Long-term variation of strong geomagnetic storms and its effect on ionospheric and telluric currents

Geomagnetic storms affect ionospheric currents, which induce electric fields at the Earth surface, and finally drive telluric currents. Tong-term variation in strong geomagnetic storms is studied through the number of days per year with aa index > 80 nT, and the number of hours per year with Akasofu's epsilon parameter (ε) >1013 W. A long-term behavior consisting of the Suess cycle modulated by the Gleissberg cycle is observed. Considering that these storms involve also high frequency variations with magnetic inductive effects, and the direct relation between e and geoelectric fields, this result can be extended to ionospheric and telluric currents.