Preseismic alteration of atmospheric electrical conditions due to anomalous radon emanation

Preseismic radon (Rn-222) emanation is considered to be one of the main physical mechanisms of electromagnetic precursory phenomena. In this paper, we analyze the atmospheric radon concentration around the occurrence time of the 1995 Kobe earthquake, and evaluate the influence on the atmospheric electrical conditions based on the global electric circuit under the quasistatic state. The radon emanation increases the number density of small ions and the atmospheric conductivity and decreases the atmospheric electric field by about 40–50% in the lower atmosphere, while they show little change in the upper atmosphere. The difference in the lower and upper atmosphere induces the reduction of the ionospheric potential. The estimated quasistatic changes near the ground can explain the reported seismic precursors such as the ion number density and the electric field. At the ionospheric altitude, however, not only the quasistatic process but also the shorter-term process, i.e., a transient electrodynamic process in the atmosphere is required for the understanding of the lithosphere–atmosphere–ionosphere interaction due to the radon emanation associated with large earthquakes.