Computer production of Kindices: review and comparison of methods

Abstract

The K indices are to be hand-scaled from analogue magnetograms following morphological criteria expressed in the so-called Mayaud rules. All observatories used to follow these rules in the routine hand-scaling of K indices from analogue magnetograms. As a result of digitalization and automation of observatories, however, observers in charge who are experienced in K scaling from analogue magnetograms are becoming increasingly rare. Therefore the organizations running observatories were forced to find compensating machine methods for the production of K indices. The basic features of the K indices and the Mayaud rules are first reviewed, and then the methods developed for computer derivation of K indices are discussed.In order to decide which of the proposed algorithms was suitable, a comparison among them was organized by the IAGA Working Group on geomagnetic indices. The comparison was made with a common data set, using the same statistical tests. The results are summarized and discussed. Further comparisons between the K indices produced by computer with the different methods are presented in this paper. Four methods were acknowledged by the Working Group during the Vienna IUGG general Assembly in 1991. The results confirm that these methods provide acceptable results in comparison to the average quality of hand-scaled indices.A comparison between computer-produced K indices and reference K indices handscaled following the Bartels-Mayaud rules is also presented. It appears that only two acknowledged methods provide computer-produced K in good agreement with handscaled K. The best computer method (the FMI method) is found to be good enough to allow the continuation of the long tradition of producing K indices without any serious jump in the statistics.

Correction of artificial jumps in the historical geomagnetic measurements of Coimbra Observatory, Portugal

“…The K index values, available since October 1951, were hand-scaled during the period of classic variographs (from October 1951 to August 2006), and automatically scaled afterwards using the 1 min digital FGE variometer files and FMI algorithm (Menvielle et al, 1995). Besides this change in the calculation methods, the following instrumental and methodological changes (see also Table 1 of metadata information) could have affected the integrity and homogeneity of the K index series: (1) according to the observatory's logbook, a change in the hand-scaled calculation method was applied in MayJune of 1985, though no further information is given; (2) the magnet suspension wire (originally of quartz) of the H variograph broke for the first time in December 1979 and was replaced in the following month by a tungsten wire.…”

An automated procedure for near‐real‐time Kp estimates

“…The definition of the term "regular" is subjective, but in concept the regular part should not contain disturbances generated by storms or substorms. According to Mayaud [1967], there are established rules that should be followed in defining QDC [see also Menvielle et al, 1995]. 4.…”

Results of Russian geomagnetic observatories in the 19th century: magnetic activity, 1841–1862

“…The index algorithm applied to the Russian data (Menvielle et al, 1995) (based on hourly values) is the same that has been utilized in the analysis of Helsinki (Finland) magnetic observatory data 1844-1897 (Nevanlinna, 2004).…”