Search

A novel tool utilizing machine learning techniques was designed to forecast ap index values for the next three consecutive days (24 values). The tool employs time series data from the 3 h ap index of solar cycles 23 and 24 to train the Long Short-Term Memory (LSTM) model, predicting ap index values for the next 72 h at three-hour intervals. During periods of quiet geomagnetic activity, the LSTM model's performance is sufficient to yield favorable outcomes. Nevertheless, during geomagnetically disturbed conditions, such as geomagnetic storms of different levels, the model needs to be adapted in order to provide accurate ap index results. In particular, when coronal mass ejections occur, the ap Prediction tool is modulated by inserting predominant features of coronal mass ejections such as the date of the event, the estimated time of arrival and the linear speed. In the present work, this tool is described thoroughly; moreover, results for G2 and G3 geomagnetic storms are presented. [ABSTRACT FROM AUTHOR]

As the current solar cycle 25 progresses and moves towards solar maxima, solar activity is increasing and extreme space weather events are taking place. Two severe geomagnetic storms accompanied by two large Forbush decreases in galactic cosmic ray intensity were recorded in March and May, 2024. More precisely, on 24 March 2024, a G4 (according to the NOAA Space Weather Scale for Geomagnetic Storms) geomagnetic storm was registered, with the corresponding geomagnetic indices Kp and Dst equal to 8 and −130 nT, respectively. On the same day, the majority of ground-based neutron monitor stations recorded an unusual Forbush decrease. This event stands out from a typical Forbush decrease because of its high amplitude decrease phase and rapid recovery phase, i.e., 15% decrease and an extremely rapid recovery of 10% within 1.5 h, as recorded at the Oulu neutron monitor station. Furthermore, on 10–13 May 2024, an unusual G5 geomagnetic storm (geomagnetic indices Kp = 9 and Dst = −412 nT) was registered (the last G5 storm had been observed in 2003). In addition, the polar neutron monitor stations recorded a Ground Level Enhancement (GLE74) during the recovery phase of a large Forbush decrease of 15%, which started on 10 May 2024. In this study, a detailed analysis of these two severe events in regard to the accompanying solar activity, interplanetary conditions and solar energetic particle events is provided. Moreover, the results of the NKUA "GLE Alert++ system", the NKUA/IZMIRAN "FD Precursory Signals" method and the NKUA "ap Prediction tool" concerning these events are presented. [ABSTRACT FROM AUTHOR]

Over the last years the Athens Cosmic Ray Group of the National & Kapodistrian University of Athens has implemented a warning tool called GLE Alert, which is a highly credible application that issues alerts when a ground level enhancement (GLE) starts due to very high energy solar energetic particles reaching the Earth. This application warns of a high intensity solar energetic particle event up to several minutes before it reaches near the near-Earth space environment. In this work, an assessment of the latest updated version of GLE Alert, GLE Alert++, is presented. GLE Alert++ is a federated product of the ESA S2P SWE Space Radiation Expert Service Centre, which is part of the ESA Space WEather Service NETwork (SWESNET) project. The assessment of the GLE Alert++, which was finalized in October 2022, focused on: (a) the availability of the real-time data provided by the neutron monitor stations that contribute to the GLE Alert++, (b) the behaviour of each station regarding the different Alert levels status (Watch, Warning and Alert), and (c) the definition of the real-time assessment index. The results of this work are of essential importance since they ensure a reliable and trustworthy warning tool, and can be highly useful in protecting humans during extreme solar energetic events. [ABSTRACT FROM AUTHOR]

In recent decades, the exposure of aviation crews and passengers to cosmic radiation has been progressively increasing due to the growing need of flights. Therefore, there is a need for radiation assessment software tools to act as a complement to other radiation protection techniques and countermeasures. In this work, the exposure to cosmic radiation is estimated for flights from Athens International Airport to various international destinations, by performing Monte Carlo simulations with the validated tool DYASTIMA/DYASTIMA-R. The results of the estimated total ambient dose equivalent, as well as the ambient dose equivalent rate for different flights, applying a typical flying level and constant atmospheric conditions, are presented for the first time. This study is carried out for different phases of solar activity for the time period from 1996 to 2019, which includes the two recent Solar Cycles 23 and 24. [ABSTRACT FROM AUTHOR]

Visible light positioning systems (VLP) have attracted significant commercial and research interest because of the many advantages they possess over other applications such as radio frequency (RF) positioning systems. In this work, an experimental configuration of an indoor VLP system based on the well-known Lambertian light emission, is investigated. The corresponding results are also presented, and show that the system retains high enough accuracy to be operational, even in cases of low transmitted power and high background noise. [ABSTRACT FROM AUTHOR]

