KOSTECKÝ, Pavel, et al. “Observations of the Anomalous Third Schumann Resonance Peak During October 21 to 25, 2004 at Modra Observatory and a Possible Explanation”. Contributions to Geophysics and Geodesy, vol. 35, no. 1, May 2021, pp. 33-51, https://journal.geo.sav.sk/cgg/article/view/364.