Stelios M POTIRAKIS

• PhD
• Professor at University of West Attica

Spike structures appear in several phenomena, whereas spike trains (STs) are of particular importance, since they can carry temporal encoding of information. Regarding the STs of the biological neuron type, several models have already been proposed. While existing models effectively simulate spike generation, they fail to capture the dynamics of hi...

This study explores how the variation of Gravity Waves (GWs) is modified and intensified during tropical cyclones using high-resolution ERA5 reanalysis data. GWs play a vital role in understanding tropical cyclone dynamics due to their connection with energy and momentum transfer in the atmosphere. Different issues related to six tropical cyclones...

The present study examines the negative ionospheric response over Europe during two geomagnetic storms on 10–13 May 2024, known as the Mother’s Day geomagnetic superstorm. The first storm, with a peak SYM-H value of −436 nT, occurred in the interval 10–11 May, while the second, less intense storm (SYM-H~−103 nT), followed in the interval 12–13 May....

Solar eclipses present a valuable opportunity for controlled in situ ionosphere studies. This work explores the response of the upper atmosphere’s F-layer during the total eclipse of 8 April 2024, which was primarily visible across North and South America. Employing a multi-instrument approach, we analyze the impact on the ionosphere’s Total Electr...

The purpose of this paper is to discuss the effect of earthquake (EQ) preparation on changes in meteorological parameters. The two physical quantities of temperature (T)/relative humidity (Hum) and atmospheric chemical potential (ACP) have been investigated with the use of the Japanese meteorological “open” data of AMeDAS (Automated Meteorological...

This study explores how the variation of atmospheric gravity waves (AGWs) get modified and intensified during tropical cyclones using high-resolution ERA5 reanalysis data. AGWs play a vital role in understanding tropical cyclone dynamics due to their connection with energy and momentum transfer in the atmosphere. Different issues related to six tro...

Solar eclipses present a valuable opportunity for controlled in-situ ionosphere studies. This work explores the response of the upper atmosphere’s F-layer during the total eclipse of April 8, 2024, which was primarily visible across North and South America. Employing a multi-instrument approach, we analyze the impact on the ionosphere’s Total Elect...

In this article, we examine whether incorporating complexity measures as features in deep learning (DL) algorithms enhances their accuracy in predicting forex market volatility. Our approach involved the gradual integration of complexity measures alongside traditional features to determine whether their inclusion would provide additional informatio...

In this study, we present a six-year (2014–2020) statistical analysis of VLF subionospheric propagation data at 19 VLF receivers from the VLF transmitter with call name JJI and frequency 22.2 kHz, all located in Japan. Moderate and strong earthquakes (EQs) (ML≥4.5 and depth ≤ 50 km) that occurred in the wider area around Japan during the same time...

Launched on 22 November 2013, Swarm is the fourth in a series of pioneering Earth Explorer missions and also the European Space Agency’s (ESA’s) first constellation to advance our understanding of the Earth’s magnetic field and the near-Earth electromagnetic environment. Swarm provides an ideal platform in the topside ionosphere for observing ultra...

It is well established that pre-and co-seismic irregularities in the earth's atmosphere highly depend on a set of parameters. According to the Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) mechanism, these parameters are associated with various channels (thermal, chemical, acoustic, electromagnetic, etc.) through which an earthquake (EQ) prepar...

This study examines the response of the thermal channel within the Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) mechanism during the notable earthquake in Crete, Greece, on 27 September 2021. We analyze spatio-temporal profiles of Surface Latent Heat Flux (SLHF), Outgoing Longwave Radiation (OLR), and Atmospheric Chemical Potential (ACP) using...

The hELlenic Seismo-ElectroMagnetics Network (ELSEM-Net, http://elsem-net.uniwa.gr) is a network of telemetric stations spanning all over Greece for the monitoring of fracture-induced electromagnetic emissions (FEME), often also referred to as electromagnetic radiation (FEMR). At the laboratory scale, it has been proved that FEME/FEMR are produced...

In this work we present the multi-method analysis of very low frequency (VLF) data, acquired by the radio receiver with call name UWA, located in Athens (Greece), in the University of West Attica, focusing on two strong ( ) earthquakes (EQs) that occurred in north-central mainland Greece sequentially, on 3 and 4 March 2021, with very close epicente...

