Abstract. The plan of this study is to construct analytical exact solutions of space-time fractional Calogero-Degasperis equation and space-time fractional Sharma- Tasso-Olver equation. Our approach stems mainly on applying the generalized (G'/G) expansion method. Fractional complex transform, along with fractional derivative version of Jumarie's modified Riemann-Liouville method, are employed to transform fractional differential equations into the corresponding nonlinear ordinary differential equations. As a result, three types of exact analytical solutions, namely, the generalized hyperbolic function solutions, generalized trigonometric function solutions and rational solutions with free parameters are successfully furnished. A variety of the obtained results are new and reported here for the first time, to the best of our knowledge. We provide proper graphs to illustrate the obtained results. The distinct properties of the obtained solutions, such as soliton characteristics, are enumerated corresponding to the behavior of each solution.

Abstract. In this work, a high efficiency and stable numerical method, which is called exponential time differencing fourth-order Runge-Kutta (EDT4RK) method, is introduced for the investigation of strong nonlinear and dispersive integrable and non-integrable differential equations. This method is applied for studying higher-order nonlinear structures that arise and propagate in plasma. The fluid equations of a plasma consisting of two electrons with different temperatures and immobile positive ions are reduced to a damped Kawahara equation via the reductive perturbation method. The damping term in the evolution equation appears due to taking the effect of electron-ion collision into account. If the damping term is ignored, the evolution equation reduces to the integrable Kawahara equation. The last equation has a hierarchy of exact solutions such as solitons and cnoidal waves. For modeling the dissipative solitons in the present plasma model, the numerical solution to the damped Kawahara equation is obtained via operating the EDT4RK method. The influence of the physical parameters related to the model under consideration on the higher-order dissipative solitons is investigated. The obtained results are novel solutions for non-integrable/damped Kawahara equation. The EDT4RK method is a powerful and a fourth-order accurate technique for obtaining numerical solutions with high accuracy and longtime stable.

Abstract. We study two types of discrete nonlinear waves based on discrete optical waveguides. One is the scattering problem of the spatial discrete dark solitons, and the other one is the excitation of the discrete X-waves. The unidirectional flow of the discrete dark solitons is found by the interaction between a mobile discrete dark soliton and an embedded defect created by a pair of parity-time (PT) symmetric waveguides with mutually balanced gain and loss. The scattering is an inelastic process, where the discrete dark soliton can be reflected, transmitted or trapped depending on its transversal speed and strengths of the embedded defects. We show numerically the unidirectional flow of the discrete dark solitons and the trapping of discrete dark soliton by two of PT-symmetric defects. Thus it is possible to perform the diode effect and the storage of single and multiple discrete dark solitons. We numerically demonstrate that the optical discrete X-waves can be excited for a wide range of initial conditions, namely by the interplay between discrete diffraction, normal temporal dispersion, and nonlinearity in PT-symmetric waveguide arrays.

Abstract. In this article, two- and three-dimensional nonlinear fractional partial differential systems are solved by employing the methods of double and triple Laplace- Adomian decomposition. The applicability of these methods is examined on some known examples. The numerical simulations of the obtained results reveal that these methods are simple and effective on finding the exact and approximate solutions for some mathematical models. Moreover, they show that these methods are fully harmonious with the complexity of nonlinear problems.

Abstract. The collisions between two unmodulated (waves that propagate at a phase velocity) and modulated (waves that propagate at a group velocity) dust kinetic Alfvén solitons (DKASs) have been analyzed in a low-β dusty plasma consisting of inertial negatively dust charged grain in addition to Boltzmann-Maxwellian distributed electrons and ions. In this work we study two important types of solitons collision. The first one is the unmodulated solitons collision. For studying the collisions between the unmodulated solitons, the extended Poincaré-Lighthill-Kuo (PLK) technique has been adopted to derive the two Korteweg-de Vries (KdV) equations for the right and left moving solitons and their phase shifts. The analysis showed that for small amplitudes, i.e., weakly nonlinearity and quasi-elastic collisions, the colliding solitons amplitude at the colliding point nearly equals to the sum of individual amplitudes. Moreover, the impact of the related physical parameters on the phase shifts is examined. In the second part of this paper, the KdV equation is converted to the nonlinear Schrödinger equation (NLSE) for investigating the modulated dark solitons. Then for studying the dark soliton collisions, the PLK technique is introduced. Furthermore, the impact of relevant plasma parameters on the dark soliton phase shift is reported. This work may apply to understand the creation of nonlinear coherent structures and their collisions in astrophysical and space plasma domain where dust grains exist.

