Katsue Misawa's sunspot observations in 1921–1934: a primary reference for the Wolfer-Brunner transition

Abstract

Abstract Our knowledge of the long-term solar variability is largely based on the International Sunspot Number time series (SN), a composite index based on multiple visual sunspot observers from the 18th century onward and maintained by the World Data Center ‘Sunspot Index and Long-term Solar Observations’ (SILSO). However, over the period 1919–1944, our capacity to diagnose the homogeneity of this time series is currently limited because most of the archived source data of the Zürich Observatory are presently missing over that interval and were not published. Therefore, the recovery of any long-duration series from an individual sunspot observer active during this period is essential to bridge this Zürich data gap. In this context, Katsue Misawa has conducted regular sunspot observations from 1921 to 1934 (mean coverage of 25.4 days/month), which were not accessible for the Zürich Observatory and thus form a valuable addition to the database maintained by the WDC-SILSO. In this study, we digitized his observational records, documented his observing technique, and reconstructed his total and hemispheric sunspot numbers. We compared his data with the International Sunspot Number (current version V2) and evaluated their stability. Misawa's data series generally agrees well with SN V2. However, Misawa's data show a significant transitory drift in 1925–1928, when the Zürich pilot observer changed from Alfred Wolfer to William Otto Brunner.