Measurement of atmospheric ionic current

Known as the global circuit, air-earth conductive current or atmospheric ion current (AIC) is flowing into the ground. In this study, the result of continuous measurement of AIC has been presented, with possible relation with major earthquakes. The AIC flowing into an electrode on the roof has been measured as a potential by using a high input impedance electrometer. As a result, typical daily change in V
_m
in fair weather showed a very small value after sunset until sunrise. It then gradually increased with time reaching a peak around noon followed by a gradual decrease to a very small value in the evening. Since it has been reported that the AIC is affected by solar radiation, normal daily variation of V
_m
should be mainly due to the AIC. To analyze a correlation between the observed potential and AIC, time-course of the potential was analyzed. From the potential observation before and after the huge earthquake occurred on March 11th 2011 in Japan, two types of signals of AIC clearly and significantly different from normal one mentioned above were observed. One is high potential in midnight started to be observed about 10 days before the earthquake occurrence and the other is large burst pulses observed a few hours before the earthquake. Because the abnormal behavior of the measured AIC was observed before some other large earthquakes, it can be ascribed to crustal movement such as preslip. Further study and international joint measurement of AIC is necessary to study the correlation between AIC and earthquake.