Author name code: olshevsky ADS astronomy entries on 2022-09-14 author:"Olshevsky, Vyacheslav" ------------------------------------------------------------------------ Title: A Database of MMS Bow Shock Crossings Compiled Using Machine Learning Authors: Lalti, A.; Khotyaintsev, Yu. V.; Dimmock, A. P.; Johlander, A.; Graham, D. B.; Olshevsky, V. Bibcode: 2022JGRA..12730454L Altcode: 2022arXiv220304680L Identifying collisionless shock crossings in data sent from spacecraft has so far been done manually or using basic algorithms. It is a tedious job that shock physicists have to go through if they want to conduct case studies or perform statistical studies. We use a machine learning approach to automatically identify shock crossings from the Magnetospheric Multiscale (MMS) spacecraft. We compiled a database of 2,797 shock crossings, spanning a period from October 2015 to December 2020, including various spacecraft-related and shock-related parameters for each event. Furthermore, we show that the shock crossings in the database are spread out in space, from the subsolar point to the far flanks. On top of that, we show that they cover a wide range of parameter space. We also present a possible scientific application of the database by looking for correlations between ion acceleration efficiency at shocks with different shock parameters, such as the angle between the upstream magnetic field and the shock normal θBn and the Alfvénic Mach number MA. We find no clear correlation between the acceleration efficiency and MA; however, we find that quasi-parallel shocks are more efficient at accelerating ions than quasi-perpendicular shocks. Title: Automated Classification of Plasma Regions Using 3D Particle Energy Distributions Authors: Olshevsky, Vyacheslav; Khotyaintsev, Yuri V.; Lalti, Ahmad; Divin, Andrey; Delzanno, Gian Luca; Anderzén, Sven; Herman, Pawel; Chien, Steven W. D.; Avanov, Levon; Dimmock, Andrew P.; Markidis, Stefano Bibcode: 2021JGRA..12629620O Altcode: 2019arXiv190805715O We investigate the properties of the ion sky maps produced by the Dual Ion Spectrometers (DIS) from the Fast Plasma Investigation (FPI). We have trained a convolutional neural network classifier to predict four regions crossed by the Magnetospheric Multiscale Mission (MMS) on the dayside magnetosphere: solar wind, ion foreshock, magnetosheath, and magnetopause using solely DIS spectrograms. The accuracy of the classifier is >98%. We use the classifier to detect mixed plasma regions, in particular to find the bow shock regions. A similar approach can be used to identify the magnetopause crossings and reveal regions prone to magnetic reconnection. Data processing through the trained classifier is fast and efficient and thus can be used for classification for the whole MMS database. Title: Kinetic Modeling in the Magnetosphere Authors: Markidis, Stefano; Olshevsky, Vyacheslav; Tóth, Gábor; Chen, Yuxi; Peng, Ivy Bo; Lapenta, Giovanni; Gombosi, Tamas Bibcode: 2021GMS...259..607M Altcode: 2020arXiv201206669M This paper presents the state of the art of kinetic modeling techniques for simulating plasma kinetic dynamics in magnetospheres. We describe the critical numerical techniques for enabling large-scale kinetic simulations of magnetospheres: parameter scaling, implicit Particle-in-Cell schemes, and fluid-kinetic coupling. We show an application of these techniques to study particle acceleration and heating in asymmetric magnetic reconnection in the Ganymede magnetosphere. Title: A comparison of methods for finding magnetic nulls in simulations and in situ observations of space plasmas Authors: Olshevsky, V.; Pontin, D. I.; Williams, B.; Parnell, C. E.; Fu, H. S.; Liu, Y.; Yao, S.; Khotyaintsev, Y. V. Bibcode: 2020A&A...644A.150O Altcode: 2021arXiv210102014O Context. Magnetic nulls are ubiquitous in space plasmas, and are of interest as sites of localised energy dissipation or magnetic reconnection. As such, a number of methods have been proposed for detecting nulls in both simulation data and in situ spacecraft data from Earth's magnetosphere. The same methods can be applied to detect stagnation points in flow fields.
