Speaker
Description
Sunspot drawings are a unique source of information to study the long-term manifestation of the magnetic activity on the solar surface. The Royal Observatory of Belgium (ROB) started such drawings around 1940 and continues today on a daily basis, making the whole collection spanning over more than 80 years.
In this presentation, we discuss two important limitations to the full scientific exploitation of sunspot drawings: (1) the need for a fast and reliable software to analyse the drawings and (2) an estimation of the inherent uncertainty, in particular in position calculations. In order to overcome the first issue, the use of a standard analysis tool for the parameter measurement would make it easier to analyse contemporary sunspot drawings. In addition, it would help to fill the inevitable gaps from a single observation station by merging homogeneous data. Secondly, obtaining an appropriate estimate of the uncertainty would give us more comprehensive information and enable us to combine measurements from different drawing collections.
In the first part of the presentation, we describe DigiSun, a software developed by our team for measuring ROB sunspot drawings and for creating a catalog of sunspots. An important feature is the calculation of a pixel-wise true area, corrected for foreshortening. This calculation is more precise than most current software, which only considers one centroid position to correct foreshortening. Another strength of DigiSun is its ability to handle different drawing formats, which allows it to analyse drawings from other collections. It has been shared with other observatories such as the Specola Observatory in Locarno since 2019, Tapei and Kandili since 2024. In addition, DigiSun can be used to analyse historical drawings and extend the series of detailed solar parameters further back in time.
In the second part of the presentation, we aim to improve our understanding of the source of uncertainty present in sunspot drawings and provide a first global estimate of it. We estimate the uncertainties in heliographic coordinates, which is a crucial factor in the analysis of long-term solar differential rotation and the distribution of heliographic longitude and latitude across various solar cycles.