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II. The effect of axisymmetric and spatially varying equilibria and flow on mhd wave modes: Cylindrical geometry

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arXiv 2021年

作者： Skirvin, Samuel Fedun, Viktor Silva, Suzana Verth, Gary Plasma Dynamics Group Department of Automatic Control & Systems Engineering University of Sheffield SheffieldS3 7RH United Kingdom Plasma Dynamics Group Department of Mathematics and Statistics University of Sheffield SheffieldS3 7RH United Kingdom

Magnetohydrodynamic (MHD) waves are routinely observed in the solar atmosphere. These waves are important in the context of solar physics as it is widely believed they can contribute to the energy budget of the solar atmosphere and are a prime candidate to contribute towards coronal heating. Realistic models of these waves are required representing observed configurations such that plasma properties can be determined more accurately which can not be measured directly. This work utilises a previously developed numerical technique to find permittable eigenvalues under different non-uniform equilibrium conditions in a Cartesian magnetic slab geometry. Here we investigate the properties of magnetoacoustic waves under nonuniform equilibria in a cylindrical geometry. Previously obtained analytical results are retrieved to emphasise the power and applicability of this numerical technique. Further case studies investigate the effect that a radially non-uniform plasma density and non-uniform plasma flow, modelled as a series of Gaussian profiles, has on the properties of different MHD waves. For all cases the dispersion diagrams are obtained and spatial eigenfunctions calculated which display the effects of the equilibrium inhomogeneity. It is shown that as the equilibrium non-uniformity is increased, the radial spatial eigenfunctions are affected and extra nodes introduced, similar to the previous investigation of a magnetic slab. Furthermore, azimuthal perturbations are increased with increasing inhomogeneity introducing vortical motions inside the waveguide. Finally, 2D and 3D representations of the velocity fields are shown which may be useful for observers for wave mode identification under realistic magnetic waveguides with ever increasing instrument resolution. © 2021, CC BY.

关键词： Magnetohydrodynamics

Waves in weakly ionised solar plasmas

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arXiv 2022年

作者： Alharbi, Abdulaziz Ballai, I. Fedun, V. Verth, G. Plasma Dynamics Group School of Mathematics and Statistics University of Sheffield Hounsfield Road Hicks Building SheffieldS3 7RH United Kingdom Department of Mathematics Jamoum University College Umm Al-Qura University Jamoum Makkah25375 Saudi Arabia Plasma Dynamics Group Department of Automatic Control and Systems Engineering The University of Sheffield Mappin Street SheffieldS1 3JD United Kingdom

Here we study the nature and characteristics of waves propagating in partially ionised plasmas in the weakly ionised limit, typical for the lower part of the solar atmosphere. The framework in which the properties of waves are discussed depends on the relative magnitude of collisions between particles, but also on the relative magnitude of the collisional frequencies compared to the gyro-frequency of charged particles. Our investigation shows that the weakly ionised solar atmospheric plasma can divided into two regions and this division occurs, roughly, at the base of the chromosphere. In the solar photosphere the plasma is non-magnetised and the dynamics can described within the three-fluid framework where acoustic waves associated to each species can propagate. Due to the very high concentration of neutrals, the neutral sound waves propagates with no damping, while for the other two modes the damping rate is determined by collisions with neutrals. The ion and electron-related acoustic modes propagate with a cut-off determined by the collisional frequency of these species with neutrals. In the weakly ionised chromosphere only electrons are magnetised, however, the strong coupling of charged particles reduces the working framework to a two-fluid model. The disassociation of charged particles creates electric currents that can influence the characteristic of waves. The propagation properties of waves with respect to the angle of propagation are studied with the help of polar diagrams. Copyright © 2022, The Authors. All rights reserved.

关键词： Damping

Empirical Risk Minimization with Relative Entropy Regularization: Optimality and Sensitivity Analysis

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arXiv 2022年

作者： Perlaza, Samir M. Bisson, Gaetan Esnaola, Iñaki Jean-Marie, Alain Rini, Stefano INRIA 2004 Route des Lucioles Sophia Antipolis06902 France The Laboratoire de Mathématiques GAATI Université de la Polynésie Française BP 6570 Faaa98702 French Polynesia The Department of Automatic Control and Systems Engineering University of Sheffield Sheffield United Kingdom The Department of Electrical and Computer Engineering National Chao Tung University Hsinchu Taiwan The Department of Electrical and Computer Engineering Princeton University PrincetonNJ08544 United States

The optimality and sensitivity of the empirical risk minimization problem with relative entropy regularization (ERM-RER) are investigated for the case in which the reference is a σ-finite measure instead of a probability measure. This generalization allows for a larger degree of flexibility in the incorporation of prior knowledge over the set of models. In this setting, the interplay of the regularization parameter, the reference measure, the risk function, and the empirical risk induced by the solution of the ERM-RER problem is characterized. This characterization yields necessary and sufficient conditions for the existence of regularization parameters that achieve arbitrarily small empirical risk with arbitrarily high probability. Additionally, the sensitivity of the expected empirical risk to deviations from the solution of the ERM-RER problem is studied. Dataset-dependent and dataset-independent upper bounds on the absolute value of the sensitivity are presented. In a special case, it is shown that the expectation (with respect to the datasets) of the absolute value of the sensitivity is upper bounded, up to a constant factor, by the square root of the lautum information between the models and the datasets. © 2022, CC BY.

