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- ItemSimultaneous observations of a Mesospheric Inversion Layer and turbulence during the ECOMA-2010 rocket campaign(Göttingen : Copernicus, 2013) Szewczyk, A.; Strelnikov, B.; Rapp, M.; Strelnikova, I.; Baumgarten, G.; Kaifler, N.; Dunker, T.; Hoppe, U.-P.From 19 November to 19 December 2010 the fourth and final ECOMA rocket campaign was conducted at Andøya Rocket Range (69 N, 16 E) in northern Norway. We present and discuss measurement results obtained during the last rocket launch labelled ECOMA09 when simultaneous and true common volume in situ measurements of temperature and turbulence supported by ground-based lidar observations reveal two Mesospheric Inversion Layers (MIL) at heights between 71 and 73 km and between 86 and 89 km. Strong turbulence was measured in the region of the upper inversion layer, with the turbulent energy dissipation rates maximising at 2 W kg-1. This upper MIL was observed by the ALOMAR Weber Na lidar over the period of several hours. The spatial extension of this MIL as observed by the MLS instrument onboard AURA satellite was found to be more than two thousand kilometres. Our analysis suggests that both observed MILs could possibly have been produced by neutral air turbulence.
- ItemInnovative and automated method for vortex identification(Les Ulis : EDP Sciences, 2022) Canivete Cuissa, J. R.; Steiner, O.Context. As a universally accepted definition of a vortex has not yet been established, the community lacks an unambiguous and rigorous method for identifying vortices in fluid flows. Such a method would be useful for conducting robust statistical studies on vortices in highly dynamical and turbulent systems such as the solar atmosphere. Aims. We aim to develop an innovative and robust automated methodology for the identification of vortices based on local and global characteristics of the flow, while avoiding the use of a threshold that could potentially prevent the detection of weak vortices in the process. Methods. We present a new method that combines the rigor of mathematical criteria with the global perspective of morphological techniques. The core of the method consists of an estimation of the center of rotation for every point of the flow that presents some degree of curvature in its neighborhood. For this purpose, we employed the Rortex criterion and combined it with morphological considerations of the velocity field. We then identified coherent vortical structures based on clusters of estimated centers of rotation. Results. We demonstrate that the Rortex is a more reliable criterion than the swirling strength and the vorticity for the extraction of physical information from vortical flows, because it measures the rigid-body rotational part of the flow alone and is not biased by the presence of pure or intrinsic shears. We show that the method performs well in the context of a simplistic test case composed of two Lamb-Oseen vortices. We combined the proposed method with a state-of-the-art clustering algorithm to build an automated vortex identification algorithm. The algorithm was applied to an artificial flow composed of multiple Lamb- Oseen vortices, with a random noisy background, and to the turbulent flow of a simulated magneto-hydrodynamical Orszag-Tang vortex test. The results demonstrate the reliability and accuracy of the method. Conclusions. The present automated vortex identification method can be considered a new tool for the detection and study of vortices in dynamical and turbulent (magneto)hydrodynamical flows. By applying the implemented algorithm to numerical simulations and observational data, as well as comparing it to existing detection methods, we seek to successively improve the reliability of the detections and, ultimately, our knowledge on swirling motions in the solar, stellar, and planetary atmospheres.
- ItemObserving Mesospheric Turbulence with Specular Meteor Radars: a novel Method for Estimating Second-Order Statistics of Wind Velocity(Malden, Mass. : American Geophysical Union, 2019) Vierinen, J.; Chau, J.L.; Charuvil, H.; Urco, J.M.; Clahsen, M.; Avsarkisov, V.; Marino, R.; Volz, R.There are few observational techniques for measuring the distribution of kinetic energy within the mesosphere with a wide range of spatial and temporal scales. This study describes a method for estimating the three-dimensional mesospheric wind field correlation function from specular meteor trail echoes. Each radar echo provides a measurement of a one-dimensional projection of the wind velocity vector at a randomly sampled point in space and time. The method relies on using pairs of such measurements to estimate the correlation function of the wind with different spatial and temporal lags. The method is demonstrated using a multistatic meteor radar data set that includes ≈105 meteor echoes observed during a 24-hr time period. The new method is found to be in good agreement with the well-established technique for estimating horizontal mean winds. High-resolution correlation functions with temporal, horizontal, and vertical lags are also estimated from the data. The temporal correlation function is used to retrieve the kinetic energy spectrum, which includes the semidiurnal mode and a 3-hr period wave. The horizontal and vertical correlation functions of the wind are then used to derive second-order structure functions, which are found to be compatible with the Kolmogorov prediction for spectral distribution of kinetic energy in the turbulent inertial range. The presented method can be used to extend the capabilities of specular meteor radars. It is relatively flexible and has a multitude of applications beyond what has been shown in this study.
