Rodriguez, J. B., Okunev, O. V., Puschmann, K. G., Kneer, F., & Sánchez-Andrade Nuño, B. (2007). On the properties of faculae at the poles of the Sun. Astronomy and Astrophysics, 474(1), 251-259. doi:10.1051/0004-6361:20077739.