Sunspot observations by Hisako Koyama: 1945-1996

Hisashi Hayakawa, Frédéric Clette, Toshihiro Horaguchi, Tomoya Iju, Delores J. Knipp, Huixin Liu, Takashi Nakajima

Research output: Contribution to journalArticlepeer-review

10 Citations (Scopus)


Sunspot records are the only observational tracer of solar activity that provides a fundamental, multicentury reference. Its homogeneity has been largely maintained with a succession of long-duration visual observers. In this article, we examine observations of one of the primary reference sunspot observers, Hisako Koyama. By consulting original archives of the National Museum of Nature and Science of Japan (hereafter, NMNS), we retrace the main steps of her solar-observing career, from 1945 to 1996. We also present the reconstruction of a full digital data base of her sunspot observations at the NMNS, with her original drawings and logbooks. Here, we extend the availability of her observational data from 1947-1984 to 1945-1996. Comparisons with the international sunspot number (Version 2) and with the group sunspot number series show a good global stability of Koyama's observations, with only temporary fluctuations over the main interval 1947-1982. Identifying drawings made by alternate observers throughout the series, we find that a single downward baseline shift in the record coincides with the partial contribution of replacement observers mostly after 1983.We determine the correction factor to bring the second part (1983-1996) to the same scale with Koyama's main interval (1947-1982). We find a downward jump by 9 per cent after 1983, which then remains stable until 1995. Overall, the high quality of Koyama's observations with her life-long dedication leaves a lasting legacy of this exceptional personal achievement.With this comprehensive recovery, we now make the totality of this legacy directly accessible and exploitable for future research.

Original languageEnglish
Pages (from-to)4513-4527
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - Mar 1 2020

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'Sunspot observations by Hisako Koyama: 1945-1996'. Together they form a unique fingerprint.

Cite this