López Fuentes, M., C. Mandrini, P. Démoulin, and L. Van Driel-Gesztelyi. “Inferring the Writhe of Magnetic Flux Tubes from the Evolution of Active Solar Regions”. Geofisica Internacional, vol. 43, no. 1, Jan. 2004, pp. 83-88, doi:10.22201/igeof.00167169p.2004.43.1.1417.