Aims: In this paper we describe a systematic comparison of different methods for finding magnetic nulls. The Poincaré index method, the first-order Taylor expansion (FOTE) method, and the trilinear method are considered.
Methods: We define a magnetic field containing fourteen magnetic nulls whose positions and types are known to arbitrary precision. Furthermore, we applied the selected techniques in order to find and classify those nulls. Two situations are considered: one in which the magnetic field is discretised on a rectangular grid, and the second in which the magnetic field is discretised along synthetic "spacecraft trajectories" within the domain.
Results: At present, FOTE and trilinear are the most reliable methods for finding nulls in the spacecraft data and in numerical simulations on Cartesian grids, respectively. The Poincaré index method is suitable for simulations on both tetrahedral and hexahedral meshes.
Conclusions: The proposed magnetic field configuration can be used for grading and benchmarking the new and existing tools for finding magnetic nulls and flow stagnation points. Title: Extending the FOTE Method to Three-dimensional Plasma Flow Fields Authors: Wang, Z.; Fu, H. S.; Olshevsky, V.; Liu, Y. Y.; Liu, C. M.; Chen, Z. Z. Bibcode: 2020ApJS..249...10W Altcode: In this study, we examine whether the First-Order Taylor Expansion (FOTE) method can be applied to steady-state plasma flow fields in space. We particularly examine whether this method (termed FOTE-V) can be used to identify the flow critical points (including both stagnation point and vortex center) and reconstruct the flow patterns around these points. Quantitatively, we test the accuracy of this method using 3D kinetic simulation data, and find the FOTE-V method can give accurate reconstruction results within an area about 2 times the size of the spacecraft tetrahedron, particularly when there are no clear nonlinear flow structures in the simulation box. With simulation data, we also reveal the ability of the FOTE-V method on reconstructing 3D flow field topology of both radial-type null and spiral-type nulls. We further test the accuracy of this method using measurements from NASA's Magnetospheric Multi-scale (MMS) mission. In a current sheet crossing event, the FOTE-V method successfully identifies the spiral-type nulls in the reconnection exhaust region. In an EDR crossing event, the FOTE-V method detects the stagnation point near the reconnection center. We find these 3D flow structures are quasi-linear at the MMS separation scale. Utilizing the continuity equation of the steady flow, we define a parameter, $\alpha =\tfrac{{\rm{\nabla }}\cdot \left(n{\boldsymbol{V}}\right)}{{\rm{\nabla }}\times \left(n{\boldsymbol{V}}\right)}$ , to quantify the error of this method—the smaller this parameter the better the results. This study demonstrates that the plasma flows at small scale are indeed linear, and thus the FOTE-V method can be applied to such flow fields. In particular, this method will be useful to study stagnation points and electron vortices in space plasmas. Title: A Fully Kinetic Perspective of Electron Acceleration around a Weakly Outgassing Comet Authors: Divin, Andrey; Deca, Jan; Eriksson, Anders; Henri, Pierre; Lapenta, Giovanni; Olshevsky, Vyacheslav; Markidis, Stefano Bibcode: 2020ApJ...889L..33D Altcode: The cometary mission Rosetta has shown the presence of higher-than-expected suprathermal electron fluxes. In this study, using 3D fully kinetic electromagnetic simulations of the interaction of the solar wind with a comet, we constrain the kinetic mechanism that is responsible for the bulk electron energization that creates the suprathermal distribution from the warm background of solar wind electrons. We identify and characterize the magnetic field-aligned ambipolar electric field that ensures quasi-neutrality and traps warm electrons. Solar wind electrons are accelerated to energies as high as 50-70 eV close to the comet nucleus without the need for wave-particle or turbulent heating mechanisms. We find that the accelerating potential controls the parallel electron temperature, total density, and (to a lesser degree) the perpendicular