optical 

pumping magnetometer was first used and then replaced with an overhauser 

magnetometer made by GEM, Canada in 1993. The total force is measured every 

minute at Memambetsu and Kanoya by proton precession magnetometers. 

(observed F by Overhauser) – (calculated F by FLUXGATE)

calculated F by FLUXGATE

observed F by Overhauser

0.5nT

0.5nT

0.5nT

(UT)

Fig.4. Differences between the observed total force and calculated total force (top), time derivatives 

of the calculated (middle) and observed (bottom) total force at Kakioka on May 9, 2008 

193.1 
KAK

0.5nT

(UT)

MMB

KNY

0.5nT

0.5nT

Fig.5. Time derivatives of the calculated total force at Kakioka (top), Memambetsu (middle) and 

Kanoya (bottom) on May 9, 2008 

The total force data are available to the user together with the three vector 

components of the geomagnetic field. In addition, the total force is used to check the 

quality of the measurement by the fluxgate magnetometer. 

Comparison between the fluxgate magnetometer data and the overhauser 

magnetometer data suggests that baselines of the fluxgate magnetometer are stable even 

for short time periods. The top panel of Figure 4 shows differences between total force 

measured by the overhauser magnetometer and the total force calculated from the H and 

Z components of fluxgate magnetometer data at Kakioka on May 9, 2008. The 

sampling interval is one-second. Peak-to-peak amplitudes of the total force difference 

between the measured and the calculated are smaller than 0.3 nT during the day. 

The middle panel of Figure 4 is the time derivative of the total force calculated from 

the fluxgate magnetometer data, while the bottom is that observed by the overhauser 

magnetometer. Fluctuations of the calculated total force were significantly smaller 

than those observed by the overhauser magnetometer. That suggests the fluxgate 

magnetometer is stable with a high sensitivity and resolution. 

One-second measurements at Memambetsu and Kanoya have a resolution and 

stability that compares well with Kakioka. The time derivatives of the calculated total 

force at Kakioka, Memambetsu, and Kanoya show the same stability and small 

fluctuations (Fig.5). 

Calibrations of fluxgate magnetometers 

Careful checks and regular maintenance are indispensable for accurate observations. 

As a check procedure, the fluxgate magnetometers for the one-second measurement are 

routinely calibrated by using an external DC/AC voltage source that generates magnetic 

fields at the inside of the sensor unit (Fig.1). Figure 6 shows results of amplitude 

193.2 
0.9985

0.9990

0.9995

1.0000

1.0005

1.0010

1.0015

M ar-03M ar-05M ar-07

H Z

D

repair

calibrations using the DC signal, which is conducted twice a year at Kakioka. The 

variations of the calibration factors are rather small except two data points just after two 

repairs.

Fig.6. Calibration factors of the DC amplitude to the three vector components at Kakioka, from 

Mar. 2003 to Jun. 2007 

The fluxgate magnetometer was damaged in September 2006 and repairs took about 

five months. A set of three-axis square Helmholtz coils at Kakioka (Fig.7) is used to 

calibrate repaired or new magnetometers (Koike et.al., 1990). Magnetometers are 

calibrated by generating artificial magnetic fields with the coils. Each side of the coils 

is three meters long and errors in axis alignment are less than six minutes of arc. This 

large Helmholtz coils can generate a uniform magnetic field whose maximum amplitude 

is 500nT for each axis. 

3m

Fig.7. A Three axis large Helmholtz coils at Kakioka. 

193.3 
Summary 

JMA has observed and recorded one-second geomagnetic field one-second data for 

about 25 years at Kakioka and for more than 10 years at Memambetsu and Kanoya. 

Three-axis fluxgate magnetometers are the main instruments for the one-second value 

measurements. 

The total force (F) calculated from the fluxgate magnetometer measurements are 

sufficiently stable that the total force is recorded every second at Kakioka by JMA. 

The fluxgate magnetometers for the one-second value measurement are routinely 

calibrated using the external DC/AC voltage source signals. Repaired or new 

instruments are calibrated by generating the artificial magnetic fields with a set of the 

large Helmholtz coils at Kakioka.

References 

Kakioka Magnetic Observatory, Report of The Kakioka Magnetic Observatory, Geomagnetism KAKIOKA, 

1983, Kakioka Magnetic Observatory, Kakioka Japan, 1984. 

Kakioka Magnetic Observatory, Report of The Kakioka Magnetic Observatory, Geomagnetism KAKIOKA, 

1984, Kakioka Magnetic Observatory, Kakioka Japan, 1985. 

Kakioka Magnetic Observatory, Report of The Kakioka Magnetic Observatory, Geomagnetism KAKIOKA 

MEMAMBETSU KANOYA CHICHIJIMA, 1996, Kakioka Magnetic Observatory, Kakioka Japan, 

1997. 

Kakioka Magnetic Observatory, Report of The Kakioka Magnetic Observatory, Geomagnetism KAKIOKA 

MEMAMBETSU KANOYA CHICHIJIMA, 1997, Kakioka Magnetic Observatory, Kakioka Japan, 

1998. 

Koike K., S. Nakajima and Y. Shimizu, Performance of fluxgate magnetometer-Sensitivity and calibration 

accuracy (in Japanese with English abstract), Memoirs of Kakioka Magnetic Observatory, 24(1), 

1990. 

Korepanov V., A. Marusenkov , J. Rasson , S. Leonov , A. Prystai , The new INTERMAGNET 1-second 

standard fluxgate magnetometer, Geophysical Research Abstracts,Vol. 10, EGU2008-A-00114, 

2008. 

Owada T., T. Tokumoto, Y. Yamada, M. Ozima, N. Kumasaka, M. Yokoyama, M. Sugawara, K. Koike, 

and Y. Shimizu, Introduction to our New System: Magnetometer for Wide Frequency Range (in 

Japanese with English abstract), Memoirs of Kakioka Magnetic Observatory, 26(2), 1-14, 1998 

(translation is in Technical Report of the Kakioka Magnetic Observatory, Supplement -Selected 

Translations, No. 1, 1-14, March 2003). 

Tsunomura S., A. Yamazaki, T. Tokumoto, and Y. Yamada, The New System of kakioka Automsatic 

Standard Magnetometer, Memoirs of Kakioka Magnetic Observatory, 26(1), 1-14, 1998.

193.4 
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