A laser beam propagating in the free space suffers numerous degradation effects. In the context of free space optical communications (FSOCs), this results in reduced availability of the link. This study provides a comprehensive comparison between six machine learning (ML) regression algorithms for modeling the refractive index structure parameter ( C n 2) . A single neural network (ANN), a random forest (RF), a decision tree (DT), a gradient boosting regressor (GBR), a k-nearest neighbors (KNN) and a deep neural network (DNN) model are applied to estimate C n 2 from experimentally measured macroscopic meteorological parameters obtained from several devices installed at the Naval Postgraduate School (NPS) campus over a period of 11 months. The data set was divided into four quarters and the performance of each algorithm in every quarter was determined based on the R2 and the RMSE metric. The corresponding RMSE were 0.091 for ANN, 0.064 for RF, 0.075 for GBR, 0.073 for KNN, 0.083 for DT and 0.085 for DNN. The second part of the study investigated the influence of atmospheric turbulence in the availability of a notional FSOC link, by calculating the outage probability (Pout) assuming a gamma gamma (GG) modeled turbulent channel. A threshold value of 99% availability was assumed for the link to be functional. A DNN classification algorithm was then developed to model the link status (On-Off) based on the previously mentioned meteorological parameters. [ABSTRACT FROM AUTHOR]

The new space era has expanded the exploration of other planets of our solar system. In this work, radiation quantities are estimated in the Venusian atmosphere using the software tool DYASTIMA/DYASTIMA-R, such as the energy deposit and the ambient dose equivalent rate. Monte Carlo simulations of the secondary particle cascades for different atmospheric layers were performed during solar minimum and solar maximum conditions, as well as during the extreme solar particle event that took place in October 1989, with a focus on the so-called Venusian zone of habitability. [ABSTRACT FROM AUTHOR]

A ground-level enhancement (GLE) event is a sudden increase in cosmic ray intensity originated by solar sources and recorded by ground-based detectors. GLEs are invariably associated with large solar flares that can release and accelerate solar particles at high energies. The minimum kinetic energy of particles reaching the Earth's surface is >433 MeV at sea level and about 300 MeV/n at high-mountain altitude of about 3000 m a.s.l. Even though these abrupt events linked to solar activity are quite rare, they can have a great impact on technological systems and human health when recorded. Therefore, the accurate and effective prognosis of such events is of great importance. In this paper, an overview of the most recently recorded GLE event and the first of solar cycle 25, i.e., GLE73, as well as a post-event analysis is presented. GLE73 was detected on 28 October 2021 and was associated with the active region AR12887 on the central part of the solar disk, which produced an X1.0 solar flare. The event was registered by several stations of the worldwide ground-based neutron monitor network. An accurate alert was issued successfully by the ESA R-ESC federated product GLE Alert Plus, as well as the updated GLE Alert++ System of the Athens Neutron Monitor Station (A.Ne.Mo.S.). It should be emphasized that the GLE Alert++ signal by NKUA/A.Ne.Mo.S. was issued 45 min earlier than the one issued by GOES. A short description and the advantages of this last system are provided. [ABSTRACT FROM AUTHOR]

The main source of the ionization of the Earth's atmosphere is the cosmic radiation that depends on solar activity as well as geomagnetic activity. Galactic cosmic rays constitute a permanent radiation background and contribute significantly to the radiation exposure inside the atmosphere. In this work, the cosmic-ray-induced ionization of the Earth's atmosphere, due to both solar and galactic cosmic radiation during the recent solar cycles 23 (1996–2008) and 24 (2008–2019), was studied globally. Estimations of the ionization were based on the CRAC:CRII model by the University of Oulu. The use of this model allowed for extensive calculations from the Earth's surface (atmospheric depth 1033 g/cm2) to the upper limit of the atmosphere (atmospheric depth 0 g/cm2). Monte Carlo simulations were performed for the estimation quantities of radiobiological interest with the validated software DYASTIMA/DYASTIMA-R. This study was focused on specific altitudes of interest, such as the common flight levels used by commercial aviation. [ABSTRACT FROM AUTHOR]

Recently, wireless communications started incorporating FSO systems to a large extent due to their high bandwidth and the low installation and operational cost. However, their performance deteriorates significantly due to the atmosperic conditions. A very significant issue is the atmospheric turbulence which causes the scintillation effect and results in fast power fluctuations at the receiver which can be modelled through the appropriate stochastic models. In this work, for first time to the best of our knowledge, the average block error rate (BLER) performance of a terrestrial FSO communication link with receivers' diversity is estimated and new mathematical expressions are derived, for weak to strong turbulence channels with the Mixture-Gamma distribution which emulate accurately the most of the well-known turbulence models. The derived expressions are significant, especially for high data rates FSO communication systems and networks, because by estimating accurately the BLER performance, the appropriate coding scheme, for links designing, can be selected. [ABSTRACT FROM AUTHOR]

