Deterministic Variability of the Ionosphere on the Eve of Strong (M ≥ 6) Earthquakes in the Regions of Greece and Italy According to Long-Term Measurements Data

Abstract

Abstract—Specific features of the variation in the electron concentration in the ionosphere in front of strong (M ≥ 6) earthquakes in the most seismically active areas of Europe, Greece and Italy, are considered on the basis of long-term measurements. Vertical sounding data (variations of the critical frequency foF2) are analyzed to cover long periods of time before the GPS/GLONASS era, and variations of the vertical total electronic content are also analyzed with the advent of global navigation satellite systems receivers in the considered regions. This analysis is based on the results of previous publications that revealed features of ionospheric precursors such as self-similarity, which is expressed in the exceptional stability of electron concentration variation as dependence on the local time. Its graphic representation is called the ionospheric precursor mask. This stability, which is defined as deterministic variability, is confirmed by the multiyear measurement data presented in this work. One new result involves the regional features of the ionospheric precursor mask found for Italy, where the mask for earthquakes in central Italy (within a radius of 300 km north of Rome) and the mask for earthquakes in southern Italy (within a radius of 300 km south of Rome) differ in phase. A possible explanation of the observed regional features is proposed.