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Change in the Nuclear Gyromagnetic Ratio γ_p for Proton Magnetometers

Observation of total magnetic force became much easier and quicker with the advent of proton magnetometers in the mid-20th century, and these instruments were introduced at Kakioka in 1963.
The monitoring principle is based on the fact that protons move precessionally at a frequency proportional to ambient magnetic strength, as given by

  2 π f = γ_p • F

Here, f is the frequency of precession [Hz (hertz)], F is the total magnetic force [T (Teslas)] and γ_p is a physical constant called the nuclear gyromagnetic ratio. Using this relationship, total magnetic force can be determined by monitoring the frequency.
Naturally, if the nuclear gyromagnetic ratio adopted is inaccurate, the total magnetic force value will also be affected.

The value  2.67513×10⁸ [T ^-1sec^-1]  was long used in accordance with the recommendation of IAGA (*2), a sub-organization of ICSU (*1).
Recently, however, CODATA (*3), also a subordinate body of ICSU, issued a new value of  2.67515333×10⁸ [T ^-1sec^-1] . With the worldwide trend being to switch to the more accurate γ_p value, JMA decided to adopt it for more correct measurements.

The new value has been used since 1 January 2005. This change results in a slight apparent shift of almost a constant amount by 0.4 nT for measurement, for example, around Japan (approx. 46,000 [nT]).
However, the total magnetic field value also includes a correction term for various other reasons.

Taking the importance of data continuity into account, JMA processes geomagnetic observations to avoid discontinuities in published data.
 (*1) International Council for Science (now the International Science Council)
 (*2) International Association of Geo-Electromagnetics and Ultra-High-Temperature Physics
 (*3) Scientific and Technical Data Committee