Abstract Sunspots are the most striking and easily observed magnetic structures of the Sun, and statistical analysis of solar historical data could reveal a wealth of information on the long-term variation of solar activity cycle. The hand-drawn sunspot records of Yunnan Observatories, Chinese Academy of Sciences have been accumulating for more than 60 years, and nearly 16 000 images have been preserved. In the future, the observation mode of recording sunspots by hand-drawing will be replaced inevitably by digital images observed either at ground or in space. To connect the hand-drawn sunspot data and the purely digital sunspot data in future, it is necessary to analyze the systematic errors of the data which are observed by the two observation modes in the period of transition. In this paper, we choose 268 round sunspots (H-type in modified Zurich sunspot classification) from the drawing of Yunnan Observatories to compare their positions and areas with the CCD observations made by Helioseismic and Magnetic Imager (HMI) on board Solar Dynamic Observatory (SDO) and Global Oscillation Network Group (GONG). We find that the latitude and longitude accuracy of hand-drawn sunspot are within −0.127 and 2.29 degree respectively, and the area accuracy is about 16.36 sunspot unit (μHem). Systematic errors apparently decrease with large sunspot.
Keywords sunspots — methods: data analysis — techniques: image processing
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