SOHO/MDI and SDO/HMI sunspot area measurement and analysis

ABSTRACT Sunspot area is a key indicator of solar activity, providing an alternative to traditional empirical sunspot number. This paper presents a new catalogue of total sunspot area derived from full-disc white-light images acquired by the SOHO Michelson Doppler Imager (MDI) and the SDO Helioseismic and Magnetic Imager (HMI) from 1996 to the present. The methodology involves a multistep threshold approach to identify sunspots which has been calibrated against the Debrecen Photoheliographic Data (DPD) for consistency with previous sunspot area catalogues. The data set includes daily sunspot area measurements, with projected and corrected values, spanning solar Cycles 23, 24, and 25. Long-term trends in sunspot area are analysed, including hemispheric asymmetries and the relationship with the International Sunspot Number. Significant changes in sunspot area, including extreme daily variations and cycle maxima, are identified. Additionally, the study includes a comparison, during Cycle 25, with area measurements from the Kislovodsk solar station of the Pulkovo observatory. Finally, example high-cadence sunspot group area evolution is shown for four large sunspot groups, as is the different split of sunspots into groups by several observatories.