The Kakioka Magnetic Obsevatory - Past and Present
Yasuhiro NINAMOTO
Abstract
Geomagnetic observations were conducted by the Central Meteorological Observatory (CMO) in Tokyo from 1883 to 1913. With the progress of urbanization in the metropolitan city of Japan, artificial noise, such as that generated by street cars, increased and consequently CMO relocated the geomagnetic observatory to Kakioka in Ibaraki prefecture, about 75 km northeast of Tokyo. Although the observation data measured at the Kakioka Magnetic Observatory (KMO) were sent to CMO, all the written records and field notes stored at CMO were destroyed in a fire after the 1923 Kanto Earthquake . The geomagnetic records taken before 1923 that still survive today are publications of the records from 1897 to 1915, but the data from 1916 to 1923 was unfortunately lost.
In 1950, KMO developed a new observation instrument that incorporated a temperature compensation function and achieved a remarkable improvement in observation accuracy, and which replaced the conventional variation observation instrument. For absolute observation instruments, KMO developed in 1956 the A-56 universal magnetometer and the H-56 sine galvanometer. In 1965, KMO installed the MO-P vector proton magnetometer to drastically enhance the quality of its absolute observation, probably making it world class at the time.
In 1976, the Kakioka automatic standard magnetometer (KASMMER) was installed (Yanagihara et al., 1973). KASSMER allowed KMO to provide observation data values with a one-minute resolution. With regard to data with a resolution measured in seconds, KMO started providing observation data with a three-second resolution in 1985 and with a one-second resolution in 1987.
Today, KMO conducts absolute observations with DI-72, an angle measuring instrument, and a proton magnetometer, and variation observations with a high-sensitivity triaxial fluxgate magnetometer and four Overhauser magnetometers. The main measuring instrument is a high-sensitivity triaxial fluxgate magnetometer, which measures the values of three components every second and observation values at 10 Hz. Although the fluxgate magnetometer is equipped with a monitoring device that checks inclination and temperature, the annual temperature variation is kept within 3?C, and the inclination variation is also kept stable.
KMO is surrounded by residential and farm land, and its observations can be affected by artificial disturbances such as those caused by vehicles, buildings, other magnetic bodies or construction work. To cope with the increasing level of artificial disturbances such as these, an artificial disturbance measurement system was established in 2008. This system calculates the magnetic moments of sources of artificial disturbances from the measurement values of the geomagnetic measuring instruments installed inside the KMO campus.
According to a re-examination of past data, it was revealed that since the data obtained at Kakioka until 1947 was not processed appropriately, the published data lacked adequate quality. The reports delivered at the IATME Oslo Meeting held in 1948 concluded that the observation accuracy before 1947 was very much poorer than that thereafter, but when the observation data in question were reviewed and re-processed appropriately, its accuracy turned out to be almost equal to that after 1948. Now the work of re-examining and re-processing of absolute observation results from the 1920s to the 1940s is under way.
Until the optical pumping magnetometer was introduced in 1976, only the hourly values among the observation data of geomagnetic variation were disclosed. But in order to provide the past data in a more accessible and usable form, KMO developed a technique to convert the geomagnetic data recorded on bromide paper into digital values with a one-minute resolution.
Received 8 March 2010; received in revised form 11 March 2010; accepted 16 March 2010
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