This Special Issue of Annals of Geophysics is dedicated to present recent developments in the field of earthquake (EQ) precursors. Due to their potentially catastrophic nature, EQs have early attracted the attention of humans. During the last decades, a considerable amount of scientific effort has been devoted to the study of EQ precursors, aiming...

The COVID-19 pandemic has had an unprecedented impact on the global economy and financial markets. In this article, we explore the impact of the pandemic on the weak-form efficiency of the cryptocurrency and forex markets by conducting a comprehensive comparative analysis of the two markets. To estimate the weak-form of market efficiency, we utiliz...

In this work we focus on eight strong (M ≥ 5.5) earthquakes (EQs) that happened very recently (2021-2022) in southeastern Mediterranean by analyzing the very low frequency (VLF) subionospheric propagation data, acquired from three receivers installed at the prefecture of Attica in Greece. We used the VLF data received from many transmitters, mainly...

In this work, first, it is confirmed that a recently introduced symbolic time-series-analysis method based on the prime-numbers-based algorithm (PNA), referred to as the “PNA-based symbolic time-series analysis method” (PNA-STSM), can accurately determine the exponent of the distribution of waiting times in the symbolic dynamics of two symbols prod...

In 2023, the ESA’s Swarm constellation mission celebrates 10 years in orbit, offering one of the best ever surveys of the topside ionosphere. Among its achievements, it has been recently demonstrated that Swarm data can be used to derive space-based geomagnetic activity indices, similar to the standard ground-based geomagnetic indices monitoring ma...

It is known that in thermal systems of finite size that are subject to second order phase transitions and until the spontaneous symmetry breaking is completed, the fluctuations of the order parameter obey to the dynamics of critical intermittency. Beyond the SSB, critical intermittency does not hold. Thus, it is not expected that the distribution o...

In this work, we present the analysis of VLF/LF sub-ionospheric propagation data to study anomalies possibly related to very recent strong (M > 5.5) earthquakes (EQs) that occurred in the southeastern Mediterranean in September–October 2021 and January 2022. We used the signal of one transmitter located at Negev in Israel (29.7 kHz) as received by...

We present the atmospheric anomalies instigated through seismogenic sources by a multichannel observation using ground- and satellite-based systems. This study emphasizes the seismic event which happened on the east coast of Japan, near the Fukushima Prefecture on November 21, 2016 (in UTC), with a magnitude of 6.9 and a depth of 11.4 km. We mainly...

The ELSEM-Net (hELlenic Seismo-ElectroMagnetics Network, http://elsem-net.uniwa.gr) is a telemetric network of ground-based monitoring stations for the study of fracture-induced electromagnetic emissions. It comprises 11 telemetric stations, spanning all over Greece, and has continuously been operated for almost 30 years. In this paper we present t...

This article investigates the dynamical complexity and fractal characteristics changes of the Bitcoin/US dollar (BTC/USD) and Euro/US dollar (EUR/USD) returns in the period before and after the outbreak of the COVID-19 pandemic. More specifically, we applied the asymmetric multifractal detrended fluctuation analysis (A-MF-DFA) method to investigate...

This work presents a new analysis method for two-symbol symbolic time series based on the time-to-space mapping achieved through a device of current carrying circular rings. An algorithm based on the theory of prime numbers is proposed for the approximate estimation of the stratified magnetic field produced by the aforementioned device. The main pr...

We study the magnetization dynamics in finite 2D and 3D Ising lattices just below the pseudo-critical temperature $T_{pc}$ when the free energy, as a function of the mean magnetization $M$, possesses doubly degenerate minima at $\pm M_0 \neq 0$. We calculate the jump probability $P_{LR}$ between the microstate-subspaces with $M < 0$ (L) and $M > 0$...

Lithosphere–atmosphere–ionosphere coupling (LAIC) is studied through various physical or chemical quantities, obtained from different sources, which are observables of the involved complex processes. LAIC has been proposed to be achieved through three major channels: the chemical, the acoustic, and the electromagnetic. Accumulated evidence supporti...

The cryptocurrency market is an emerging market that is characterized by intense volatility and therefore the study of its dynamics presents increased interest. The present work investigates the issue of the critical dynamics of a financial complex system approaching a crash, by using the Method of Critical Fluctuation (MCF) which is known for its...

The ground-based monitoring of the lower ionosphere by studying the perturbations of the subionospheric propagation of very-low-frequency/low-frequency (VLF/LF) signals is important in the research of a wide variety of geophysical and Sun/space extreme phenomena. Such perturbations are identified as anomalies in the signal received from the VLF/LF...