Free space optics (FSO) technology has demonstrated an increasingly scientific and commercial interest over the past few years. However, due to signal propagation in the atmosphere, the operation depends strongly on the atmospheric conditions and some random impairments, including turbulence and pointing error (PE) effects. In the present study, a single-input multiple-output FSO system with wavelength, spatial, or time diversity over the turbulence and non-zero boresight PE effects is thoroughly investigated. A versatile mixture composite model which accurately describes both impairments is employed for the performance evaluation. Novel mathematical expressions of the outage probability and the average bit-error rate assuming intensity modulation/direct detection and optimal combining at the reception are provided. [ABSTRACT FROM AUTHOR]

The authors consider a point‐to‐point optical wireless communication link with on–off keying (OOK) modulation scheme. The propagation is deteriorated by the joint effects of gamma–gamma atmospheric turbulence and misalignment induced fading described by a non‐zero boresight pointing error model. Mathematical expressions are derived for the average block error rate and their accuracy is verified through numerical simulations with Monte Carlo method. [ABSTRACT FROM AUTHOR]

The authors consider a point-to-point optical wireless communication link with on-off keying (OOK) modulation scheme. The propagation is deteriorated by the joint effects of gamma-gamma atmospheric turbulence and misalignment induced fading described by a non-zero boresight pointing error model. Mathematical expressions are derived for the average block error rate and their accuracy is verified through numerical simulations with Monte Carlo method. [ABSTRACT FROM AUTHOR]

Free-Space Optical (FSO) technology can effectively transfer multiple radiofrequency signals, and this new technique is commonly referred in the technical literature as radio on FSO (RoFSO). In the present study, we investigate the evaluation of an RoFSO scenario employing the orthogonal frequency-division multiplexing scheme. Both phase shift keying and quadrature amplitude modulation (QAM) formats are considered, and the performance is investigated over the weak to strong atmospheric turbulence regime characterized by the recently launched Málaga or M-distribution probability density function (pdf). This pdf acts as a unifying statistical model, which incorporates several common irradiance scintillation distributions extensively used by researchers working in the FSO area. We particularly obtain useful mathematical expressions of some critical metrics, i.e., the average bit error rate (BER) and the probability of outage. Proper numerical results are also provided and depicted using suitable graphs to verify the accuracy of the derived expressions. [ABSTRACT FROM AUTHOR]

This paper elaborates on the end-to-end capacity of dual-hop free-space optical (FSO) communication systems employing amplify-and-forward (AF) relaying, assuming channel state information is only known at the receiving terminals. The relay is assumed to either possess perfect channel state information or have a fixed gain. The performance of the considered system is affected by the combined effects of atmospheric turbulence-induced fading, pointing errors (i.e., misalignment fading), and path loss. Atmospheric turbulence conditions are modeled using the gamma-gamma distribution. For the system under consideration, accurate analytical approximations as well as upper bounds to the ergodic capacity are derived. In addition, bound approximations in the high signal-to-noise ratio regime are deduced that provide valuable insights into the impact of model parameters on the capacity of AF FSO dual-hop relaying systems. Numerically evaluated and computer simulation results are further provided to demonstrate the validity of the proposed mathematical analysis. [ABSTRACT FROM PUBLISHER]

In this paper, we derive mathematical expressions for the evaluation of the average (ergodic) capacity of free-space optical (FSO) communication systems. Atmospheric turbulence conditions are modeled using the I-K distribution. Our newly derived expressions provide an efficient tool to assess the spectral efficiency of FSO communication systems. Numerically evaluated and computer simulation results are further provided to demonstrate the validity of the proposed mathematical analysis. [ABSTRACT FROM PUBLISHER]

Terrestrial free space optical telecommunication systems offer license-free very high bandwidth access characteristics with significantly low installation and operational cost. On the other hand, the performance of such a system depends strongly on the weather conditions in the area between the transmitter and the receiver due to the fact that the transmission media is the atmosphere. A very significant phenomenon which decreases a FSO links' performance is atmospheric turbulence. In order to counterbalance this reduction, in this work, we consider a free space optical system with a multiple receivers' scheme and diversity in space that operates under weak to strong atmospheric turbulence conditions modeled by the gamma–gamma distribution. We derive closed form mathematical expressions for the estimation of the outage probability and the average bit error rate of the multiple receivers' system. Additionally, we compare the performance capabilities of the single point to point link with those of the spatial diversity scheme under the assumption that the total aperture surface of the receivers of the latter is the same as the aperture surface of the single receiver of the former. Moreover, we demonstrate numerical results for a variety of common practical cases for both cases, with and without, spatial diversity. [ABSTRACT FROM PUBLISHER]

