Studying Earth's ionosphere applying very low radio frequency (VLF) ground-based networks or only single station instrumentation provides spatially and temporal limited information. The majority of the VLF experiments utilize strong naval communication transmitters as their signal sources. In most of those cases, the signal is relatively unknown, except perhaps for its frequency and field strength. Other than ionosondes (vertical ionospheric sounder), VLF radio transmitters are usually located at different locations than their receiver stations. This results in very different radio wave propagation paths and because of different ionospheric regimes (longitudinal, latitudinal, or seasonal variations).

Because VLF radio transmission can be influenced by regular natural sources, e.g. sunrise or sunset, or irregular natural sources, e.g. solar X-ray flare or earthquake, it is non-trivial to provide accurate ionospheric weather forecasts or detect precursors for possible hazards. For natural hazards, such as earthquakes, ionospheric disturbances are often found only after the occurrence of the hazard. Therefore, it is essential to experiment with modified approaches, other than the common ionospheric investigation methods.

The aim of this study is a case-based analysis of the two VLF transmissions from Iceland at 37.5 kHz and 57.4 kHz originating from the Naval Radio Transmitter Facility at Grindavik. Utilizing passive broadband VLF ground-based measurements, we compare observed delay times of the signal behaviors at sunrise and sunset, as well as sudden ionospheric disturbances (SID). Other than the zenith angle dependence, which causes changes of the photoionization at the ionospheric D-region, SID is caused by solar X-ray flare radiation. The solar X-ray flare flux data are provided by NOAA's GOES satellite. The temporal difference in the VLF signal level is of the order of minutes for dusk and dawn. The difference between the two VLF signal levels can not be caused by significant differences in their propagation paths. However, it is assumed that this temporal delay reflects vertical ionospheric composition changes.

Usually, VLF monitoring networks are used for comparing an observed SID event at various VLF frequencies and recorded from various VLF monitoring stations. The D-region enhancement during daytime is stronger during solar flare events. Especially, by comparing the VLF signal levels the spatial effects of solar flares are studied. However, the present study focuses mainly on temporal variation of the signal levels during dusk, dawn, and during SID events, which again could be caused by vertical ionospheric composition changes.

How to cite: Danielides, M. and Skripachev, V. O.: Monitoring of Ionospheric D-Region Behavior Utilizing the Dual VLF Experiment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-840, https://doi.org/10.5194/egusphere-egu21-840, 2021.

The ionospheric electron density reacts to a change of ionization condition by a time delay Δt. Appleton (1953) demonstrated that this time delay is inversely proportional to the product of the electron density Ne and recombination rate coefficient α. Thus, the evaluation of the time difference between the peak time of VLF emission, which is supposed to represent the instant of maximum ionization, and the ionization source's peak time provides an easy way to estimate α Ne. First used to evaluate the increase of electron density at noon from H α peak emission, this technic was also employed to estimate the recombination rate during solar flares. The GOES Soft X-ray emissions (i.e. in the range 1.5-12keV) are then considered to determine the ionising source peak time.

Based on VLF measurements obtained from the SUPERSID antenna installed at the Meudon site of the Paris Observatory (France), we computed each flare's time delay from January 2017. We benefit from the events of September 2017, the strongest from the last 10 years. We thus demonstrate the prominent role of Hard X-Rays in ionizing the D-layer of the ionosphere.  

How to cite: Briand, C., Inturi, S., and Cecconi, B.: Hard X-ray impact on the ionosphere D-layer: new results from VLF measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5623, https://doi.org/10.5194/egusphere-egu21-5623, 2021.

We 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 M_w 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. 

How to cite: Boudjada, M. Y., Eichelberger, H. U., Biagi, P. F., Schwingenschuh, K., Nico, G., Solovieva, M., Ermini, A., Moldovan, I. A., Contadakis, M. E., Nina, A., Katzis, K., Bezzeghoud, M., Lammer, H., Galopeau, P. H. M., Besser, B., and Aydogar, Ö.: Ray paths of VLF/LF transmitter radio signals in the seismic Adriatic regions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7659, https://doi.org/10.5194/egusphere-egu21-7659, 2021.

A new VLF/LF (10 - 47.5 kHz) radio receiver has recently been installed in the University of West Attica, in Athens (Greece), and has been operating in trial mode since April 2020 for the study of sub-ionospheric propagation variations, mainly aiming at the identification of possible earthquake (EQ) precursors or signatures of other extreme geophysical phenomena. The receiver is monitoring signals from a number of transmitters. Most of them are located in Europe, while some are located in Asia, Australia and North America. The recorded data (amplitude and phase) from this receiver are sampled at a rate of 1 sample per second. In this paper we present information about the new VLF/LF receiver as well as preliminary results concerning a very recent, strong (Mw = 6.7), shallow (focal depth = 12 km), EQ that occurred in Greece (epicenter located in the Aegean Sea, off-coast of the Samos island, close to the Greece-Turkey borders) on 30/10/2020, hereafter referred to as Samos’ EQ. The subionospheric propagation data associated with two specific transmitters were analyzed. Τhe first transmitter, with call sign TBB, is located in Denizköy (Turkey) and the location of Samos’ EQ epicenter is within of 5^th Fresnel zone of the corresponding propagation path. The second transmitter, with call sign ISR, is located in Negev (Israel) and the location of Samos’ EQ epicenter is in close distance to the borders of the 5^th Fresnel zone, so that, considering the magnitude of the specific EQ, the corresponding propagation path could possibly be disturbed. In this paper we present the analysis of the receiver’s amplitude data by means of the Terminator Time Method (TTM) in order to reveal any possible pre-seismic anomaly in the lower ionosphere. Our preliminary results show that there are indications for disturbance of the lower ionosphere a few days before the EQ occurrence.

