Comparative analysis of Debrecen sunspot catalogues

TL;DR

This study compares multiple sunspot catalogues from Debrecen and Greenwich to analyze measurement differences, calibration factors, and the effects of observational conditions on sunspot area data.

Contribution

It provides a detailed comparison and calibration of sunspot area measurements across different catalogues and observational conditions.

Findings

Poor seeing increases individual spot areas but reduces small spot counts.

DPD areas are 5% smaller than HMIDD; umbra areas are 9% smaller.

Greenwich areas are a few percent smaller than DPD areas.

Abstract

Sunspot area data are important for studying solar activity and its long term variations. At the Debrecen Heliophysical Observatory, we compiled three sunspot catalogues: the Debrecen Photoheliographic Data (DPD), the SDO/HMI Debrecen Data (HMIDD), and the SOHO/MDI Debrecen Data. For comparison, we also compiled an additional sunspot catalogue, the Greenwich Photoheliographic Data, from the digitized Royal Greenwich Observatory images for the 1974-76. By comparing these catalogues when they overlap in time, we can investigate how various factors influence the measured area of the sunspots, and, in addition, we can derive area cross-calibration factors for these catalogues.The main findings are as follows. The poorer seeing increases the individual corrected spot areas and decreases the number of the small spots. Interestingly, the net result of these two effects for the total corrected spot area is zero. DPD daily total corrected sunspot areas are 5 per cent smaller than the HMIDD ones. Revised DPD daily total corrected umbra areas are 9 per cent smaller than those of HMIDD. The Greenwich Photoheliographic Result areas are only a few per cent smaller than DPD areas. A $0.2^{\circ}$ difference between the North directions of the DPD and MDI images is found. This value is nearly the same as that was found ($0.22^{\circ}$) by us in a previous paper comparing HMI and MDI images. The Solar Observing Optical Network area measurement practice (spots smaller than $10 mh$ were not directly measured but an area of $2 mh$ were assigned to each) can not explain the large area deficit of the Solar Observing Optical Network.