Magnetic fields of opposite polarity in sunspot penumbrae M. Franz, M. Collados, C. Bethge, R. Schlichenmaier, J. M. Borrero, W. Schmidt, A. Lagg, S. K. Solanki, T. Berkefeld, C. Kiess et al. (18 more) A&A, 596 (2016) A4 Published online: 30 November 2016 DOI: 10.1051/0004-6361/201628407