O. Molchanov

Pre-earthquake ULF electromagnetic perturbations as a result of inductive seismomagnetic phenomena during microfracturing

Abstract This paper proposes a generation mechanism of ULF electromagnetic noise observed several hours before some strong earthquakes. It is assumed that this effect results from the energization of crack formation and fracture process in a focal zone before the main shock. Electromagnetic noise is considered as a by-product of the summation of electromagnetic micro-fields appearing in the conductive medium ahead of acoustic micro-waves radiated by an ensemble of tensile cracks. These micro-fields and currents are generated due to the movement of conductive medium in the geomagnetic field. The electromagnetic perturbations depend on the direction diagram of radiation and the attenuation of acoustic waves emitted by opening cracks. We investigate two different zones and regimes of ULF magnetic field propagation. In the near zone we expect the prevailing diffusion regime, where the sign of components of electromagnetic fields is found to be the same for all the cracks independent of their space orientation. A similar coherent effect takes place at the far or acoustic zone. These surprising effects occur, because the effective magnetic moments of current systems generated by the cracks are always directed opposite to the vector of geomagnetic field; i.e. these moments are strongly paralleled. It was established that the coherent amplification of ULF signals generated by the cracks could appear if only their size exceeds several critical values. Such an effect might appear at the background of low seismic activity because the acoustic emission of cracks is incoherent. The averaging of the fields over the ensemble of opening cracks allowed us to relate the parameters of electromagnetic noise to porosity, volumetric strain, dilatational coefficient and average size of macro-cracks at the earthquake hypocenter. Our theory predicts magnetic and electric perturbations to be of the order of 1– 10 nT and 1– 10 μV/m , respectively, at the distance 10– 50 km from the epicenter. These values are very close to those observed prior to the occurrence of earthquakes at Loma Prieta and Spitak.

Analysis of lightning ‐ induced ionization from VLF Trimpi events

Detailed analysis of Trimpi phenomena (Trimpis) revealed two different regimes in their temporal behavior. One time regime is the initial “nose” part of Trimpis of about 1–5 s, and the second is the “tail” of Trimpis of about 50–100 s. Using a rather simple theoretical model of VLF wave scattering from abating ion and electron density perturbations, we discovered two fitting formulas to describe the temporal evolution of Trimpis. An exponential-hyperbolic evolution in amplitude a/a0 = (exp{−t/τ0})/[1 + b/2(1 − exp{−t/τ0})] for weak scattering and a logarithmic dependence a/a0 = [ln τ2/(t + τ1)]/(ln τ2/τ1) for strong scattering. Parameters b, τ0, τ1, τ2 were calculated for all the observed 50 Trimpi events. Maximal ion and electron density perturbation (Δnmi and Δnme) were estimated together with the background ion and electron density (ni0 and ne0) and vertical length of the perturbation column (ΔH). For the conventional model of background electron density and horizontal scale of perturbations L ≃ 5 – 10 km, we find the perturbation parameters to be as follows: Δnme ≃ 103 – 104 cm−3, Δnme/ne0 ∼ Δnmi/ni0 ≃ 10 – 100, ni0 ≃ 3 × 103 − 3 × 104 cm−3, and ΔH ∼ 10 – 15 km.

Propagation characteristics of whistler waves in the Jovian ionosphere and magnetosphere

The purpose of this paper is to carry out extensive reexamination of the propagation characteristics of unducted whistlers in the Jovian magnetosphere. In addition to the previous work on the dispersion of whistlers, our emphasis is placed on the detailed study of their amplitude variations in order to investigate the plasma structure of the magnetosphere and characteristics of causative lightning discharges. The two-dimensional ray-tracing computations are used as before to investigate magnetospheric whistler propagation paths, but with the first attempt of estimating the whole attenuation factors (polarization effect, focusing/defocusing and interactive damping (Landau and cyclotron)). The total attenuation due to three effects is estimated, which is compared with the amplitude of whistlers observed by the Voyager spacecraft. The following conclusions are reached. (1) Some bias in magnetospheric plasma density is required to explain the high-frequency cutoff of whistlers, (2) the magnetospheric temperature is about 20 eV for the interpretation of low-frequency cutoff of whistlers, and (3) estimated total radiated power per flash is about 4.5×106 W(0∼20 kHz) on the assumption of unducted propagation and nearly the same even in the case of ducted propagation.

Method of modeling of VLF wave propagation in the Earth's magnetosphere and upper ionosphere

A method of ray tracing (RT) with amplitude variations for simulating VLF wave propagation in a weakly inhomogeneous upper ionosphere and magnetosphere is discussed in detail. This ray tracing plus amplitude (RTA) technique is different from the existing ones in the selection of the most complete model with all its plasma parameters dependent on geomagnetic activity, allowance of thermal correction to the refractive index, and calculations of amplitude factors (coefficients of focusing-defocusing, polarization, and linear amplification-attenuation). The presented examples of calculations show that the introduction of the above new elements in the RT technique makes it possible to clarify the wave propagation properties and to account for numerous experimental phenomena.

A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes

The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over seven years from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion and nighttime fluctuation) and earthquakes with magnitude greater than 6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40km). Finally some comments on the mechanism of seismo-ionospheric perturbations are discussed.

Effect of rectification of vlf waves in the earth's magnetosphere

This paper discusses the results of an experiment on the detection of a rectified magnetospheric signal (RMS) arising in the action of a strong, modulated, low-frequency wave on the magnetospheric plasma. The signal develops at the modulation frequency of the transmitter and arrives at the receiver with a delay approx. = 3-5 sec. A possible mechanism of RMS generation in the region of resonance absorption of a vlf wave in the magnetosphere is discussed.