Lithosphere–ionosphere coupling mechanism and its application to the earthquake in Iran on June 20, 1990. A review of ionospheric measurements and basic assumptions

Abstract

Analysis of the reliable ionospheric data that have been obtained by different research groups near a seismically active region in western Iran has enabled us to infer that: (1) Disturbances in the lower ionosphere ( h∼90 km, radio wave phase anomalies), F region ( h∼270 km, airglow anomaly), and upper ionosphere ( h∼2400 km, VLF noise anomalies measured by the satellite) were recorded at distances of 250 km, 1000 km, and 2000 km, respectively, from the epicentral zone. These might be considered as precursors (h/days before the event) and/or as after-effects of seismical activity. (2) Disturbances in the lower ionosphere correspond to density enhancement in this part of the ionosphere. (3) Decreasing the 630-nm emission intensity in the F region may be interpreted as a formation of the region with plasma depletion. (4) Anomalous impulse VLF-emissions in the upper ionosphere, apparently, may be related with whistler trapping into the ducts, formed in the ionosphere above the seismically active region. (5) LH-noise frequency rising corresponds to increasing the plasma density in the upper ionosphere. To interpret these anomalies, we exploit a new lithosphere–ionosphere coupling mechanism related to internal gravity wave generation in seismically active regions. Under favourable conditions, these waves can reach the ionosphere. The quantitative analysis of ionospheric disturbances caused by internal gravity wave interaction with ionospheric plasma will be published elsewhere.