Seismogenic Field in the Ionosphere before Two Powerful Earthquakes: Possible Magnitude and Observed Ionospheric Effects (Case Study)

Abstract

A retrospective analysis of complex geophysical data around the time of the two most powerful earthquakes that occurred in Alaska and had magnitudes M = 8.2 (29 July 2021) and M = 9.2 (28 March 1964), respectively, is carried out. The aim of the research is to assess the maximum possible magnitude of the electric field of a seismogenic nature that penetrated the ionosphere/plasmasphere, which could cause the ionospheric effects observed experimentally. Theoretical calculations have shown that under the geophysical conditions that existed before these earthquakes (favorable for the penetration of the seismogenic field into the ionosphere), the maximum value of a quasi-static electric seismogenic field in the ionosphere, perpendicular to geomagnetic field lines (tens of hours/units of days before the earthquake) for earthquakes with magnitudes M = 8–9 could reach 1–2 mV/m. Such values are sufficient for the formation of a plasmaspheric ULF-ELF-VLF-duct, which is formed in the vicinity of the geomagnetic field-line passing through the epicenter of the earthquake under the influence of a seismogenic electric field that penetrated into the ionosphere/plasmasphere. This leads to an anomalous amplification of the captured ULF-ELF-VLF waves, ULF (DC-16 Hz), ELF (6 Hz–2.2 kHz), VLF (1.8–20 kHz), not only above the epicenter of the future earthquake, but also at the point magnetically conjugated with the epicenter of the earthquake, testifying to the formation of such a duct, stretched along the geomagnetic field from one hemisphere to another, and formed on closed L-shells shortly before the earthquake. This result is confirmed by the measurements of the mission of the CSES satellite (China-Seismo-Electromagnetic Satellite) for the 29 July 2021 earthquake with magnitude M = 8.2.