Recovering the unsigned photospheric magnetic field from Ca II K observations Theodosios Chatzistergos, Ilaria Ermolli, Sami K. Solanki, Natalie A. Krivova, Fabrizio Giorgi and Kok Leng Yeo A&A, 626 (2019) A114 Published online: 21 June 2019 DOI: 10.1051/0004-6361/201935131