Possible relation of air ion density anomalies with earthquakes and the associated precursory ionospheric perturbations: An analysis in terms of criticality

The air ion density observed at Sendai during a period of 10/02/2013 to 30/04/2016 (more than 3 years) has been analyzed in terms of natural time critical analysis. It is then found that the air ions exhibit criticalities at some specific time periods, as a signature of pre-earthquake (EQ) fracturing process in the lithosphere. So it is likely to conclude that this criticality analysis enables us to associate the criticality in ion density to an EQ as a general tendency. The detection of air ions at a particular place suggests that those ions may be a precursor to an EQ (magnitude greater than 5.0) occurring within a radius of 200 km from the ion observatory. However, based on a comparison with the corresponding VLF/LF sub-ionospheric propagation data, we cannot say anything about a certain relationship of those air ions with the lower ionospheric perturbation as seen by VLF/LF, because the temporal scale of ions is medium-termed, but VLF/LF perturbation is short-termed, which makes it extremely difficult to correlate between the two.