Abstract. Machine Learning (ML) has shown a substantial impact on computational sciences in recent years. The adaptation of ML techniques to deal with various systems in physical sciences has gained ground in addition to the existing numerical methods. In this work, we introduce the readers to machine learning with special reference to Artificial Neural Networks (ANNs) that can solve ordinary differential equations (ODEs) and partial differential equations (PDEs) including those which are subject to specific symmetries. This paper will be helpful for graduate and undergraduate students as an introductory material to early career researchers interested in applying ML techniques to solve problems in computational sciences. In particular, we choose elementary differential equations that describe systems from various fields of science to illustrate the proficiency of ANNs to capture the regularities that underlie such systems in the hope of adding ML techniques to the physicists' toolbelt.

Abstract. Ultra-high energy cosmic rays (UHECRs) are subatomic charged particles accelerated at astounding energies in violent astrophysical processes. They travel vast distances through the outer space and every so often enter the Earth's atmosphere. Here they interact and initialize the development of cascading secondary elementary particles, known as extensive air showers (EASs). From characteristics of the induced air showers, properties of primary cosmic rays are deduced, such as mass, energy and arrival direction, and so they serve in complementary indirect measurements of UHECRs. The radio detection technique is used to record radio signals emitted by the electromagnetic component of air showers, employing arrays of radio antennas. In this paper we look at the response of a model hexagonal layout of 37 antennas, by using a sample of CoREAS simulated proton events at two different energies: 10¹⁸ and 10¹⁹ eV. First we look at the general characteristics of the registered polarized radio signals, in correlation with the position of the observer, with respect to the shower core and distance to the showers axis. In a second step we explore some particular characteristics of the radio emission.

Abstract. Based on the previously proposed model of accounting for quantum fluctuations for the Born-Infeld models, which arise from the string theories (strings and superstrings) and Heisenberg-Euler Lagrangian, the dynamics of an ultrashort optical pulse is considered. The Lagrangian contains the fourth degree in field strength. Higher derivatives are also taken into account in the Lagrangian. The dynamics of an ultrashort pulse is considered in the approximation of slowly varying amplitudes and phases. The terms arising due to the presence of higher derivatives in the Lagrangian stabilize the pulse and increase its lifetime until collapse.

Abstract. We report the results obtained on recording of light-induced gratings in As₂S₃ films and in the monitoring, analyzing and modelling of this process. The gratings were recorded by the illumination of the chalcogenide film with an interference pattern of two Ar laser beams (Λ = 514.5 nm) at the As₂S₃ band-gap energy, E_g = 2.39 eV. At this wavelength the light modifies both refractive index and absorption coefficient of As₂S₃, and also the film thickness, inducing a mixed phase-amplitude grating with two components (refractive index and relief) of the phase grating. The dynamics of the complex recording process is modelled in the frame of the Raman-Nath diffraction theory using a simplified model of the energy-band structure of As₂S₃. The magnitudes of the absorption coefficient, refractive index, surface relief modulations and time constants of the recording process have been determined.

Abstract. After more than 10 years in development, the nonlinear topological photonics is emerging as a new branch of physics. One of the most interesting subjects in the nonlinear topological photonics are the topological edge solitons. These solitary structures move along the edges of photonic crystals with constant speed, are immune to disorders/defects along the way, and maintain their profiles unchanged during long-distance propagation. In this paper, we present bright and dark valley Hall edge solitons in the kagome photonic lattice. These solitons emerge at domain walls that exist between different types of kagome lattices. We are interested in the wall between two specific types: the squeezed and expanded kagome photonic lattices. The solitons move along the wall without change in their profiles, thanks to the self-action effect of nonlinearity, and can circumvent sharp corners, thanks to the topological protection. Advances achieved in this paper represent new progress in the nonlinear topological photonics and may lead to applications in the development of novel photonic chips.

