Owing to the fact that a Living Reviews in Solar Physics article may evolve over time, we recommend to cite the article as follows:
Juan M. Borrero and Kiyoshi Ichimoto,
"Magnetic Structure of Sunspots",
Living Rev. Solar Phys. 8, (2011), 4. URL (accessed <date>):
The date given as <date> then uniquely identifies the version of the article you are referring to.