Dr. Theodosios Chatzistergos is an outstanding young scientist with a truly pan-European profile. He obtained his MSc in Astrophysics, with distinction, from Queen Mary University, London, in 2013, and his PhD in 2017 from Georg-August University of Göttingen, Germany. From 2018 to 2020 he was a postdoctoral fellow at the Istituto Nazionale di Astrofisica (INAF) in Rome, Italy, and he is currently a senior postdoc at the Max Planck Institute for Solar System Research in Göttingen, Germany.

Dr. Chatzistergos has been working predominantly in the field of the long-term variability of the Sun, with a substantial contribution to the quality improvement and recalibration of the international Sunspot Number Index and other solar variability indicators, which have been a topic of much interest and debate recently. The key input to studies of long-term solar activity and variability are historical – mainly solar – observations. Unlike the vast richness of data available for studies of the present-day Sun and heliosphere, understanding the physics and the behaviour of the Sun on longer time scales has to rely on data of significantly poorer coverage and quality. Therefore, historical archives, such as sunspot observations or photographs of the Sun at various wavelengths, are of irreplaceable value for understanding the long-term changes in the behaviour of the Sun and Space Climate generally.

The historical data must often be cleaned of artefacts and calibrated before they can be used in a meaningful way. Dr. Chatzistergos has made significant contributions to this effort, unlocking the unique potential of precious historical solar data. The work of Dr. Chatzistergos is thus providing us with historical data sets of inestimable value for understanding past solar activity.

Dr. Chatzistergos has an excellent publication record, with in particular an impressive number of publications of which he is the first author. His publication record also includes many single-author papers, demonstrating that he is fully autonomous in his research. He has made innovative contributions in his research field since the very beginning of his career and receives wide acclaim for his contributions from the scientific community. This wide support for his work was in particular very visible in the enthusiastic support for his nomination for the Chizhevsky Medal.

For all of these reasons, the ESWAN Awards Committee has decided to attribute the 2024 Alexander Chizhevsky Medal to Dr. Theodosios Chatzistergos.