electron temperature and the magnetic field magnitude. Our self-consistent approach enables us to better understand the underlying plasma processes that govern the near-comet plasma environment. Title: Accelerating Magnetospheric Modeling with Heterogeneous Hardware Authors: Markidis, S.; Chien, S. W. D.; Olshevsky, V. Bibcode: 2019AGUFMSM12B..07M Altcode: The advent of supercomputers with multiple accelerators per computational node is impacting the development of codes for magnetospheric modeling. The two current largest supercomputers in November 2018 Top500 list, Summit and Sierra, feature six and four V100 NVIDIA GPUs per node respectively providing a theoretical peak performance of 750 and 500 Tops/s (operations in mixed precision) per node [1]. However, it is still unclear how codes for magnetospheric modeling could take advantage of new heterogeneous architectures. Widely-used massively parallel codes for magnetospheric modeling are not yet capable of exploiting these new systems and need to be redesigned. Two main aspects have to be considered: first, the algorithms for magnetospheric modeling have to be reformulated to use dense matrix operations; second, new algorithms have to cope with low-precision calculations, still retaining acceptable accuracy. In this talk, we review emerging heterogeneous architectures and present our work in designing and developing new algorithmic changes in iPIC3D [2], an implicit PIC code for magnetospheric modeling, to fully exploit heterogeneous hardware.

This work has received funding from the European Commission H2020 program, Grant Agreement No. 801039 (EPiGRAM-HS, epigram-hs.eu)

[1] Markidis, Stefano, Steven Wei Der Chien, Erwin Laure, Ivy Bo Peng, and Jeffrey S. Vetter. "NVIDIA tensor core programmability, performance & precision." In 2018 IEEE International Parallel and Distributed Processing Symposium Workshops (IPDPSW), pp. 522-531. IEEE, 2018.

[2] Markidis, Stefano, Giovanni Lapenta, and Rizwan-uddin. "Multi-scale simulations of plasma with iPIC3D." Mathematics and Computers in Simulation 80, no. 7 (2010): 1509-1519. Title: SOTE: A Nonlinear Method for Magnetic Topology Reconstruction in Space Plasmas Authors: Liu, Y. Y.; Fu, H. S.; Olshevsky, V.; Pontin, D. I.; Liu, C. M.; Wang, Z.; Chen, G.; Dai, L.; Retino, A. Bibcode: 2019ApJS..244...31L Altcode: Complex magnetic structures are ubiquitous in turbulent astrophysical plasmas. Such structures can be host to many dynamic processes, such as magnetic reconnection and energy dissipation. Thus, revealing the 3D topologies of these structures is necessary. In this study, we propose a new method to reconstruct complex magnetic topologies in quasi-steady space plasmas, by utilizing eight-point measurements of magnetic fields and particles. Such a method, based on the Second-Order Taylor Expansion (SOTE) of a magnetic field, is nonlinear; it is constrained by {{\nabla }}\cdot {\boldsymbol{B}}=0 and {{\nabla }}× {\boldsymbol{B}}={μ }0{\boldsymbol{J}}, where {\boldsymbol{J}}={ne}({{\boldsymbol{V}}}{\boldsymbol{i}}-{{\boldsymbol{V}}}{\boldsymbol{e}}) is from particle moments. A benchmark test of this method, using the simulation data, shows that the method can give accurate reconstruction results within an area about three times the size of a spacecraft tetrahedron. By comparing to the previous First-Order Taylor Expansion (FOTE) method, this method (SOTE) gives similar results for reconstructing quasilinear structures but exhibits better accuracy in reconstructing nonlinear structures. Such a method will be useful to the multi-scale missions, such as the future European Space Agency's “cross-scale” mission and China's “self-adaptive” mission. Also, it can be applied to four-point missions, such as Cluster and the Magnetospheric Multiscale Mission. We demonstrated how to apply this method to the four-point missions. In principle, this method will be useful to study shocks, magnetic holes, dipolarization fronts, and other nonlinear structures in space plasmas. Title: Generation of Turbulence in Colliding Reconnection Jets Authors: Pucci, Francesco; Matthaeus, William H.; Chasapis, A.; Servidio, Sergio; Sorriso-Valvo, L.; Olshevsky, V.; Newman, D. L.; Goldman, M. V.; Lapenta, Giovanni