The History and the Present Shape of the Tokyo/Kakioka Magnetic Observatory
Yasuhiro MINAMOTO,Takeshi TOYA and Norimichi MASHIKO
Abstract
Geomagnetic observations in Japan were conducted by the Central Meteorological Observatory (CMO) in Tokyo from 1883. CMO relocated the geomagnetic observatory to Kakioka in Ibaraki prefecture, about 75 km northeast of Tokyo in 1913. All the written records and field notes stored at CMO were destroyed in a fire after the Great Kanto Earthquake of 1923. The geomagnetic records from 1916 to 1923 was unfortunately lost.
In 1951, Kakioka Magnetic Observatory (KMO), developed a new observation instrument that incorporated a temperature compensation function and achieved a remarkable improvement in variation observation accuracy, and which replaced the conventional observation instrument. For absolute observation instruments, KMO developed in 1956 the A-56 universal magnetometer. In 1964, KMO installed the MO-P vector proton precession 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. KASMMER 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 1980 and with a one-second resolution in 1983.
Today, KMO conducts absolute observations with DI-72, an angle measuring instrument, and a proton magnetometer, and variation observations with a high-sensitivity three-axis fluxgate magnetometer, which measures the values of three components every second and observation 0.1-second values. Although the fluxgate magnetometer is equipped with a monitoring device that checks inclination and temperature, the annual temperature variation is kept within 3 degrees Celsius, and the inclination variation is also kept stable.
According to the reports delivered at the International Association of Terrestrial Magnetism and Electricity Oslo Meeting held in 1948 concluded that the absolute measurement accuracy before 1947 was very much poorer than that thereafter. But the review of the observation data revealed that the accuracy before 1947 is almost equal to that of Schmidt once they have been re-processed appropriately. Now the work of re-examining and re-processing of absolute observation results from the 1920s to the 1940s is under way.
Only the hourly values among the observation data of geomagnetic variation were published before 1975. But in order to provide the past data in a more accessible and usable form, KMO developed a method to convert the geomagnetic data recorded on bromide paper into digital values with a one-minute resolution.
