Investigation of nonlinear interaction in extremely low frequency /very low frequency radio signals in the course of the two mid latitude earthquakes

This paper investigates the ionospheric turbulences generated in the Extremely Low Frequency (ELF)/Very Low Frequency (VLF) signals by wavelet bispectrum analysis and statistical methods. As in the turbulence the manifestation of nonlinearities and coherent structure indicates the frequencies coupling and phase coupling. Ionospheric plasma is very unstable and it can be easily disturbed. Development of turbulence in the ionospheric plasma is generated when instability is reached at the nonlinear stage. If turbulence is developed in geomagnetic quiet conditions then there is a high probability that the perturbations are produced by some aspects having their source at the epicenter of the earthquakes [1]. In this work we are analysing the ELF/VLF signals registered by the DEMETER over seismic region when geomagnetic condition are very quiet (Kp $\lt 3)$and thus diagnosing the disturbed ionospheric plasma response which may be derived by the earth surface. Studies show that, these disturbances are generated by growth of Atmospheric Gravity Waves (AGWs) amplitude [2, 3, 4, 5]during the earthquake preparation stage. This instability is transformed by the electric field and form horizontal irregularities [6]which affect the VLF electromagnetic waves. These commence in the area of the epicenter and then developed ionospheric plasma turbulence [7].