V. A. Liperovsky

Modifications of sporadic E-layers caused by seismic activity

Among all earthquake precursors, those related to electromagneticeffects are the most puzzling, and the many possible sources ofnoise are cause of lively controversies. A large number oflaboratory experiments clearly suggest that micro-fracturingis associated with the appearance of spontaneous charge production(electrification) and transient Electric or ElectroMagnetic (EM)Emission. Many electric and magnetic pre-seismic and co-seismiceffects have been reported in the past as well as ionosphericperturbations. The aim of this paper is to review some of theseionospheric perturbations performed in the former Soviet Union.The importance of sporadic E-layer formation is underlined.Statistical studies with the available observations are done.At the end, a physical model of lithosphere-ionosphere couplingis presented in order to explain the observations. This modeltakes into account the following effects at the Earths surface:electric charge generation, emanation of radioactive gas,temperature variation, and surface vibration. It is shown thatthese effects can trigger sporadic E-layer formation.

Physical Models of Coupling in the Lithosphere-Atmosphere- Ionosphere System before Earthquakes

The most important models of coupling in the lithosphere-atmosphere-ionosphere system are considered. In some of these models, it is assumed that atmospheric acoustic and acoustic gravity waves (AGWs), which propagate through the atmosphere and reach ionospheric altitudes (resulting in the generation of electric field disturbances and modulation of charged particle density), are generated in the near-Earth atmosphere over the earthquake preparation region. In other models it is assumed that ionospheric disturbances originate owing to the modification of electric fields and currents due to electric processes in the lithosphere or near-Earth atmosphere. It seems impossible to stress on only one model and reject the remaining models because the characteristic spatial scales of effects observed in the ionosphere before earthquakes vary from 200–300 km to several thousand kilometers, and the characteristic times vary from several minutes to several days. We can assume that there are several physical mechanisms by which the lithosphere-ionosphere coupling is actually implemented.

On the generation of modified low-frequency Farley-Buneman waves in the solar atmosphere

The possibility of the excitation of Farley-Buneman turbulence in the solar atmosphere is examined. It is found that the conditions for the generation of the modified Farley-Buneman instability can be realized in the chromosphere of the Sun 1000 km above the photosphere. While usual Farley-Buneman waves studied in relation to the Earths ionosphere are almost electrostatic, the modified Farley-Buneman waves in the solar atmosphere are electromagnetic ones. This means, that not only the potential electric field caused by the charge distribution, but also the perturbations of the magnetic field and the circularly-polarized electric field are essential. Although the physical pictures of usual and modified Farley-Buneman waves are different, their dispersion equations are almost the same. However, the increment of the modified Farley-Buneman waves is varied by additional electromagnetic effects. It is demonstrated that electromagnetic effects hinder a Farley-Buneman instability in occurring while ξ 1, no Farley-Buneman disturbances appear at all. In weakly-ionized solar regions, the modified (ξ < 1) and also the usual (ξ ≪ 1) Farley-Buneman turbulence could make “electromagnetic” contributions to the process of energy dissipation of nonstationary streams of neutral gases. Besides, they may modify the low-frequency acoustic noise. It seems that the modified Farley-Buneman turbulence contributes to the sporadic radiation of the Sun. It is possible, that such an effect takes not only place in the chromosphere of the Sun, but also in the atmospheres of other stars.

Stabilization of the Farley-Buneman instability by three-wave interaction as consequence of the modification of the speed of energy transfer from an external electric field

The energetics of the stabilization of Farley-Buneman (FB) waves by three-wave interaction is studied. Considering decay processes of three FB waves in an open system, which are supposed to be called “quasi-decay processesŔ, it is shown, that during wave stabilization the direct energy transfer between the waves, which is essential for the stabilization of instabilities by normal decay processes in closed plasma systems, is less effective. According to the estimates, a decreasing growth rate of a FB wave, and thus a possible stabilization of the FB instability, may occur as a consequence of the change of the speed of energy transfer from an external electric field, which is caused by the action of two other waves.