关键词： Sensitivity analysis

A two-level identification model for selecting the coordination strategy for the urban arterial road based on fuzzy logic

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International Journal of Simulation and Process Modelling 2019年第6期14卷 478-487页

作者： Sun, Haochen Qiao, Feng Wang, Zhaoyan Guo, Lingzhong Faculty of Information and Control Engineering Shenyang Jianzhu University 9 Hunnan East Road Shenyang China Department of Automatic Control and Systems Engineering University of Sheffield SheffieldS1 3JD United Kingdom

A novel model for identifying the traffic condition of urban arterial roadways is proposed in this paper to improve the operational efficiency and safety of the urban traffic arterial road system. During the identification process, fuzzy analytic hierarchy process and fuzzy integrated evaluation are employed to identify the traffic condition on the arterial road;according to the fuzzy logic scheme, a proper coordination strategy is then generated based on the resulting identification of each way of the artery. To verify the effectiveness of the proposed method, a numerical experiment is carried out by using the microscopic traffic simulation software VISSIM, where a traffic flow simulation system is generated according to the real-time traffic data. The comparison results show that the proposed model works well to fit with the actual operating condition of the arterial traffic and the proposed coordination strategy can provide a better performance for the traffic management. Copyright © 2019 Inderscience Enterprises Ltd.

关键词： Fuzzy logic

Denial of Service Attacks on control systems with Packet Loss

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IFAC-PapersOnLine 2020年第2期53卷 3488-3495页

作者： William Casbolt Iñaki Esnaola Bryn Jones Department of Automatic Control and Systems Engineering University of Sheffield Sheffield S1 3JD UK Department of Electrical Engineering Princeton University Princeton NJ 08544 USA

The performance of control systems with packet loss as a result of an attack over the actuation communication channel is analysed. The operator is assumed to monitor the state of the channel by measuring the average number of packet losses and an attack detection criteria is established based on the statistic. The performance of the attacker is measured in terms of the increase of the linear quadratic cost function of the operator subject to a given detection constraint. Within that setting, the optimal denial of service (DoS) attack strategy is formulated for UDP-like and TCP-like communication protocols. For both communication protocols, DoS attack constructions that are independent and identically distributed (IID) are compared to those that are non-stationary. The main contributions of this paper are (i) explicit characterisation of the expected cost increase of the optimal attack constructions and the associated packet loss parameter for the IID case, (ii) proof, by example, that non-stationary random attacks outperform IID attacks in the presence of detection constraints.

关键词： Secure networked control systems control estimation with data loss control under communication constraints

Geometrical Features based Visual Relocalization for Indoor Service Robot

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Geometrical Features based Visual Relocalization for Indoor ...

作者： Jikai Wang Deyun Dai Peng Wang Hao Zhao Meng Xu Zonghai Chen Department of Automation University of Science and Technology of China Department of Automatic Control and Systems Engineering The University of Sheffield

In traditional SLAM methods, the environment map is the simply assembled points or lines, which makes it difficult to directly perform relocalization using such map. This paper presents a new implementation method for indoor environment representation and visual relocalization using RGB-D *** method is developed for indoor service robots to perform relocalization using the observed point and line features. In this paper, the sparse feature map, line segment map, and dense point cloud map of an environment are learned by a random forest to regress the correspondences between visual features and 3 D world locations, geometrical features and 3 D world locations. Using the learned forest, landmark positions are efficiently predicted and the camera poses are then estimated in a probabilistic framework. The performance of the proposed method is demonstrated under various challenging environments using public benchmark dataset and our own dataset collected in an office environment. These conditions contain ambiguous areas,long corridor, moving people, viewpoint changes, or illumination variation. The proposed method is thoroughly evaluated against several strong state-of-the-art baselines. Experimental results prove the efficacy of our method.

关键词： Camera relocalization regression forest line segment RGB-D sensor

Case studies on pre-eruptive X-class flares using R-value in the lower solar atmosphere

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arXiv 2024年

作者： Biswal, Shreeyesh Korsós, Marianna B. Georgoulis, Manolis K. Nindos, Alexander Patsourakos, Spiros Erdélyi, Robertus School of Mathematics and Statistics University of Sheffield Hounsfield Road SheffieldS3 7RH United Kingdom Department of Physics Section Astrogeophysics University of Ioannina Ioannina45110 Greece University of Sheffield Department of Automatic Control and Systems Engineering Amy Johnson Building Portabello Street SheffieldS1 3JD United Kingdom Department of Astronomy Eötvös Loránd University Pázmány Péter sétány 1/A BudapestH-1117 Hungary Petőfi tér 3. GyulaH-5700 Hungary Johns Hopkins University Applied Physics Laboratory LaurelMD20375 United States Research Center for Astronomy and Applied Mathematics The Academy of Athens Athens11527 Greece

