A Sunspot Database from SDO/HMI Observations: Methodology and Current Status

Sunspots are the longest continuously observed manifestations of solar activity and form the basis of modern indices of solar variability. Systematic sunspot catalogues, beginning with the Greenwich Photoheliographic Results and later continued by the Debrecen Observatory, provide a unique long-term record of solar activity. Space-based full-disk observations now allow these records to be extended using data from modern instruments.

We present a work-in-progress methodology for the construction of a sunspot database based on observations from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), covering the period from 2010 onward. The database is developed within an ongoing project running until 2028 and is intended to be continuously extended rather than finalized at a single endpoint. The current implementation uses one flattened SDO/HMI intensitygram per day, prioritizing long-term homogeneity over high temporal cadence.

Sunspot detection is performed using a threshold-based method applied to flattened HMI intensity images. Umbrae and penumbrae are detected and treated as separate features. The detection approach can be extended to non-flattened full-disk images through the optional application of limb-darkening compensation, enabling future use with other instruments/observations. For each detected sunspot, the current database structure includes observation time, positional information in both image-plane (x–y) and heliographic coordinates, area measurements, and a provisional identification number. The database format itself remains under active development.

The assignment of persistent identification numbers across consecutive observations is under development and is based on the near preservation of relative distances and angular relationships between sunspots on the solar sphere (pattern preservation). The association of individual sunspots with sunspot groups and active regions is planned as a subsequent step.

This contribution focuses on the detection methodology, evolving database design, and tracking concept, and presents the current status of the pipeline. The resulting database is intended as a community resource for future studies of sunspot evolution, long-term solar activity, and solar rotation.