Influence of collisional heating on the current generation in moving mid-latitude sporadic E-layers

Abstract A theoretical analysis of local heating processes above the borders of night-time midlatitude dense sporadic E-layers moving under the action of strong acoustic impulses in the ambient plasma of lower density at altitudes of 90 km to 130 km is done. It is seen, that a large velocity of motion of the layer generates a strong Hall current in the layer closed by an external circuit. The field-aligned currents of the external circuit can cause remarkable heating of the electron component by collisions with the neutral particles. As a consequence the field-aligned conductivity in the external circuit is considerably reduced and the Hall current in the sporadic layer becomes smaller. But the estimates show, that the Hall current can nevertheless reache values above the threshold of the excitation of Farley-Buneman waves. Besides it can be possible, that the field-aligned currents generate ion-acoustic and drift waves near the borders of the sporadic layers.

Ion-acoustic instability caused by neutral wind action on sporadic E-layers

Abstract The local occurrence of the ion-acoustic instability in the night-time ionosphere at altitudes between 80 km and 150 km is investigated. The ion-acoustic instability can be excited in the ionosphere if the electron field-aligned velocity exceeds a threshold of about 10 4 m/s. It is shown that such conditions can be locally fulfilled for field-aligned elements of a current system generated by the action of intensive acoustic impulses on sporadic E-layers. Here the occurrence of one sporadic layer or of two sporadic layers connected by the same magnetic field lines is considered. The estimates show that under different rare geomagnetic conditions the excitation of ion-acoustic waves is possible.

Sporadic E-layers as current generators

Quasi two- and three-dimensional models of dense night-time sporadic E-layers of the earths ionosphere as current generators are developed. The layers are assumed to be situated in an ambient plasma of about fifty times lower density and their neutral particles possess a rather high bulk velocity relative to the ions. The quasi-two dimensionality of the first model results from the assumption that relative electron-ion drifts in the neutral wind direction are almost compensated. The ion current in the neutral wind direction is caused by collisions with neutrals, but the electron current is a result of the appearing electrical polarization field. Within the proposed model the electron current is closed by an external circuit, for which a rough description is developed. The electrical polarization field is considered and estimated taking into account the altitude profile of the electrical conductivity in the external circuit. Further, a quasi-three-dimensional model of local current generation caused by the action of short-duration intense neutral winds on sporadic E-layers of finite horizontal dimensions is presented. In the analysis, the Hall and Pedersen currents in the sporadic layer as well as in the less dense plasma above the sporadic layer are taken into account. Thus, the currents in two horizontal plasma layers of different density and the field-aligned currents connecting the borders of these two layers are considered. A two-fluid hydrodynamic analysis of the system shows that maximum Hall currents occur if the sporadic layer-generator is situated at altitudes of about 120 km. The bulk electron velocity in these currents can reach values of the order of the neutral wind velocity.

Quasi-three-dimensional model of current generation in the ionosphere caused by neutral wind action on Es-clouds

The paper is devoted to questions connected with the possible effects of excitation of local current systems in the E-region of the ionosphere on the basis of a quasi-three-dimensional model. A hypothesis of local current generation caused by the action of short-time intense neutral winds on sporadic E-layers of finite horizontal dimensions is accepted. Currents in two horizontal Es-layers and field-aligned currents connecting the borders of these two layers are considered. The Hall currents in sporadic layers can be strong enough to generate Farley-Buneman turbulence. Some heating effects of local electric fields and the effects on the electron distribution function are found which cause effective excitation of some ionosphere neutral components.

Weekly anthropogenic phenomena in the F2 layer of the ionosphere

A statistical analysis of the behavior of the characteristic frequency f0F2 during the seven days of the week in 1987 is performed. Italian data from a station in Rome situated in an industrial center and from a station in Gibilmana, which is located in a rural area, are used. Since industrial activity in European countries decreases rapidly during weekends, the behavior of the density of the F2 layer shows weekly periodicity. By expansion of this quantity in a Fourier series it is found that the intensity of the second harmonic with period T=12 h increases at the end of the work week. Moreover, the correlation coefficient between pairs of successive days increases monotonically during the week. Since this correlation coefficient is directly connected with the phase shift of the harmonics from one day to the next, it can be concluded that industry influences systematically the behavior of the ionospheric density over industrial centers. Conceivably, this effect is due to local random heating of the ionosphere by anthropogenic infrasound waves.