The R-value is a measure of the strength of photospheric magnetic Polarity Inversion Lines (PILs) in Active Regions (ARs). This work investigates the possibility of a relation between R-value variations and the occurrence of X-class flares in ARs, not in the solar photosphere, as usual, but above it in regions, closer to where flares occur. The modus operandi is to extrapolate the Solar Dynamic Observatory’s (SDO) Helioseismic and Magnetic Imager (HMI) magnetogram data up to a height of 3.24 Mm above the photosphere and then compute the R-value based on the extrapolated magnetic field. Recent studies have shown that certain flare-predictive parameters such as the horizontal gradient of the vertical magnetic field and magnetic helicity may improve flare prediction lead times significantly if studied at a specific height range above the photosphere, called the Optimal Height Range (OHR). Here we define the OHR as a collection of heights where a sudden but sustained increase in R-value is found. For the eight case studies discussed in this paper, our results indicate that it is possible for OHRs to exist in the low solar atmosphere (between 0.36 - 3.24 Mm), where R-value spikes occur 48-68 hrs before the first X-class flare of an emerging AR. The temporal evolution of R-value before the first X-class flare for an emerging AR is also found to be distinct from that of non-flaring ARs. For X-class flares associated with non-emerging ARs, an OHR could not be found. Copyright © 2024, The Authors. All rights reserved.

关键词： Solar concentrators

Comparison of exact and approximate MHD slow body mode solutions in photospheric waveguides

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arXiv 2022年

作者： Aldhafeeri, Anwar A. Verth, Gary Fedun, Viktor Lennard, Matthew Ballai, Istvan Mathematics and Statistic Department Faculty of Science King Faisal University Al-Hassa P.O. Box 400 Hofuf31982 Saudi Arabia Plasma Dynamics Group School of Mathematics and Statistics University of Sheffield Hicks Building Hounsfield Road SheffieldS3 7RH United Kingdom Plasma Dynamics Group Department of Automatic Control and Systems Engineering The University of Sheffield Mappin Street SheffieldS1 3JD United Kingdom

In this study we explore the possibility of simplifying the modeling of magnetohydrodynamic (MHD) slow body modes observed in photospheric magnetic structure such as the umbrae of sunspots and pores. The simplifying approach assumes that the variation of the eigenvalues of slow body waves can be derived by imposing that the longitudinal component of velocity with respect to the tube axis is zero at the boundary of the magnetic flux tube, which is in a good agreement with observations. To justify our approach we compare the results of our simplified model for slow body modes in cylindrical flux tubes with the model prediction obtained by imposing the continuity of the radial component of the velocity and total pressure at the boundary of the flux tube. Our results show that, to a high accuracy (less than 1% for the considered model), the conditions of continuity of the component of transversal velocity and pressure at the boundary can be neglected when modelling slow body modes under photospheric conditions. © 2022, CC BY.

关键词： Magnetohydrodynamics

New approach for analysing dynamical processes on the surface of photospheric vortex tubes

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arXiv 2022年

作者： Aljohani, Yasir Fedun, Viktor Ballai, Istvan Silva, Suzana S.A. Shelyag, Sergiy Verth, Gary Plasma Dynamics Group School of Mathematics and Statistics University of Sheffield Hicks Building Hounsfield Road SheffieldS3 7RH United Kingdom Department of Mathematics Jamoum University College Umm Al-Qura University Jamoum Makkah25375 Saudi Arabia Plasma Dynamics Group Department of Automatic Control and Systems Engineering University of Sheffield Mappin Street SheffieldS1 3JD United Kingdom School of Information Technology Deakin University Geelong Australia

The majority of studies on multi-scale vortex motions employ a two-dimensional geometry by using a variety of observational and numerical data. This approach limits the understanding the nature of physical processes responsible for vortex dynamics. Here we develop a new methodology to extract essential information from the boundary surface of vortex tubes. 3D high-resolution magnetoconvection MURaM numerical data has been used to analyse photospheric intergranular velocity vortices. The Lagrangian Averaged Vorticity Deviation (LAVD) technique was applied to define the centers of vortex structures and their boundary surfaces based on the advection of fluid elements. These surfaces were mapped onto a constructed envelope grid that allows the study of the key plasma parameters as functions of space and time. Quantities that help in understanding the dynamics of the plasma, e.g. Lorentz force, pressure force, plasma-β were also determined. Our results suggest that, while density and pressure have a rather global behaviour, the other physical quantities undergo local changes, with their magnitude and orientation changing in space and time. At the surface, the mixing in the horizontal direction is not efficient, leading to appearance of localized regions with higher/colder temperatures. In addition, the analysis of the MHD Poynting flux confirms that the majority of the energy is directed in the horizontal direction. Our findings also indicate that the pressure and magnetic forces that drive the dynamics of the plasma on vortex surfaces are unbalanced and therefore the vortices do not rotate as a rigid body. Copyright © 2022, The Authors. All rights reserved.

关键词： Vortex flow