Sunspot Observations by Hisako Koyama: 1945-1996

Abstract

The sunspot record is the only observational tracer of solar activity that provides a fundamental, multi-century reference. Its homogeneity has been largely maintained with a succession of long-duration visual observers. In this paper, 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 database 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% after 1983, which then remains stable until 1996. 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.

arXiv commentMain text 31 pages, references 6 pages, 13 figures, 3 tabes, accepted for publication in Monthly Notices of the Royal Astronomical Society, 2020

References in corpus (28)

• The Solar Cycle
• Revisiting the Sunspot Number
• The new Sunspot Number: assembling all corrections
• Reconstruction of the Sunspot Group Number: the Backbone Method
• The Maunder minimum (1645--1715) was indeed a Grand minimum: A reassessment of multiple datasets
• Historical Sunspot Observations: A Review
• A Revised Collection of Sunspot Group Numbers
• A New Calibrated Sunspot Group Series Since 1749: Statistics of Active Day Fractions
• Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo
• On-line Tools for Solar Data Compiled at the Debrecen Observatory and their Extensions with the Greenwich Sunspot Data
• Visualization of the challenges and limitations of the long-term sunspot number record
• Sunspot positions and sizes for 1825-1867 from the observations by Samuel Heinrich Schwabe
• New reconstruction of the sunspot group number since 1739 using the direct calibration and "backbone" methods
• Temporal and Spatial Evolutions of a Large Sunspot Group and Great Auroral Storms around the Carrington Event in 1859
• Revisited sunspot data: A new scenario for the onset of the Maunder minimum
• Predicting the Amplitude and Hemispheric Asymmetry of Solar Cycle 25 with Surface Flux Transport
• Solar cycle 25: another moderate cycle?
• Comparative analysis of Debrecen sunspot catalogues
• Level and length of cyclic solar activity during the Maunder minimum as deduced from the active day statistics
• The revised Brussels-Locarno Sunspot Number (1981-2015)
• Low-Latitude Aurorae during the Extreme Space Weather Events in 1859
• Improvement of solar cycle prediction: Plateau of solar axial dipole moment
• Thaddäus Derfflinger's sunspot observations during 1802-1824: A primary reference to understand the Dalton Minimum
• A Great Space Weather Event in February 1730
• Reconstructing solar magnetic fields from historical observations V. Sunspot magnetic field measurements at Mount Wilson Observatory
• Sunspot Observations and Counting at Specola Solare Ticinese in Locarno since 1957
• Sunspot Observations by Hisako Koyama: 1945-1996
• Eric Strach: four decades of detailed synoptic solar observations (1969-2008)

Cited by in corpus (5)

• Re-calibration of the Sunspot Number: Status Report
• Thaddäus Derfflinger's sunspot observations during 1802-1824: A primary reference to understand the Dalton Minimum
• Reconstructing solar irradiance from historical Ca II K observations. I. Method and its validation
• Scrutinizing the relationship between plage areas and sunspot areas and numbers
• Sunspot Observations by Hisako Koyama: 1945-1996