We present the variation of unusual atmospheric parameters, aerosols, to understand the pre-seismic irregularities for two major EQs in Japan. Here, we consider Aerosol Optical Depth and Angstrom Exponent data retrieved from Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard TERRA satellite to establish the possible connection...

In this work, we demonstrate the Natural time (NT) analysis of potential energy data that have been computed from temperature profiles that have been obtained from the "sounding atmosphere using broadband emission radiometry" (SABER) instrument, which is a part of a satellite called "thermosphere-ionosphere-mesosphere energetics and dynamics" (TIME...

In this study, we investigate the seasonal variation of sunrise and sunset terminator times of several VLF propagation paths based on 2-year long recordings (2020-2022) from the newly installed VLF/LF receiver UWA, installed in Athens (University of West Attica), by finding their relationship with local sunrise and sunset time. More specifically, i...

A new VLF/LF (10-47.5 kHz) radio receiver, established in 2020 at the University of West Attica (call sign UWA), is continuously monitoring the lower ionosphere, mainly aiming to find pre-seismic indications before earthquakes (EQs), as well as precursors associated with other extreme geophysical and space phenomena. The receiver records signals (a...

A VLF (10-47.5 kHz) radio receiver with call sign UWA has recently been installed at the University of West Attica in Athens (Greece) and is continuously monitoring the lower ionosphere by means of the receptions from many transmitters, in order to identify any possible pre-seismic signatures or other precursors associated with extreme geophysical...

Electromagnetic emissions (EMEs) associated with earthquake (EQ) epicenters are found to be one of the most reliable phenomena for the short-term pre-seismic mechanism of earthquakes. The detection mechanism of such emissions can have both direct and indirect techniques. In this manuscript, we attempted to present the signature of such emissions be...

Random telegraph noise (RTN) owns its very name to its assumed stochastic nature. In this paper, we follow up previous works that questioned this stochastic nature, and we investigate this assumption using experimentally measured noise coming from properly biased Ni/HfO2 unipolar Resistive RAM memristor nanodevices. We have used established, well–k...

Lower ionospheric perturbation during earthquake activity is well recognized by the anomalous behavior of Very Low or Low Frequency (VLF/LF) radio signal. In this study, we present the sub-ionospheric VLF signal modulation during the Samos earthquake that took place on October 30, 2020 (M = 6.9). We use the conventional terminator time method to ch...

The recently introduced [Contoyiannis et al., 2021] hybrid artificial neural network can simulate the dynamics of membrane potential fluctuations of real neurons based on fundamental principles of Physics. Here, we propose a temporal description of the membrane potential fluctuations, which resembles the soliton solutions in φ⁴ field theory. Within...

In the present work we propose an algorithm based on the theory of prime numbers for the estimation of the magnetic field in a device of current carrying circular rings. Using the proposed algorithm, the magnetic field can be determined in a very good agreement with that resulting from an algorithm based on the Biot-Savart law. In addition, the pri...

In this paper, we present a new method for successfully simulating the dynamics of COVID-19, experimentally focusing on the third wave. This method, namely, the Method of Parallel Trajectories (MPT), is based on the recently introduced self-organized diffusion model. According to this method, accurate simulation of the dynamics of the COVID-19 infe...

Atmospheric disturbances caused by seismic activity are a complex phenomenon. The Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) (LAIC) mechanism gives a detailed idea to understand these processes to study the possible impacts of a forthcoming earthquake. The atmospheric gravity wave (AGW) is one of the most accurate parameters for explaining s...

Random telegraph noise (RTN) owns its very name to its assumed stochastic nature. In this paper, we follow up previous works that questioned this stochastic nature, and we investigate this assumption using experimentally measured noise coming from properly biased Ni/HfO 2 unipolar Resistive RAM memristor nanodevices. We have used established, well–...

We focus on the possible thermal channel of the well-known Lithosphere–Atmosphere–Ionosphere Coupling (LAIC) mechanism to identify the behavior of thermal anomalies during and prior to strong seismic events. For this, we investigate the variation of Surface Latent Heat Flux (SLHF) as resulting from satellite observables. We demonstrate a spatio-tem...

In this work we suggest that the production of bosons (mesons) in a cascade process such as the one provided by the Schwinger and Bjorken models for the quark confinement, can be considered as a natural feedback process. Within this feedback process, a recurrent operator acts on the initial boson field function, which is mathematically expressed by...