How to cite: Politis, D., Potirakis, S., Biswas, S., Sasmal, S., and Hayakawa, M.: A new VLF radio receiver in Greece for the detection of lower ionosphere anomalies before strong seismic events and the preliminary results for a strong (6.7 Mw) earthquake that occurred in Samos (Greece) on 30/10/2020., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12516, https://doi.org/10.5194/egusphere-egu21-12516, 2021.

This presentation discusses physical processes related to potentially seismic and non-seismic disturbances along VLF/LF paths measured with two different receivers located side by side at the Space Research Institute (IWF) facility in Graz, Austria. At the same time both systems are embedded in international networks which gives the unique opportunity to probe the waveguide cavity over a large area.
In general, a variety of VLF/LF amplitude and phase variations are ubiquitous at wide scales throughout the cavity. We analyse such signals observed in the period 2018-2020 (solar minimum, i.e. less external forcing of the upper ionospheric boundary) in the time- and frequency-domain for several paths. In this attempt we aim to single out natural disturbances, characterise the source event, and figure out the lithosphere-atmosphere-ionosphere coupling mechanism. For known seismic events we consider the so-called Dobrovolsky-Bowman relationship [1,2] allowing to estimate the pre-seismic zone crossed by the VLF/LF paths.
The findings open up good prospects for an automated monitoring and characterisation of source phenomena who affect the electric field of VLF/LF sub-ionospheric links.

Ref:
[1] Dobrovolsky, I.P., Zubkov, S.I., and Miachkin, V.I., Estimation of the size of earthquake preparation zones, PAGEOPH 117, 1025–1044, 1979.
https://doi.org/10.1007/BF00876083
[2] Bowman, D.D., Ouillon, G., Sammis, C.G., Sornette, A., and Sornette, D., An observational test of the critical earthquake concept, JGR Solid
Earth, 103, B10, 24359-24372, 1998. https://doi.org/10.1029/98JB00792

How to cite: Eichelberger, H., Schwingenschuh, K., Boudjada, M. Y., Besser, B. P., Wolbang, D., Solovieva, M., Biagi, P. F., Stachel, M., Aydogar, Ö., Pitterle, M., Muck, C., Grill, C., and Jernej, I.: Synoptic view on sub-ionospheric VLF/LF amplitude and phase variations at the Graz seismo-electromagnetic facility, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10123, https://doi.org/10.5194/egusphere-egu21-10123, 2021.

Since 2009, several radio receivers have been installed throughout Europe in order to realize the INFREP European radio network for studying the VLF (10-50 kHz) and LF (150-300 kHz) radio precursors of earthquakes. Precursors can be related to “anomalies” in the night-time behavior of  VLF signals. A suitable method of analysis is the use of the Wavelet spectra.  Using the “Morlet function”, the Wavelet transform of a time signal is a complex series that can be usefully represented by its square amplitude, i.e. considering the so-called Wavelet power spectrum.

The power spectrum is a 2D diagram that, once properly normalized with respect to the power of the white noise, gives information on the strength and precise time of occurrence of the various Fourier components, which are present in the original time series. The main difference between the Wavelet power spectra and the Fourier power spectra for the time series is that the former identifies the frequency content along the operational time, which cannot be done with the latter. Anomalies are identified as regions of the Wavelet spectrogram characterized by a sudden increase in the power strength.

On January 30, 2020 an earthquake with Mw= 6.0 occurred in Dodecanese Islands. The results of the Wavelet analysis carried out on data collected some INFREP receivers is compared with the trends of the raw data. The time series from January 24, 2020 till January 31, 2000 was analyzed. The Wavelet spectrogram shows a peak corresponding to a period of 1 day on the days before January 30. This anomaly was found for signals transmitted at the frequencies 19,58 kHz, 20, 27 kHz, 23,40 kHz with an energy in the peak increasing from 19,58 kHz to 23,40 kHz. In particular, the signal at the frequency 19,58 kHz, shows a peak on January 29, while the frequencies 20,27 kHz and 23,40 kHz are characterized by a peak starting on January 28 and continuing to January 29. The results presented in this work shows the perspective use of the Wavelet spectrum analysis as an operational tool for the detection of anomalies in VLF and LF signal potentially related to EQ precursors.

How to cite: Nico, G., Biagi, P. F., Ermini, A., Boudjada, M. Y., Eichelberger, H. U., Katzis, K., Contadakis, M., Skeberis, C., Moldovan, I. A., Bezzeghoud, M., and Nina, A.: Wavelet analysis applied on temporal data sets in order to reveal possible pre-seismic radio anomalies and comparison with the trend of the raw data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5154, https://doi.org/10.5194/egusphere-egu21-5154, 2021.

The effects of climate change are causing atypical changes dynamics of tropical glaciers. Conventional methods and optical images were ineffective in measuring these changes periodically due to the complexity of remote mountainous regions and cloud cover. In this research, a Differential Interferometric Synthetic Aperture Radar (DInSAR) analysis has gone performed with Sentinel-1 data from February 2019 to March 2020 in the Cordillera Blanca and Vilcabamba for Mapping displacement and subsidence. The measurements were compared with surface temperature and precipitation, providing zonal statistics to identify and assess regions associated with Glacial Lake Outburst Floods (GLOFs) hazards and enhanced understanding of the glacier dynamics in response to changing climatic conditions.

How to cite: Riveros Lizana, C., Espinoza Villar, R., Jara Infantes, H., and Torres Lazaro, J. C.: DInSAR monitoring of glacier dynamics in Cordillera Blanca and Vilcabamba, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12434, https://doi.org/10.5194/egusphere-egu21-12434, 2021.