Study of VLF/LF wave propagations above seismic areas

Abstract: We report on radio transmitter signals recorded in Europe by INFREP network which is mainly devoted to search for earthquakes electromagnetic precursors (Biagi et al., 2011). We consider in this analysis the detection of transmitter signals recorded by INFREP receivers located in different regions of Europe, i.e. Romania, Italy, Greece and Austria. The aim is the investigation of the electromagnetic environment above earthquakes regions. We selected seismic events which occurred in the year 2016 and characterized by a moment magnitude (Mw) above 5.0 and a depth of less than 50 km. A common method is applied to all events and which involves the analysis of the VLF/LF signal detection taking into consideration the following parameters: (a) the distance transmitters-receivers, (b) the signal to noise ratio during the diurnal and night observations, (c) the daily and night averaged amplitude and (d) the sunset and sunrise termination times. This leads us to specify the key factors which can be considered as criteria to distinguish and to identify earthquakes precursors. We discuss in this contribution the radio wave propagation in the D- and E-layers and their impacts on the VLF/LF amplitude signal. We show that the 'seismic anomaly' requests a more precise analysis of the 'quiet' and 'disturbed' ionospheric conditions and their corresponding spectral traces on the VLF/LF transmitter signals

EFD experiment onboard CSES satellite: Characterization of hiss and chorus whistler emissions during geomagnetic activity

International audienceWe study the geomagnetic activity effects on whistler emissions recorded by the Electric Field Detector (EFD) experiment onboard the China Seismo-Electromagnetic Satellite (CSES). This mission is devoted to investigate the ionospheric disturbances linked to the seismic activity. The satellite has a circular sun-synchronous orbit with a descending node at 14 LT and an altitude of 507 km [1]. Four probes are used to measure the electric field recorded by EDF instrument covering a frequency range from DC up to 3.5 MHz [2]. We consider in this analysis geomagnetic events which occurred in the year 2018 after the launch of the CSES satellite, i.e. on 02nd February 2018. The Kp-index leads us to estimate the variation of the geomagnetic activity which is found to have sudden enhancements on the following days: 21st April, 05th May, 26th Aug. and 10th Sept. We show in this analysis that the whistler emissions, i.e. hiss and chorus occurring in the frequency bandwidth 1 kHz to 20 kHz, are influenced by the Earth's magnetic activity. Hence whistler spectral shapes are globally found to develop towards higher frequencies. Two aspects are discussed: (a) the way to characterize an ionospheric disturbance index taking into consideration the CSES geographical configuration orbit and (b) the comparison of the electric field power levels as derived from EFD/CSES instrument and from ICE/DEMETER experiment [3]

Development of Space Magnetometers in Austria

With spaceborne magnetic field measurements it is possible to investigate the interior of planets,moons and asteroids which have either an intrinsic or a crustal magnetic field. Furthermore, preciseknowledge of the magnetic field is essential to derive fundamental information about theenvironment surrounding different bodies in the solar system as well as to explore the interplanetaryspace. [...]<br /

Sub-ionospheric VLF/LF waveguide variations related to magnitude M>5 earthquakes in the eastern Mediterranean area

International audienceIn this study we examine earthquakes with magnitude M>5 in the year 2022 where the epicenters are crossed by sub-ionospheric narrowband VLF/LF radio links. The study regions are Italy, Aegean area, and the Balkan Peninsula. Ideal suited for this task are paths from the transmitters TBB (26.70 kHz, Bafa, Turkey), ITS (45.90 kHz, Niscemi, Sicily, Italy), and ICV (20.27 kHz, Tavolara, Italy) to the seismo-electromagnetic receiver facility GRZ (Graz, Austria). The receiver is part of a wider network, this gives the opportunity to have multiple simultaneous crossings of an earthquake event.We investigate electric field amplitude variations in the time span a few days around the main shock, in particular we apply the so-called night-time amplitude method. All electric field data sets have 1 sec temporal resolution. A crucial point is a certain threshold magnitude to obtain statistically significant results, but to firm up the results additional complementary investigations are necessary.In summary, VLF/LF investigations of strong earthquakes show the complex interplay between the lithospheric events and electric field amplitude waveguide variations, multi-parametric observations in a network could be a tool to derive robust results

Ray paths of VLF/LF transmitter radio signals in the seismic Adriatic regions

International audienceWe analyze the radio wave propagations of VLF/LF transmitter signals along subionospheric paths using two different reception systems localized in the Graz seismo-electromagnetic facility (15.43E,47.06N). Those systems allow the simultaneous detection of more than fifteen transmitter signals emitting in the northern (i.e. France, Germany and United Kingdom) and southern (i.e. Italy and Turkey) parts of Europe. In this work, we investigate the transmitter radio wave propagations associated with two earthquakes (EQs) which occurred, at two occasions, in nearly the same Croatian regions (Geo. Long.=16°E; Geo. Lat.=45°N). The first and second EQs happened, respectively, on March 22 and December 29, 2020, with magnitudes Mw equal to 5.4 and 6.4. The use of two complementary reception systems, i.e. INFREP (Biagi et al., Open Journal of Earthquake Research, 8, 2019) and UltraMSK (Schwingenschuh et al., Nat. Hazards Earth Syst. Sci., 11, 2011), and the proximity to the epicenters lead us to characterize the behavior of the transmitter signal amplitudes particularly above the Croatian seismic regions. We analyze the amplitude variation for a given transmitter frequency starting few weeks before the earthquakes occurrences. We discuss the observed anomalies in the transmitter signals which may be considered as precursors due to the ionospheric disturbances of the transmitter ray paths above the earthquakes preparation zones

Investigation of VLF/LF electric field variations related to magnitude Mw&#8805;5.5 earthquakes in the Mediterranean region for the year 2023

International audienceStrong natural hazards together with their societal impact are usually accompanied by multiple physical phenomena which can be an important information source about the underlying processes. In this study we statistically analyze the lithosphere–atmosphere–ionosphere couplings of magnitude Mw5.5+ earthquakes (EQs) in the year 2023 with the aid of sub-ionospheric VLF/LF radio links. The electric field amplitude and phase measurements with a temporal resolution of one second are from the seismo-electromagnetic receiver facility in Graz (GRZ), Austria (Galopeau et al., 2023), which is part of the INFREP network. The spatial extend of the study area has the range [-10°E ≤ longitude ≤ 40°E] and [20°N ≤ latitude ≤ 50°N], in total are 17 EQs according to the United States Geological Survey (USGS) data base, among them the Turkey–Syria EQs (main shocks Mw7.8 and Mw7.5) and the Morocco Mw6.8 EQ. We apply the night-time amplitude method (Hayakawa et al., 2010) for all available paths, of particular importance are the transmitter links TBB (26.70 kHz, Bafa, Turkey), ITS (45.90 kHz, Niscemi, Sicily, Italy), and ICV (20.27 kHz, Tavolara, Italy). Relevant crossings are determined by the size of the Dobrovolsky-Bowman relationship (Dobrovolsky et al., 1979; Bowman et al., 1998).A major finding is the statistically significant electric field variation of the TBB-GRZ link related to the Turkey–Syria EQ sequence. A physical interpretation is based on atmospheric gravity waves (AGWs) which could alter the E-layer in the lower ionosphere during nighttime and modulate the height of the waveguide cavity