Reception conditions of low frequency (LF) transmitter signals onboard DEMETER micro-satellite

https://doi.org/10.1016/j.pce.2016.07.006Get rights and content

Highlights

  • We study the LF transmitters signals observed by DEMETER/ICE electric field experiment.

  • We select LF stations localized around the Mediterranean and the black seas.

  • We examine the solar and geomagnetic effects on LF signals.

  • We show the drop of the flux density few days before earthquakes occurrences.

Abstract

We analyse the flux density variation associated to low frequency (LF) broadcasting transmitters observed by the ICE electric field experiment onboard DEMETER micro-satellite, observed from 01st Jan. to 09th Dec. 2010. We select five stations localized around the Mediterranean and the Black seas: Tipaza (252 kHz, 02°28′E, 36°33′N, Algeria), Roumoules (216 kHz, 06°08′E, 43°47′N, Monte Carlo), Polatli (180 kHz, 32°25′E, 39°45′N, Turkey), Nadour (171 kHz, 02°55′W, 35°02′N, Morocco) and Brasov (153 kHz, 25°36′E, 45°40′, Romania). The detection of the LF transmitter signals by DEMETER micro-satellite is found to depend on the radiated power, the emitted frequency, and the orbit paths with regard to the location of the stations. This leads us to characterize the reception condition of the LF signals and to define time intervals where the detection probability is high. We show that LF signal are regularly recorded, each 12 days, when the satellite is above the broadcasting station. The signal intensity levels are principally significant during the solar activity. Hence we find that the solar and the geomagnetic activities are slightly correlated to the maxima of LF signal as recorded by DEMETER. Also we note a drop of the intensity level several days before the occurrence of earthquakes in/around the Mediterranean and Black seas.

Introduction

Ground-based low frequency broadcasting stations operate near continuously with a power of about 100 kW. Measurements of the amplitude of low frequency signals propagating in the Earth-ionosphere waveguide have long been used effectively for remote sensing of the lower ionosphere. Such signals are particularly appropriated to investigate D- and E-layers of Earth's ionosphere. In this study we consider the low frequency signals emitted by broadcasting stations localized around the Mediterranean and Black seas. Hereafter we introduce three aspects which are related to the analysis of the transmitter signals: (a) the very low frequency (VLF) and the low frequency (LF) European network, (b) the solar and geomagnetic activities and (c) the seismo-ionospheric perturbations over earthquakes. The frequency bands associated to VLF and LF signals are, respectively, 3–30 kHz and 30–300 kHz.

Since 2009 a network of VLF and LF radio receivers is operating in Europe in order to study the disturbances produced by the earthquakes on the propagation of these signals. The network was formed by nine receivers located in Italy, Austria, Greece, Portugal, Romania, Russia and Turkey (Biagi et al., 2011). The receiver is an equipment working in VLF and LF bands which can monitor 10 frequencies selected in these bands. A list of the radio signals sampled by each European receiver of the network is given in Table 2 of Moldovan et al. (2012). The receiver has two standard XLR antenna connectors, one for each band, with four poles. The two antennas are rectilinear ones, long 1.5 m (LF band) and 2.0 m (VLF band), respectively. The configuration of the equipment, its status and the download of the collected data are possible by a standard powerful and ubiquitous Ethernet interface that can be connected to a PC with an Ethernet port. The collected data is daily recorded with a sampling period of one minute.

Another type of VLF/LF receiver is used in Austria which is part of the low frequency European network. The VLF station is localized in Graz (Austria) and operating continuously throughout the year with a network-wide selected temporal resolution of 20 s. The eleven VLF and LF signals are synchronised with a GPS device. The selected transmitters and broadcast signals are emitted by stations in Europe and USA, eight are common within the collaboration network (Schwingenschuh et al., 2011). The VLF receiving chain is equipped with a wind load, thermal, and lightning protected 4 m antenna installed on the roof of a building and a low noise amplifier mounted in its immediate vicinity. The VLF signal and the GPS signal (from which a pulse per second is finally derived) are routed with shielded cables into the A/C room. The signals are digitised with a MAudio Delta 44 sound-card and a special purpose software (UltraMSK) delivers the raw format of the data. The daily data files from each transmitter are post-processed, validated, and together with quick-look plots are transferred to a file server in front of the firewall, where they are available for the European teams.

The geomagnetic activity results from two main causes (Hathaway and Wilson, 2006): (a) transient phenomena (such as solar flares, prominence eruptions, and coronal mass ejections) and follows the sunspot cycle, while (b) the recurrent phenomena (like high-speed solar wind) is almost 180° out of phase with the sunspot cycle.

The low frequency waves were exploited to investigate the ionospheric perturbation associated with solar and geomagnetic activities because the ionosphere is sensitive to the solar-terrestrial effects. Several parameters are used to estimate the degree of activity of the Sun and the Earth's magnetic field: the sunspot numbers, the solar radio flux, Kp-index, Ap-index, Dst-index. Boudjada et al. (2012) investigated the solar and geomagnetic effects on VLF signals emitted by ground-based stations. Like the low frequency broadcasting signals most of the energy radiated by such transmitters is trapped between the ground and the lower ionosphere, which forms the Earth-ionosphere waveguide. The authors showed that the degree of correlation in periods of high geomagnetic and solar activities is, on average, about 40%. Such effects can be fully neglected in the period of weak activity. Also it was found that the solar activity can have a more important effect on the VLF transmitter signal than the geomagnetic activity.