- ItemNonlinear dynamical properties of frequency swept fiber-based semiconductor lasers(Bristol : IOP Publishing, 2021) Slepneva, Svetlana; Pimenov, AlexanderWe investigate dynamics of semiconductor lasers with fiber-based unidirectional ring cavity that can be used as frequency swept sources. We identify key factors behind the reach dynamical behavior of such lasers using state-of-the-art experimental and analytical methods. Experimentally, we study the laser in static, quasi-static and synchronization regimes. We apply experimental methods such as optical heterodyne or electric field reconstruction in order to characterize these regimes or study the mechanisms of transition between them. Using a delay differential equation model, we demonstrate that the presence of chromatic dispersion can lead to destabilization of the laser modes through modulational instability, which results in undesirable chaotic emission. We characterize the instability threshold both theoretically and experimentally, and demonstrate deterioration of the Fourier domain mode locking regime near the threshold.
- ItemA Multiwavelength Dynamical State Analysis of ACT-CL J0019.6+0336(Basel : MDPI, 2021) Pillay, Denisha S.; Turner, David J.; Hilton, Matt; Knowles, Kenda; Kesebonye, Kabelo C.; Moodley, Kavilan; Mroczkowski, Tony; Oozeer, Nadeem; Pfrommer, Christoph; Sikhosana, Sinenhlanhla P.; Wollack, Edward J.In our study, we show a multiwavelength view of ACT-CL J0019.6+0336 (which hosts a radio halo), to investigate the cluster dynamics, morphology, and ICM. We use a combination of XMM-Newton images, Dark Energy Survey (DES) imaging and photometry, SDSS spectroscopic information, and 1.16 GHz MeerKAT data to study the cluster properties. Various X-ray and optical morphology parameters are calculated to investigate the level of disturbance. We find disturbances in two X-ray parameters and the optical density map shows elongated and axisymmetric structures with the main cluster component southeast of the cluster centre and another component northwest of the cluster centre. We also find a BCG offset of ~950 km/s from the mean velocity of the cluster, and a discrepancy between the SZ mass, X-ray mass, and dynamical mass (MX,500 and MSZ,500 lies > 3σ away from Mdyn,500), showing that J0019 is a merging cluster and probably in a post-merging phase.
- ItemControl of transversal instabilities in reaction-diffusion systems(Bristol : Institute of Physics Publishing, 2018) Totz, S.; Löber, J.; Totz, J.F.; Engel, H.In two-dimensional reaction-diffusion systems, local curvature perturbations on traveling waves are typically damped out and vanish. However, if the inhibitor diffuses much faster than the activator, transversal instabilities can arise, leading from flat to folded, spatio-temporally modulated waves and to spreading spiral turbulence. Here, we propose a scheme to induce or inhibit these instabilities via a spatio-temporal feedback loop. In a piecewise-linear version of the FitzHugh-Nagumo model, transversal instabilities and spiral turbulence in the uncontrolled system are shown to be suppressed in the presence of control, thereby stabilizing plane wave propagation. Conversely, in numerical simulations with the modified Oregonator model for the photosensitive Belousov-Zhabotinsky reaction, which does not exhibit transversal instabilities on its own, we demonstrate the feasibility of inducing transversal instabilities and study the emerging wave patterns in a well-controlled manner.
- ItemShell models for Hall effect induced magnetic turbulence(College Park, MD : Institute of Physics Publishing, 2007) Frick, P.; Stepanov, R.; Rheinhardt, M.The Hall effect occurs in strongly magnetized conductive media and results in non-dissipative currents perpendicular to the electric field. We discuss its influence on the magnetic field dynamics ignoring fluid motion and ambipolar diffusion. The magnetic field evolution can then be basically similar to that of the velocity field in hydrodynamic turbulence resulting in a magnetic turbulence. Shell models for the induction equation with Hall effect are constructed on the basis of the conservation of magnetic energy and helicity in the dissipation-free limit. Numerical simulations of these models indicate that a magnetic energy cascade does occur, but the time behaviour and spatial spectrum of the magnetic field are very different from those of the velocity in shell models of hydrodynamic turbulence. ©IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.