Relationship between TIGRE solar S-index and USET Ca II K full disk images G. Vanden Broeck, S. Bechet, F. Clette, G. Rauw, K.-P. Schröder and M. Mittag A&A, 689 (2024) A95 Published online: 05 September 2024 DOI: 10.1051/0004-6361/202450125