It has recently been found that the evolution of the preparation of a strong earthquake (EQ), as it is monitored through fracture-induced electromagnetic emissions (EME) in the MHz band, presents striking similarity with the evolution of a thermal system as temperature drops, since distinct steps of the evolution of the phenomenon of spontaneous sy...

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a widespread chronic disease that mostly remains undetected, mainly due to the fact that it is diagnosed via polysomnography, which is a time and resource-intensive procedure. Screening the disease’s symptoms at home could be used as an alternative approach in order to alert individuals that pote...

Combining concepts of artificial neural networks (ANNs) with the stochastic dynamics of Ising spin lattices we introduce a hybrid model, the hybrid spin model (HSM). We find that the HSM carries the critical/tricritical fluctuations of the 2D Ising model and allows for an accurate estimation of the isothermal critical/tricritical exponents of 2D Is...

We present a comprehensive analysis of pre-seismic anomalies as computed from the ground and space-based techniques during the recent Samos earthquake in Greece on 30 October 2020, with a magnitude M = 6.9. We proceed with a multi-parametric approach where pre-seismic irregularities are investigated in the stratosphere, ionosphere, and magnetospher...

The sleep apnea syndrome is a chronic condition that affects the quality of life and increases the risk of severe health conditions such as cardiovascular diseases. However, the prevalence of the syndrome in the general population is considered to be heavily underestimated due to the restricted number of people seeking diagnosis, with the leading c...

The Electrocardiogram (ECG) is a standard diagnostic tool for heart's health condition, while the “gold standard” in ECG interpretation is still by observation by the cardiologist. In this article we show that the analysis of the high frequency ECG amplitude fluctuations, which are ignored during the conventional interpretation of the ECG by the ca...

This paper presents our study of the presence of the unstable critical point in spontaneous symmetry breaking (SSB) in the framework of Ginzburg–Landau (G-L) free energy. Through a 3D Ising spin lattice simulation, we found a zone of hysteresis where the unstable critical point continued to exist, despite the system having entered the broken symmet...

The most common index for diagnosing Sleep Apnea Syndrome (SAS) is the Apnea-Hypopnea Index (AHI), defined as the average count of apnea/hypopnea events per sleeping hour. Despite its broad use in automated systems for SAS severity estimation, researchers now focus on individual event time detection rather than the insufficient classification of th...

In this brief, the spontaneous symmetry breaking (SSB) of the φ4 theory in phase space, is studied. This phase space results from the appropriate system of Poincare maps, produced in both the Minkowski and the Euclidean time. The importance of discretization in the creation of phase space, is highlighted. A series of interesting, novel, unknown beh...

In the frame of complex systems research, methods used to quantitatively analyze specific dynamic phenomena are often used to analyze phenomena from other disciplines on the grounds that are governed by similar dynamics. Technical analysis is considered the oldest, currently omnipresent, method for financial markets analysis, which uses past prices...

In this work we present the statistical and criticality analysis of the very low frequency (VLF) sub-ionospheric propagation data recorded by a VLF/LF radio receiver which has recently been established at the University of West Attica in Athens (Greece). We investigate a very recent, strong (M6.9), and shallow earthquake (EQ) that occurred on 30 Oc...

We show that the spontaneous symmetry breaking can be defined also for finite systems based on the properly defined jump probability between the ground states in the 2d and 3d Ising models on a square and a cubic lattice respectively. Our analysis reveals the existence of an interval in the temperature (control parameter) space within which the spo...

It has recently been found that the evolution of the preparation of a strong earthquake (EQ), as it is monitored through fracture-induced electromagnetic emissions (EME) in the MHz band, presents striking similarity with the evolution of a thermal system as temperature drops, since distinct steps of the evolution of the phenomenon of spontaneous sy...

In this brief, the spontaneous symmetry breaking (SSB) of the $\varphi^4$ theory in phase space, is studied. This phase space results from the appropriate system of Poincare maps, produced in both the Minkowski and the Euclidean time. The importance of discretization in the creation of phase space, is highlighted. A series of interesting, novel, un...

The aim of this work was to investigate a wide range of grain sizes of sand in the pre-yield regime during compression through the combined study of ultrasound (US) wave speed and acoustic emission (AE). The specific study was performed using modified oedometer and uniaxial compression experimental set-ups. The studied samples were natural dune san...