Abstract. This work reports the effect of the rapid solidification technique and thermal treatment on the martensitic transformation (MT), magnetic and magnetostrictive properties on the off-stoichiometric Ni₄₉Mn₃₁Ga₂₀ and Ni₅₁Mn₂₈Ga₂₁ ferromagnetic shape memory ribbons. The samples were investigated by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, magnetic and magnetostrictive measurements. The temperature dependence of the X-ray phases analysis shows the presence of martensite structures, both tetragonal and monoclinic, at room temperature and allowed to study their evolution through MT. The thermal treatment induces changes in the microstructure with implications in MT and Curie temperatures evolution. The competition between the magnetization orientation and twin boundary motion within martensitic variants under magnetic field evidenced in the magnetic-strain curves was discussed and correlated with the magnetic data.

Abstract. Pollen levels in rapidly developing urban areas are of particular interest due to their negative impact on human health, being responsible for the increasing prevalence of seasonal allergic diseases. This study analyzed multiyear data (2014–2019) of the pollen concentrations in correlation with major air pollutants PM₁₀, PM_2.5, NO_x, CO, VOC_s, O₃, SO₁ and meteorological parameters from Bucharest, in order to find potential links between them. The pollen monitoring performed at Colentina Clinical Hospital using a Hirst-type pollen trap showed that maximum values of pollen concentration from trees are reached in early spring, from grasses in spring and early summer and from weeds in late summer and fall. The correlation analysis was performed using the Spearman correlation coefficient on annual and seasonal basis and revealed the influence of air pollutants and meteorological parameters on pollen concentrations. No monotonic decreasing or increasing trend was detected for Bucharest during the investigated 6-year period, but a general constant behavior.

Abstract. Clear Air Turbulence (CAT) is a weather phenomenon often met in the atmosphere, creating disturbances in air traffic efficiency and regularity. This work analyses CAT occurrence probability in Romanian airspace considering the number of in-flight pilot reports (AIREP) and air traffic density. The study used AIREPs between June 2017 and May 2019. In 24 months, 496 moderate or severe turbulence encounters at altitudes equal to or higher than 6100 m were reported, amongst which 19% severe turbulence reports. The results confirm the significant role of topography as a CAT generating factor within the Romanian airspace.

Abstract. Stratocumulus clouds represent one of the key components of the Earth's radiative balance because it generally reflects incident solar radiation. The aim of the study is to understand the occurrence and characteristics of stratocumulus clouds using satellite data collected from Dec 2019 to Feb 2021. A series of macrophysically and microphysical cloud parameters (cloud cover fraction, cloud types, cloud geometrical depth, cloud top temperature, cloud top pressure, cloud height, cloud optical depth, liquid water path) were extracted from the Clouds and the Earth's Radiant Energy System (CERES) database for a region in south west Bucharest, were the Magurele Center for Atmosphere and Radiation Studies (MARS) is located.

Abstract. The PREVENT project's main objective is to systemize the research outcomes and the computational capabilities in order to develop, implement and test an integrated dynamic platform for civil structures monitoring. The present paper focuses on the description of the instrumentation setup, dataflow, the platform architecture and preliminary results for the most recent Vrancea intermediate-depth earthquakes. The expected outcomes of the project will consist of the first high-quality database recorded on structures and will provide a valuable tool to the research community, earthquake engineering community, authorities and stakeholders as well as to teachers and students, for educational purposes.

Abstract. In this work, an innovative method has been proposed to obtain the Romanian national metrological traceability chain associated with eye lens dosimetry. This metrological method is based on using modern 3D printing techniques to create a high-precision human head replica and supporting the experimental results by the Monte Carlo simulations.

Abstract. This paper reports the chemical composition of eight archaeological small finds excavated at Histria, Romania. The bulk non-destructive analyses were performed using Prompt Gamma Activation Analysis (PGAA) at the Budapest Neutron Center (BNC). Different compositional patterns were evidenced, indicating the raw materials used in the manufacturing of these small objects.