The root of a comet tail: Rosetta ion observations at comet 67P/Churyumov–Gerasimenko E. Behar, H. Nilsson, P. Henri, L. Berčič, G. Nicolaou, G. Stenberg Wieser, M. Wieser, B. Tabone, M. Saillenfest and C. Goetz A&A, 616 (2018) A21 Published online: 07 August 2018 DOI: 10.1051/0004-6361/201832842