Recently, it has been successfully shown that the temporal evolution of the fraction of COVID-19 infected people possesses the same dynamics as the ones demonstrated by a self-organizing diffusion model over a lattice, in the frame of universality. In this brief, the relevant emerging dynamics are further investigated. Evidence that this nonlinear...

We express the Kirchhoff wave equation in terms of classic field theory. This permits us to introduce the spontaneous symmetry breaking phenomenon in the study of linear structures, such as strings in order to investigate the existence of solitons solutions. We find ϕ⁴ solitons in the space of spatial gradient of lateral displacement of a string. T...

The aim of this work is to study the relationship between the pre-earthquake emissions of radon and ULF/ELF (1–30 Hz) atmospheric electromagnetic radiation. The problem is considered on the example of the 2011 Tohoku earthquake. Radon, ionizing air, creates ions—centers of condensation of water vapor. As a result of condensation, heat is generated....

Simple Summary We demonstrate that remote, automatic monitoring of probed trees for borers at global scales is currently technologically feasible. Vibroacoustic recorders, one per tree, sample on a prescheduled basis (e.g., 20 s per hour) short clips of the internal vibroacoustic soundscene of trees. These clips are compressed and wirelessly transm...

A device consisting of an array of coaxial, identical, circular rings behaves like a solenoid when the ratio of its radius and distance between two successive rings is higher than 1. As this ratio decreases, the device significantly deviates from the solenoid behavior. At the same time, a diffraction-like phenomenon for the magnetic field appears w...

A wide variety of electromagnetic phenomena possibly related with earthquake (EQ) preparation processes have been reported in the literature during the last few decades. An interesting aspect in their study is the time series analysis of the related observables aiming at the investigation of any embedded dynamics. In this review article we focus on...

Acoustic emissions (AE) due to microcracking in solid materials permit the monitoring of fracture processes and the study of failure dynamics. As an alternative method of integrity assessment, measurements of electrical resistance can be used as well. In the literature, however, many studies connect the notion of criticality with AE originating fro...

Is there a wood-feeding insect inside a tree or wooden structure? We investigate several ways on how deep learning approaches can massively scan recordings of vibrations stemming from probed trees to infer their infestation state with wood-boring insects that feed and move inside wood. The recordings come from remotely controlled devices that sampl...

The aim of the study was to understand the changes of ultrasound-wave speed (US) and acoustic emission (AE) as a result of soil loading and unloading. The soil was dune Sand, Poorly graded, i.e., “SP” according to the Unified Soil Classification System (USCS). Natural dune sand, as well as its three extracted fractions as follows: 2.36–0.6, 0.6–0.3...

The self-organizing mechanism is a universal approach that is widely followed in nature. In this work, a novel self-organizing model describing diffusion over a lattice is introduced. Simulation results for the model's active lattice sites demonstrate an evolution curve that is very close to those describing the evolution of infected European popul...

During the last few years, various Industrial Internet of Things (IIoT) applications have emerged with numerous network elements interconnected using wired and wireless communication technologies and equipped with strategically placed sensors and actuators. This paper justifies why non-terrestrial networks (NTNs) will bring the IIoT vision closer t...

In order to have further evidence of the atmospheric oscillation channel of the lithosphere-atmosphere-ionosphere coupling (LAIC), we have studied criticality in global navigation satellite system (GNSS) surface deformation as a possible agent for exciting atmospheric gravity waves (AGWs) in the atmosphere and GNSS fluctuations in the frequency ran...

The detection of power laws in real data is a demanding task for several reasons. The two most frequently met are that (i) real data possess noise, which affects the power-law tails significantly, and (ii) there is no solid tool for discrimination between a power law, valid in a specific range of scales, and other functional forms like log-normal o...

Ionospheric anomalies possibly related to significant earthquakes (EQs) have been extensively investigated in recent years and could be considered promising for short-term EQ prediction. In this work, we study the subionospheric propagation data recorded during a time period of 3 years (1/1/2014–28/02/2017) by a network of 8 VLF/LF receivers deploy...

The previously introduced model of self-organized criticality, is adapted in the case of a virus-induced epidemic. The study presented in the following lines, highlights the critical value of virus density over a population. For low values of the initial virus density (lower than the critical value) it is proved that the virus-diffusion behavior sa...

An alternative way to represent data regarding internal damage processes, specifically the data provided by the acoustic emission (AE) technique and the pressure stimulated currents (PSCs), is attempted in this study, in terms of log‐periodic power‐law models. Both monitoring techniques have been already successfully considered widely, either indep...