Work Package 3 - VLF transmitter broadening over seismically active regions


Objectives

The main objectives of WP3 were:

  1. To find out whether or not broadening of VLF transmitter signals on board a satellite over a seismically active region or the magnetically conjugate region, may serve as a precursor of earthquakes.
  2. If the above objective is true, then try to understand the physical mechanism and to suggest a theoretical model of this phenomenon.
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Description

The physical mechanism of the observed VLF effects in satellites still remains unsolved. It might be explained by the effect of VLF signal scattering due to ionospheric turbulence which in turn has been modified by seismic activity. An example of this process was reported from a statistical analysis of plasma density measurements on the Russian satellites Cosmos-900, Intercosmos-24 (Molchanov et al., 2004) and from plasma-wave observation on the French-Russian satellite Aureol-3 (Hobara et al., 2005). The further investigation of space plasma parameters, using sophisticated mathematical analysis techniques is proposed in the project.

The possibility of interactions between the VLF signal and ionospheric turbulence can be investigated using the spectral characteristics of VLF signals. Spectrum broadening of a signal is important for the estimation of the interaction type: either linear or nonlinear scattering. Up to now, correlation between spectral broadening of the transmitter signal and seismic activity has not been established for certain on a statistically significant basis. There are a number of published mechanisms to explain observations of spectral broadening that are unrelated to seismic activity (Bell et al., 1983; Titova et al., 1984; Tanaka et al., 1987). However, a consistent mechanism, pertinent to earthquake related problems, is still missing. As proposed in this project, the development of a corresponding mechanism shall be a significant contribution to understanding of the ionosphere processes due to seismic activity.

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Highlights

Workpackage 3 originally intended to investigate the effects of the broadening of the signals from groundbased transmitters that are observed by the low altitude satellite DEMETER. The objective was to investigate whether such effects could be attributed to increased levels of seismic activity. However, during an early stage of the analysis it was discovered that the relative signal amplitudes of dual frequency transmitters could exhibit large variations. This variation occurs due to changes in the ionospheric composition, which can result from increased seismic activity. These perturbations of the ionosphere modify the characteristics of the transmitter signals as they propagate through the inner magnetosphere. Task 3.1 was therefore redirected to carry out a full survey of this phenomenon that was subsequently modelled in Task 3.2.

Task 3.1 - Data analysis

Using the measurements from the DEMETER satellite and the earthquake database from the US geological survey server, we have performed statistical analysis of the LHR frequency above seismic regions to show a statistically significant correlation between variations of the LHR frequency and gathering earthquakes. Based on this observation, we suggest monitoring of man-made VLF signals with frequencies f close to the maximum of lower hybrid resonance (LHR) frequency (fLHR)max in the hemisphere opposite to that of VLF transmitters.

The propagation of signals close to (fLHR)max may be qualitatively different for f > (fLHR)max and f < (fLHR)max, and since (fLHR)max is very sensitive to plasma distribution on the "base level" in the ionosphere, observation of unexpected transmitter signal variations can indicate unexpected variations of the LHR frequency and, thus, of plasma distribution above the receiver possibly related to seismic activity. Figure 3.1 shows the maximum LHR frequency above the satellite (green) calculated from the lower cutoff of LHR noise in the DEMETER VLF spectrograms. A sharp increase of both the local LHR frequency (blue) and maximum (green) is observed a few days before the earthquake. This is accompanied by a corresponding decrease in the spectral intensity at a frequency of 11.9kHz, the lowest of the working frequencies of the groundbased VLF Alpha transmitters. As the primary causes of changes in the LHR frequency profile may be different processes in the Earth’s atmosphere, ionosphere, and the magnetosphere, other than gathering earthquake, the unexpected variations in the transmitter signal amplitude should only be considered as one indicator in a list of possible earthquake precursors.

Changes in wave power spectra

Figure 3.1: Variation of the LHR frequency and wave spectral intensity measured by DEMETER at the time of the New Zealand earthquake on 15th July, 2009.

Task 3.2 - Development of a theoretical model.

A possible mechanism has been suggested of variation in the ion distribution and the LHR frequency profiles above seismically active regions. Such variations were studied previously (and reported under deliverable 3.1) on the basis of DEMETER measurements. It has been shown that the variations mentioned above can take place due to particle transversal drift caused by large-scale electric field arising above the region of gathering earthquake. Registrations of such electric fields were previously reported in the literature. We have undertaken a detailed analysis of plasma redistribution caused by the particle drift under reasonable assumptions about the localization and properties of the seismic-related electric field and initial particle distribution.

Ion density distributions as functions of height along the geomagnetic field line and the corresponding profiles of the LHR frequency for various concentrations on the base level are shown in Figure 3.2. In each row, the left figure shows the concentrations (the x-axis) of electrons and three ions (see the legends) as the functions of height (the y-axis), while the right figure shows the corresponding LHR frequency profile. Concentrations of each ion at the base level of 400 km can be seen in the pictures. Other parameters used in the calculations are as follows: geomagnetic latitude at the base level λ = 30° , L-shell = 1.42, electron temperature Te = 4250 K, ion temperature (equal for all types of ions) Ti = 1830 K. Essential variations of the LHR frequency maximum and the height on which this maximum is situated, in response to changes in relative ion concentrations on the base level, are clearly seen. When the concentration of heavy ions on the base level increases the maximum of the LHR frequency decreases, while its height increases. An increase of the proton concentration on the base level leads to an increase of the LHR frequency maximum with the simultaneous decrease of its height.

Changes in wave power spectra

Figure 3.2: Variation of plasma composition (left column) and the profile of the LHR frequency (right).

The present consideration amplifies the previously performed analysis and earlier suggested method of monitoring of seismic activity.

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Reports

D3.1 Variations of VLF transmitter signals above the regions of gathering earthquakes
D3.2 Modification of ion concentration profiles and the lower hybrid resonance above seismically active regions.

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