Geomagnetic 1-minute values produced by applying the Gaussian filter
Seiki ASARI, Akiko KAMIYA & Eiji HIROTA
Since Kakioka Magnetic Observatory initiated digital recording of the geomagnetic 1-second values in 1983, the geomagnetic 1-minute values have been calculated by means of simple average of sixty 1-second values. Presently most of the major magnetic observatories in the world adopt the Gaussian filter recommended by INTERMAGNET to produce their 1-minute values. In prospect of possibly conforming to the international standard method, we examine differences between the properties of the 1-minute values derived by the two methods. While the simple average and Gaussian filter are similar in their general performances as low-pass filters, the gain of the former is noticeably smaller in the period band around 1 minute. According to the frequency analysis of 1-minute values of each XYZ component derived from the Kakioka 1-second values during disturbance times, the aliasing due to the simple average appears slightly smaller, implying that the Gaussian filter has no advantage in terms of anti-aliasing. The discrepancies between the 1-minute values for the two methods can reach several nT when extreme phenomena such as ssc take place. Nevertheless, these cases are so seldom that the discrepancy is less than 0.3nT for over 99.9% of the time even during the solar maximum. No significant discontinuities should be yielded in the data quality by changing the methods. It would rather be reasonable to use a Gaussian filter from the viewpoint of data homogeneity in the international observation network.