@article{ZolotovaPonyavinMarwanetal.2009,
  author    = {Nadezhda V. Zolotova and Dmitri I. Ponyavin and Norbert Marwan and J{\"u}rgen Kurths},
  title     = {Long-term asymmetry in the wings of the butterfly diagram},
  doi       = {10.1051/0004-6361/200811430},
  url       = {http://www.aanda.org/},
  year      = {2009},
}