Since more than twenty years detailed observations of electromagnetic signals occurring before seismic events have been reported. Such precursor emissions have been found to cover a large frequency spectrum. Important aspect of these earthquakes (EQ) research is the analysis of the ionospheric disturbances observed principally over the seismic regions. EQ precursory signatures appear not only in the lithosphere, but also in the atmosphere and the ionosphere.

The original ideas have been proposed by Gokhberg et al. (1982) and Gufeld et al. (1992). The authors suggested the use of anomalies in the Earth-ionosphere waveguide propagation of VLF navigation signals for a short-term earthquake prediction. The principal studies showed anomalies (amplitude and/or phase) in the radio signal few days (from 3 days to 10 days) before the occurrence of large earthquakes with a magnitude more than 5.5 in the case of VLF (Hayakawa and Sato, 1994, Hayakawa et al., 1996, Molchanov and Hayakawa, 1998, Rozhnoi et al., 2004, Yamauchi et al., 2007) and LF bands (Biagi and Hayakawa, 2002, Biagi et al., 2001, Biagi et al., 2004, Biagi et al., 2007, Biagi et al., 2012). The conclusion of these authors are that the ionospheric D- and E-layers are affected by EQs occurrence. However the spatial area of the analysis was limited to a narrow zone along the path between the transmitter and the receiver (Rozhnoi et al., 2004).

A new method of ionospheric sounding in connection with earthquakes activity has been proposed by Molchanov et al. (2006). These authors used the transmitter signal of ground-based station as recorded by instrumentation of the electric field (Berthelier et al., 2006) and magnetic field experiments (Parrot et al., 2006) on board DEMETER microsatellite. The analysis of intensity levels of transmitter signals allowed to find that these ground signals decreased few days before the seismic event occurrence (Molchanov et al., 2006). This result has been further confirmed in several papers (Rozhnoi et al., 2007, Muto et al., 2008, Slominska et al., 2009, Rozhnoi et al., 2009, Rozhnoi et al., 2010, Rozhnoi et al., 2012). Papers were based on the monitoring of VLF/LF signals recorded by ground-based stations and/or space observations. The applied method of Molchanov et al. (2006) consists in estimation of the signal to noise ratio using a relationship between the amplitude spectrum density in the frequency band including the transmitter frequency and the signal outside of the signal band. Another and different method based mainly on the processing of the recorded dynamic spectrum, for each half-orbit, leads to estimate the maximum of intensity level through the observed channel frequencies of ICE experiment (Boudjada et al., 2008, Boudjada et al., 2010). Both methods show a decrease or a drop of the transmitter signal over seismic regions.

In this paper, we investigate the flux density variation associated to low frequency (LF) broadcasting transmitters recorded by DEMETER micro-satellite. In Section 2, we define the methodology and we report on the main solar and geomagnetic activities, and the earthquakes recorded in/around the Mediterranean and Black seas. The detection of the broadcasting transmitter signals by DEMETER satellite is presented in Section 3 and combined to solar, geomagnetic and seismic activities. Main results are discussed and summarized in Section 4 and Section 5.

Section snippets

Methodology

The LF broadcasting signal emitted by the ground-based stations is aimed to be reflected by the Earth's ionosphere. However part of the LF signal is detected by the DEMETER micro-satellite. We select five LF stations around the Mediterranean Sea and we analyse the reception conditions of the LF signal by DEMETER satellite. We apply the same method of data processing for the five stations. First, we select a geometrical area centred on the LF station and depending on its geographical coordinate

Detection of broadcasting transmitter signals by DEMETER satellite

We have selected five low frequency (LF) stations localized around the Mediterranean and Black seas. Hereafter their geographic locations (i.e. longitude λ and latitude φ) and the corresponding LF emitted frequency (frq): (a) Brasov (Romania, λB = 25°36′E - φB = 45°40′N, frq_B = 153 kHz), (b) Nadour (Morocco, λN = 02°55′W - φN = 32°02′N, frq_N = 171 kHz), (c) Roumoules (Monte Carlo, λR = 06°08′E - φR = 43°47′N, frq_R = 216 kHz), (d) Tipaza (Algeria, λN = 02°28′E - φT = 36°33′N, frq_T = 252

Discussion

We show in this work that the low frequency broadcasting signals, emitted by the ground stations, are detected by DEMETER micro-satellite. The satellite is coming back periodically above the station each 12 days. In this time interval the signal is decreasing, drops a minimum and then increases again. A regular recurrence of this periodic variation allows us to characterize the condition of reception of each broadcasting station.

Conclusion

We study the reception of the low frequency broadcasting signals by DEMETER micro-satellite. We show local time effect where the low frequency signal is mainly attenuated during day-side orbits of DEMETER. Also the absence of LF broadcasting signal occurs at frequency higher than 200 kHz. Hence the Nadour (Morocco) and Brasov (Rumania) LF stations are continuously detected on the night orbits. The dependence on solar, geomagnetic and seismic activities is, respectively, about 28%, 7% and 22%.

Acknowledgements:

We acknowledge C. N. E. S. for the use of the DEMETER data, and thankful to Jean-Jacques Berthelier who provided us with data from the electric field experiment (Instrument Champ Electrique – ICE).

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