In recent years, there has been a huge increase in air travel, both for business and leisure. For this reason, entities such as the European Commission and the International Committee on Radiological Protection have provided several recommendations for the radiation protection of aviation crews and frequent flyers, as well as highlighted the need for accurate tools for radiation assessment in the atmosphere. With a focus on the most frequent commercial flying altitudes, this work has performed dosimetry calculations in the lower atmosphere of Earth for different values of cut-off rigidity, covering the recent solar cycles 23 and 24. Results are based on Monte Carlo simulations performed with the validated Geant4 software application Dynamic Atmospheric Shower Tracking Interactive Model Application (DYASTIMA) and its extension, DYASTIMA-R. [ABSTRACT FROM AUTHOR]

Nonadherence to medication is a widespread problem in schizophrenia and is associated withpoor clinical outcomes and inappropriate management and utilization of resources. The aimof the current investigation was to assess the impact of telemonitoring of medication adherenceon symptomatology and service use in patients with schizophrenia. A total of 108 schizophreniapatients were randomized into three equal groups according to the approaches usedto assess medication adherence; self-report, pill counting, and telemonitoring. Telementoringwas achieved through an innovative new platform called @HOME. This platform offers cliniciansearly warnings about impeding nonadherence as well as information about the patternof medication taking. Patient's adherence was observed over an 8-week period, during whichpatient's clinical status and service use were recorded.In comparison to the other two groups, patients using @HOME showed improvement inthe Global Clinical Impression Scale and a significant reduction in emergency visits and medicalappointments. The @HOME platform was highly acceptable by patients, caregivers, andprofessionals, and required minimal training for implementation.The results of the study suggest that the use of telemonitoring in psychiatric settings wasboth feasible and acceptable and may be associated with significant clinical and service relatedbenefits. [ABSTRACT FROM AUTHOR]

In the recent years, Free Space Optics (FSO) technology has attracted significant research and commercial interest mostly because of its many advantages in comparison with other radio systems used for point-to-point connections. However, the reliable operation of these systems significantly depends on the conditions of the atmosphere in the area in which the optical beam propagates. The most important of these conditions are atmospheric turbulence and the misalignment between the optical beam and the receiver, which is also known as the pointing errors effect. In this work, in order to obviate the performance mitigation caused by these phenomena, we examined the most widely accepted and one of the most effective techniques, i.e., the implementation of receivers' diversity. Various metrics have been investigated to evaluate the performance of such systems, but most of them do not take into account that the ultra-fast modern optical communication systems use blocks of bits for the transmission and codes for the detection and/or correction of erroneous bits. Thus, by taking these aspects into account, in this work, we investigated the combined impact of spatial jitter and atmospheric turbulence on the total average block error rate of an optical wireless system with receivers' diversity. Novel closed-form analytical formulas were derived. [ABSTRACT FROM AUTHOR]

The modes of crack propagation, as well as the fracture surface of carbon Fiber-Reinforced composites were studied by means of a Scanning Electron Microscope (SEM). The composite specimens used were composed of an epoxy polymer matrix reinforced with unidirectional carbon fibers. In order to study the influence of the difference in the mechanical proper ties of the constituent materials on the overall crack propagation behavior of the composite, three different types of carbon fibers were used, while several amounts of plasticizer were added in the matrix material. From the crack propagation behavior observed from the microphotographs, conclu sions concerning the strength of the composite, the mode of failure, and the adhesion between the phases were derived. [ABSTRACT FROM PUBLISHER]

An experimental technique based on the optical method of caustics for the solution of the two-dimensional problem of a narrow contact between two elastic disks was developed. In particular, an elliptical pressure distribution of the Hertz type was assumed, as well as a rectilinear contact form according to Muskhelishvili's solution. The method presents the advantage of yielding directly the contact length, from which the load applied on the disks can be readily evaluated. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Clinical Periodontology is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The performance of an underwater optical wireless communication link is investigated by taking into account—for the first time and to the best of our knowledge—the simultaneous influence of the chromatic dispersion, the time jitter and the turbulence effects, by assuming chirped longitudinal Gaussian pulse propagation as information carriers. The estimation procedure is presented and a novel probability density function is extracted in order to describe the irradiance fluctuations at the receiver side. Furthermore, the availability of the link is investigated by means of its probability of fade and various numerical results are presented using typical parameters for the underwater optical wireless communication systems. [ABSTRACT FROM AUTHOR]

