1. Carrington, R.C., Description of a singular appearance seen in the Sun on September 1, 1859, Mon. Not. R. Astron. Soc., 1859, vol. 20, no. 1, pp. 13–15.
2. Coffey, H. and Erwin, E., When do the geomagnetic aa and Ap indices disagree?, J. Atmos. Sol.-Terr. Phys., 2001, vol. 63, no. 5, pp. 551–556. https://doi.org/10.1016/S1364-6826(00)00171-1
3. Gosling, J.T., McComas, D.J., Phillips, J.L., and Bame, S.J., Geomagnetic activity associated with earth passage of interplanetary shock disturbances and coronal mass ejections, J. Geophys. Res., 1991, vol. 96, no. A5, pp. 7831–7839. https://doi.org/10.1029/91JA00316
4. Reduced and extended periods of solar activity: Monitoring features and key facts, in Trudy konferentsii “Solnechnaya i solnechno–zemnaya fizika-2013” (Proceedings of the Conference;V.N. Ishkov,2013
5. Ishkov, V.N., Solar flare super-events: When they can occur and the energy limits of their realization, Sun Geosphere, 2015, vol. 10, no. 1, pp. 89‒96. http://newserver.stil. bas.bg/SUNGEO/00SGArhiv/SG_v10_No1_2015-pp-89-96.pdf.