| Popis: |
Numerical three-dimensional calculations of the ionosphere and thermosphere disturbances generated by seismogenic sources have been performed using the Upper Atmosphere Model (UAM). The external electric current with the density of 10−8 A/m2 over the area of 250 by 4000 km was used as a model input at the height of 80 km in the electric potential equation of the UAM. According to the simulation results vertical electric current flowing over the tectonic fault causes stable electric field which exerts influence on the ionospheric plasma via the electromagnetic plasma drift and leads to the total electron content (TEC) disturbances relative to the quiet background values. The calculated latitude-altitude variations of the electron density show disturbed regions in the form of plasma bubbles moving vertically over the epicenter (figure 1). It was shown that both vertical and horizontal components of the electromagnetic [E × B] plasma drift are important in formation of the TEC disturbances. The UAM calculated TEC deviations presented in figure 2 are in a good agreement with the typical disturbances observed before major earthquakes, and characteristic features of the typical earthquake precursors are reproduced: the magnitude of several tens of percents, occupation area of several thousand of kilometers, life-time of 4–6 hours, magnetic conjugation, significant depression of the anomalies up to their full disappearance after sunrise and recovery after sunset (figure 2). Charged and neutral particles' collisions caused weak traveling disturbances propagated from the epicenter and magnetically conjugated areas with characteristics of internal gravity waves (IGW). The UAM calculated relative disturbances of the neutral gas' temperature are presented in figure 3. However, the influence of IGWs on the TEC disturbances was insignificant in comparison with direct action of the electromagnetic [E × B] drift. The physical nature of the seismogenic electric current has been discussed. We consider this current is generated by the same way as vertical current charging the ionosphere positively and the Earth negatively and creating the electric background field between the Earth surface and the ionosphere. Negative charges produced by radon and cosmic rays ionization may act as a potential nucleus of water vapor condensation, and coagulation of big water drops occurs. The negatively charged heavy drops and aerosols move downwards by gravity force and charge the Earth surface negatively, while light positive charges rise upwards with warm air fluxes. Thus, due to gravitational sedimentation and convectional transport the charges separation takes place and the external electric current flowing between the Earth and ionosphere is generated. The greater the ionization rate, dust or water aerosol concentration and air humidity, the greater external electric current density because of increased condensation and coagulation. We suppose that these conditions over the acting fault favour the intensification of the vertical electric current between the Earth and ionosphere and cause observed TEC disturbances. |
