IONOSPHERIC RESPONSE TO EARTHQUAKES AND SCHUMANN RESONANCES

Abstract

The region extended from 60 km to about 1000 km altitude above the earth's surface containing free electrons, ions and neutral atoms/molecules of atmospheric gases is known as ionosphere. It is formed mainly by the ionization of neutral gas atoms/molecules present in the upper atmosphere by exposure to solar radiation. The degree of ionization changes with height leads to the formation of several distinct ionization peaks identified as D, E, F1, F2 layers. Ionospheric weather play very important role in sustaining our life, which largely dependent on present day technological systems. Understanding, monitoring and forecasting the changes in the ionosphere weather are crucial for communication, navigation, exploration of the near earth space and even exploration of electrical structure of deeper part of earth's interior. Variability of the ionospheric weather, and it's response to the phenomena occurring above and below it, can be studied by monitoring it's electrons and ions temperature, density and ion compositions. Many experimental and theoretical studies were devoted for monitoring and predicting ionospheric weather in the form of electron and ion temperature, density and ion compositions. The data used for these studies were either procured by ground based probes or by satellite observations. Some examples of dedicated satellites for these studies are: SROSS-C2 satellite recorded ionospheric temperature and density data in Indian region from 1995-2000 using RPA payload. DEMETER satellite was used to detect anomalous variations of electromagnetic waves, particle fluxes or thermal plasma parameters related to seismic activity. Numerous workers have used the data generated by these satellites for ionospheric characterization. SROSS-C2 data were used for electron, ion temperature and density variations in Indian region (Bhuyan et al., 2003; Sharma et al., 2004; Sharma et al., 2005). The study reveals that the ionospheric parameter (temperature and density) varies as a response to the anomalous phenomena occurring above and below it. Many other studies were also dedicated to investigate the ionospheric irregularities due to atmospheric gravity waves, in thunderstorms, hurricanes, tornadoes, seismological and volcanic events. Some of these events are responsible for generating observable variations in ionospheric parameters (Ahmamedov, 1993; Taranenko et al., 1993; Depuev and Zelenova, 1996; Molchanov and Hayakawa, 1998; Molchanov et al., 1998; Ondoh, 1998, 2000; Kazimirovsky et al., 2003; Pulinets et al., 1994, 2000, 2003; Liu et al., 2000, 2004; Parrot, 1994, 1995, Parrot et al., 2006; Hayakawa et al., 1996a, b, 2006; Sharma et al., 2004, 2005, 2006; Rishbeth, 2006). Gokhberg, et al., 1994; Parrot 1995; Pulinets 1998a, b and Parrot et al., 2006 have discussed the generation and propagation of seismo-electric field from epicentral zone into the ionosphere. The emission and propagation of electromagnetic radiations in a wide frequency band, covering ULF, ELF and VLF from epicenter zone of earthquake were also reported (Parrot, 1995; Koshevaya et al., 1997; Shalimov and Gokhberg, 1998; Hayakawa et al., 1996a, b, 2000; Kushwahetal.,2005). Low frequency EM (micro pulsation) field generated as a results of complex interaction of solar wind and magnetosphere are modified by ionosphere before reaching to the earth surface, where it is used as a source in Magnetotelluric (MT) method to delineated electrical structure of the earth interior (Kaufman and Keller, 1981). High frequency (>1Hz) EM field excited by thunderstormMightening discharge propagates globally in waveguide mode in the cavity formed by the earth and ionosphere as conducting boundaries. Schumann (1952) has used simplest vacuum model confined with two concentric perfectly conducting spheres (earth and ionosphere) and obtained first five Schumann resonance (SR) frequencies: 10.6, 18.4, 26.0, 35.5 and 41.1 Hz. However it was found that these were not in agreement with the observed SR frequency. Madden and Thomson, 1965, observed SR frequencies: 7.8, 14.1, 20.3, 26.3 and 32.5 Hz using finite conductivity of ionosphere which are in reasonable agreement with the observed frequencies. Amplitude and frequency characteristics of SR depend on the characteristics of their source, location of observation point with respect to source and ionospheric electron density\conductivity. For a local region assuming the