The obsolete communication systems used in the underwater environment necessitates the development and use of modern telecommunications technologies. One such technology is the optical wireless communications, which can provide very high data rates, almost infinite bandwidth and very high transmission speed for real time fast and secure underwater links. However, the composition and the optical density of seawater hinder the communication between transmitter and receiver, while many significant effects strongly mitigate the underwater optical wireless communication (UOWC) systems' performance. In this work, the influences of chromatic dispersion and time jitter are investigated. Chromatic dispersion causes the temporal broadening or narrowing of the pulse, while time jitter complicates the detection process at the receiver. Thus, the broadening of the optical pulse due to chromatic dispersion is studied and the influence of the initial chirp is examined. Moreover, the effect of the time jitter is also taken into consideration and for the first time, to the best of our knowledge, a mathematical expression for the probability of fade is extracted, taking into account the influence of both of the above-mentioned effects for a UOWC system. Finally, the appropriate numerical results are presented. [ABSTRACT FROM AUTHOR]

Visible Light Communication (VLC) systems use light-emitting diode (LED) technology to provide high-capacity optical links. The advantages they offer, such as the high data rate and the low installation and operational cost, have identified them as a significant solution for modern networks. However, such systems are vulnerable to various exogenous factors, with the background sunlight noise having the greatest impact. In order to reduce the negative influence of the background noise effect, optical filters can be used. In this work, for the first time, a low-cost optical vanadium dioxide (VO2) optical filter has been designed and experimentally implemented based on the requirements of typical and realistic VLC systems in order to significantly increase their performance by reducing the transmittance of background noise. The functionality of the specific filter is investigated by means of its bit error rate (BER) performance estimation, taking into account its experimentally measured characteristics. Numerous results are provided in order to prove the significant performance enhancement of the VLC systems which, as it is shown, reaches almost six orders of magnitude in some cases, using the specific experimental optical filter. [ABSTRACT FROM AUTHOR]

This study investigated the outage performance of a terrestrial FSO communication system that uses mixed series and parallel decode-and-forward (DF) relay-assisted (i.e., cooperative diversity) configurations, taking into account the influence of both atmospheric turbulence and pointing error effects. Turbulence-induced optical signal fading is modeled by gamma-gamma or the negative exponential distribution for weak to strong and saturated turbulence conditions, respectively. Additionally, weak to strong non-zero boresight misalignment-induced optical signal fading is modeled by the generalized Beckmann distribution. Under these conditions, an outage analysis of the examined FSO system is performed, in terms of both outage probability and mean outage duration metrics. Thus, fairly accurate closed-form mathematical expressions for both performance metrics are derived, while their corresponding analytical results demonstrate concrete performance and availability improvements for the total FSO system, especially when the number of the connected in parallel DF relays increases. Moreover, the obtained results are verified through the corresponding simulation results. [ABSTRACT FROM AUTHOR]

Abstract Nicopoulou-Karayianni K, Bragger U, Patrikiou A, Stassinakis A, Lang NP. Image processing for enhanced observer agreement in the evaluation of periapical bone changes. International Endodontic Journal , 35 , 615–622, 2002. Aim The aim of the present study was to evaluate the effect of root canal treatment on periapical lesions by conventional and subtracted digital radiographic images of clinical cases. Methodology Eleven patients who exhibited clinical or radiological signs of periapical pathology received root canal treatment. Periapical radiographs were obtained immediately postoperatively and recall radiographs at intervals of 3, 6, 9 and 12 months postoperatively were obtained. Identical exposure geometry was maintained. From the standardized radiographs digitized images were produced. Four experienced practitioners interpreted the radiographs and the digitized images. They assessed 59 pairs of images projected at random using a slide projector. On the left side, there was a reference image with no lesion, and on the right an image either with or without a lesion. Each reader was asked to rate each pair of images on a three-point scale: yes, absolutely sure that gain or loss was present; uncertain, if there was gain or loss; no, absolutely sure that no gain or loss was present. There was no time limit for each decision. Inter- and intra-examiner agreement was analyzed using the kappa-statistic for the diagnosis of periapical bone density changes in the periapical region at the different time points either in conventional pairs of radiographs or using digital subtraction images. Results The inter-examiner agreement (P ≤ 0.001) and the intra-examiner agreement (P = 0.02) when digital pictures where evaluated were significantly higher than with conventional radiographs. Conclusions A highly significantly better observer agreement was achieved by digital subtraction radiography during the evaluation of the outcome of root canal treatment... [ABSTRACT FROM AUTHOR]