Ongoing Investigation Into Long-Term Solar Variability Based On Current Sunspot Number Databases

Many calibrations of historic sunspot number observations have been carried out in the past. New improved databases of historical sunspot data are in development. A US-India collaborative group with the participation of authors (2, 3) at NSO (National Solar Observatory), have constructed a proxy of sunspot parameters based on archived spectroheliograms of daily observations in Ca K II spectral line which began in 1904 at Kodaikanal Observatory (India) and in 1915 at Mount Wilson Observatory (California).group made up of an international team at ISSI (International Space Science Institute, Bern) with participation of author (3) at NSO, is working on re-calibration of sunspot number time-series. There is an effort to recover all past records of sunspot observations and calibrate the observers to create a unified time-series. The most recent time-series of monthly sunspot numbers starting from 1818, is available via SILSO (Sunspot Index and Long Solar Observations). Author (1) investigating long-term solar variability based on historic sunspot number observations, presents a summary of historical sunspot number calibrations along with current findings from ongoing analysis.