Ouattara, Frédéric. “Relationship Between Geomagnetic classes’ Activity Phases and Their Occurrence During the Sunspot Cycle”. Annals of Geophysics 52, no. 2 (February 25, 2009): 107–116. Accessed October 27, 2021. https://www.annalsofgeophysics.eu/index.php/annals/article/view/4634.