Hale, G.E. 1908, On the probable existence of a magnetic field in sunspots, The Astrophysical Journal, 28, pps. 315–343.
Helmenstine, A. M. (2019, June 28). How Solar Flares Work. Retrieved from ThoughtCo: https://www.thoughtco.com/solar-flares-4137226
Lanzerotti, Louis & Bothmer, Volker & Daglis, Ioannis. (2007). Space weather effects on communications. https://doi.org/10.1007/978-3-540-34578-7_9