average constant source distribution, SR frequency variations can be used to determine average conductivity profile of ionosphere and vice versa (Tran &Polk, 1979a, b; Sentman, 1983). It was investigated that attenuation characteristics, frequency shift and diurnal variations are different for different field components (Sentman 1987, 1989; Bliokh et al., 1980; Nickolaenko, 1997; Roldugin et al., 2004a). SR in magnetic field components was also studied by Fullekrug (1995). Penetration of electric and magnetic field components of the SR into the ionosphere was numerically investigated by Grimalsky et al., (2005) for possible daytime and nighttime variation of conductivity in the ionospheric D- and E- layers. Hayakawa et al., (2008) showed the short term spectral modification, associated with the Moshiri (Japan) earthquake, in the SR frequency band of 2.5- 40 Hz. The study of the Schumann resonance frequency variation has become an important tool for ionospheric characterization in terms of electron density variations and monitoring sudden disturbance in the ionosphere during solar proton events (Roldugin, et al., 2001; Roldugin, et al., 2003). Precise measurements of the SR frequencies with high spectral resolution were carried out from the audiomagnetotelluric and magnetotelluric data (Beamish and Tzanis, 1986; Melnikov, et al., 2004; Tulunay et al., 2008). Present work is devoted to study the following points related to the ionospheric response to the earthquakes and Schumann resonances: (a) Ionospheric temperature and density responses to earthquakes using SROSS-C2 satellite data for Indian region. (b) Characterization of SR frequencies using MT data acquired from Himalayan region and its application in determination of electron density variation in lower ionospheric region. (c) Acquired MT data were also used to delineate electrical structure of earth crust along a selected profile in Himalayan region which is added as an appendix in the thesis. The entire work is presented systematically in the form of following five chapters. A brief description of ionospheric weather and its variability has been presented in Chapter-I. The descriptions of the solar phenomena, thunderstorms, lightning/sprites and their electromagnetic fields generated due the phenomena occurring above and below it have also been given. Chapter-ll, The studies on the diurnal, seasonal variations of the ionospheric electron and ion temperatures and their ratio (Te/Ti) using SROSS-C2 satellite data in ionospheric F2 region during the period from 1995 to 1999 is presented. The ionospheric electron temperature shows the 'morning overshoot' to about 3 to 5 times of average electron temperature and 1.5 to 2 times of average ion temperature during the sunrise hours. The temperature ratio Te/Ti, was almost unity during the nighttime and shows variations during the daytime in all seasons. The ionospheric temperature response to earthquakes has been studied. 14 earthquake events were analysed in 6 years (January 1995 to December 2000) data recorded by SROSS-C2 satellite and found that 13 events out of 14 show increase in average electron temperature during earthquake days by an amount approximately 1.1 to 1.5 times of the normal days while 14 events out of 15 show the increase in ion temperature during earthquake days by amount 1.1 to 1.3 of the normal days. Electron and ion temperature data were analyzed in such a way that the diurnal, seasonal, latitudinal, longitudinal, altitudinal, solar flare, thunderstorm and magnetic storm variations are not masked with the earthquake anomalies. The seismogenic vertical electric field and associated electromagnetic radiation from epicentral zones reaches up to ionospheric height, induces joule heating, might be responsible for the ionospheric temperature perturbation. Chapter-Ill, we have analyzed the ion density response to the earthquake for H+, He+, NO+, N02+ ions in the ionospheric F2 region. 12 earthquake events were analysis for the ion density anomalies in ionospheric F2 region for which satellite data matches with the epicenter location. Out of these, 10 events show decrease in ion density during earthquake days over normal days while only 2 events show increase during earthquake days than normal days. The observed density variation is generally consistent with the corresponding temperature anomalies. Care has been taken to remove the masking of other effects in the VI earthquake anomalies. We therefore conclude that the ion density anomalies are related to earthquake events and in reasonable agreement with the temperature anomalies. Chapter-IV discusses the Schumann resonance (SR) frequency variation using Magnetotelluric (MT) data recorded in one of the world toughest and generally inaccessible Himalayan terrain. Spectral analysis of MT time series data at frequency resolution of 0.03 Hz has been performed using Fast Fourier transform (FFT) algorithm. Spectral stabilization in Schumann resonance mode has been achieved by averaging over thirty two individual power spectral magnitude. Average amplitude and frequency variation in the Schumann resonance are presented. It has been observed that pair of same polarization components, shows a similar variation in SR frequencies. However, different frequency variations were observed for different polarizations: north-south (N-S) and east-west (E-W) magnetic field components. SR frequency variation obtained in the recorded data was used to estimate electron density in lower ionosphere. For this purpose we have reformulated Roldugin et al., (2003) analytic expression for SR frequencies in a two-layer wave guide model. We believe, however, their corresponding formula is incorrect and proposed the slightly modified alternative expression relating SR frequency with the electron density in the lower ionosphere. We have then estimated the electron density in lower ionosphere from SR frequencies. The estimated electron density has b een compared with the values derived from the International Reference ionosphere (IRI) model. Magnetotelluric (MT) data recorded in Himalayan region were also used to determine electrical structure of the upper crust to a depth of about 6 km along the selected profile. Based on the regional geology, possible geological interpretation of the Geoelectrical model is presented. The MT studies have been kept in the appendix of the present thesis. Summary and conclusion is presented in chapter V along with the recommendations for future work.