Abstract. We present a general concept of mechanisms of preseismic phenomena in the atmosphere and ionosphere. After short review of observational results we conclude: 1. Upward migration of fluid substrate matter (bubble) can lead to ousting of the hot water/gas near the ground surface and cause an earthquake (EQ) itself in the strength-weakened area; 2. Thus, time and place of the bubble appearance could be random values, but EQ, geochemistry anomaly and foreshocks (seismic, SA and ULF electromagnetic ones) are casually connected; 3. Atmospheric perturbation of temperature and density could follow preseismic hot water/gas release resulting in generation of atmospheric gravity waves (AGW) with periods in a range of 6-60 min; 4. Seismoinduced AGW could lead to modification of the ionospheric turbulence and to the change of over-horizon radio-wave propagation in the atmosphere, perturbation of LF waves in the lower ionosphere and ULF emission depression at the ground.

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[1] Local variations of the magnetic field in the ULF-ELF frequency range associated with seismicity are studied with the data of more than 3 a observations at Karimshimo complex observatory (latitude 52.83°N, longitude 158.13°E, Kamchatka, Russia). A wideband emission is found to start about 5 d before an earthquake and last until 5 d after it. Seismic ULF/ELF emission in the frequency range of 4-6 Hz as compared with the seismically quiet background has enhanced P hh /P dd spectral ratio and reduced standard deviation of ellipse orientation angle and the ellipticity, and it has a more linear polarization. Parameters of this emission are studied for more than 30 individual earthquakes and statistically with the superposed epoch method. The reliability of the earthquake predicting hypothesis is verified, and the favorable parameters for the earthquakes together with those for ELF magnetic field are selected. The following earthquake parameters are favorable for this emission: depths H < 50 km, magnitudes M S > 5.5, and epicenter distances R < 300 km. The changes of natural ULF/ELF emissions during the periods of enhanced seismic activity are interpreted as the result of the excitation of additional ULF/ELF emissions in the seismic zone to the east of the observatory or the redistribution of lightning discharges with their possible concentration near the active crust fault. The earthquake prediction hypothesis is verified for the complex field parameter DS and proved to be successful.

The nature of the spatial structure of resonant ULF waves at low latitudes has been studied as part of a joint project between the U.S. Geological Survey, the Institute of Physics of the Earth, Moscow, and the Kyrgyzian Institute of Seismology. Gradient analysis of data taken at a meridional array of three stations in Soviet Central Asia showed that Alfven field line resonances, in the Pc 3 bandwidth, do exist at L = 1.5. Resonant frequencies of 66‐84 mHz (12‐15 s) were measured. Resonance width and the radial gradient of Alfven frequency were determined from our experimental data. When compared with previous published determinations of the resonance width, the resonance width is observed to increase at lower latitudes. This is the result of an increase in ionospheric damping at lower latitudes. Ionospheric damping significantly effects both resonant frequencies and resonance widths. Initial analysis of the data showed that effects of geologic inhomogeneities between two stations can obscure resonant effects that are observed in ground‐based magnetometer data. A method was developed to address these geologic effects so that the response of the resonator can be seen in both amplitude and phase calculations. The cross‐phase spectrum was determined to be the most useful technique to identify the resonant frequency of the field line between two ground stations. The diurnal behavior of resonant frequency was examined using a cross‐phase analysis technique and is shown to agree with theoretical predictions at low latitudes. We can conclude that diurnal variations in resonant frequency are mainly due to diurnal changes in plasma density along the oscillating field line.

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An intriguing effect was found while analyzing the small-scale variations of total electron content (TEC) derived from global positioning system (GPS) signals. We found a response in TEC variations to intense global Pc5 pulsations with periods of a few millihertz covering the corrected geomagnetic latitudes~58°-75°d uring the recovery phase of the strong magnetic storms on 31 October 2003. Earlier studies demonstrated that the GPS-TEC technique is a powerful method to study the propagation pattern of transient disturbances in the ionosphere, generated by seismic or internal gravity waves. This technique has turned out to be sensitive enough to ULF waves as well. During periods with intense Pc5 geomagnetic wave activity, distinct pulsations with the same periodicity were found in the TEC data from high-latitude GPS receiving stations in Scandinavia. Wavelet and cross-spectral analysis showed a high coherence (~0.9) between the periodic geomagnetic and TEC variations. Moreover, the relative amplitude of TEC periodic fluctuations ΔTEC/TEC was about or even larger than the relative amplitude of geomagnetic variations ΔB/B. So far, the effect of TEC modulation by Pc5 waves is not well understood and is still a challenge for the MHD wave theory. Various possible modulation mechanisms have been estimated, but no mechanism has been firmly identified.

[1] The possible occurrence of a new resonator in the topside ionosphere at auroral latitudes is proposed. This resonator is formed between the E-layer of the ionosphere and the bottom boundary of the auroral acceleration region (AAR), which has a localized fieldaligned potential drop. The AAR is shown to effectively reflect Alfven waves with transverse scales less than the Alfven transit scale l A . The proposed resonator can trap and accumulate Alfvenic small-scale disturbances with periods from a few seconds to a few tens of seconds, and with transverse scales from a kilometers to a few tens of kilometers. The eigenfrequencies of the AAR-associated resonator are estimated to be lower than that of the ionospheric Alfven resonator. The small-scale Alfvenic structures commonly observed by satellites can be generated by a nonsteady field-aligned current transported by precipitating electrons and trapped in the AAR-associated resonator. Observations by ground-based magnetometers at auroral latitudes often show the occurrence of intensifications in the dynamic spectra of Pi1 pulsations in accordance with the predictions of the model.

Abstract.A new mechanism for the ionospheric Alfvén resonator (IAR) excitation at middle latitudes is considered. It is shown that the ionosphere wind system in this region is capable of sustaining the generation of geomagnetic perturbations that can be detected by ground magnetometers. The general IAR dispersion relation describing the linear coupling of the shear Alfvén and fast magnetosonic/compressional modes is obtained. The dependence of the IAR eigenfrequencies and damping rates on the perpendicular wave number and on the ground conductivity during the day-and nighttime conditions is analyzed both analytically and numerically. In order to demonstrate the IAR excitation by neutral winds the power spectra of the geomagnetic perturbation on the ground surface are calculated. Furthermore, it is found that Kolmogorov spectra of the ionospheric turbulent neutral winds and the IAR eigenfrequencies lie in the same frequency range that make it possible to enhance the IAR excitation. The relevance of the developed theoretical model to the ground-based observations is stressed.