Bibcode: 2018ApJ...867...10P Altcode: 2018arXiv181013318P The collision of magnetic reconnection jets is studied by means of a three-dimensional numerical simulation at the kinetic scale, in the presence of a strong guide field. We show that turbulence develops due to the collision of jets, producing several current sheets in reconnection outflows, aligned with the guide field direction. The turbulence is mainly two-dimensional, with stronger gradients in the plane perpendicular to the guide field and low wave-like activity in the parallel direction. First, we provide a numerical method to isolate the central turbulent region. Second, we analyze the spatial second-order structure function and prove that turbulence is confined in this region. Finally, we compute local magnetic and electric frequency spectra, finding a trend in the subion range that differs from typical cases for which the Taylor hypothesis is valid, as well as wave activity in the range between ion and electron cyclotron frequencies. Our results are relevant to understand observed collisions of reconnection jets in space plasmas. Title: Properties of turbulence in the reconnection exhaust: numerical simulations compared with observations Authors: Pucci, F.; Servidio, S.; Sorriso-Valvo, L.; Olshevsky, V.; Matthaeus, W. H.; Malara, F.; Goldman, M. V.; Newman, D. L.; Lapenta, G. Bibcode: 2018arXiv181100005P Altcode: The properties of the turbulence which develops in the outflows of magnetic reconnection have been investigated using self-consistent plasma simulations, in three dimensions. As commonly observed in space plasmas, magnetic reconnection is characterized by the presence of turbulence. Here we provide a direct comparison of our simulations with reported observations of reconnection events in the magnetotail investigating the properties of the electromagnetic field and the energy conversion mechanisms. In particular, simulations show the development of a turbulent cascade consistent with spacecraft observations, statistics of the the dissipation mechanisms in the turbulent outflows similar to the one observed in reconnection jets in the magnetotail, and that the properties of turbulence vary as a function of the distance from the reconnecting X-line. Title: Properties of Decaying Plasma Turbulence at Subproton Scales Authors: Olshevsky, Vyacheslav; Servidio, Sergio; Pucci, Francesco; Primavera, Leonardo; Lapenta, Giovanni Bibcode: 2018ApJ...860...11O Altcode: We study the properties of plasma turbulence at subproton scales using kinetic electromagnetic three-dimensional simulations with nonidentical initial conditions. Particle-in-cell modeling of the Taylor-Green vortex has been performed, starting from three different magnetic field configurations. All simulations expose very similar energy evolution in which the large-scale ion flows and magnetic structures deteriorate and transfer their energy into particle heating. Heating is more intense for electrons, decreasing the initial temperature ratio and leading to temperature equipartition between the two species. A full turbulent cascade, with a well-defined power-law shape at subproton scales, is established within a characteristic turnover time. Spectral indices for magnetic field fluctuations in two simulations are close to α B ≈ 2.9, and are steeper in the remaining case with α B ≈ 3.05. Energy is dissipated by a complex mixture of plasma instabilities and magnetic reconnection and is milder in the latter simulation. The number of magnetic nulls, and the dissipation pattern observed in this case, differ from two others. Spectral indices for the kinetic energy deviate from magnetic spectra by ≈1 in the first simulation, and by ≈0.75 in two other runs. The difference between magnetic and electric slopes confirm the previously observed value of α B - α E ≈ 2. Title: A fully kinetic perspective of electron acceleration around a weakly outgassing comet: Ohm's law Authors: Deca, Jan; Divin, Andrey; Henri, Pierre; Eriksson, Anders; Olshevsky, Vyacheslav; Markidis, Stefano; Horányi, Mihály Bibcode: 2018EGUGA..20.6571D Altcode: When a comet is sufficiently close to the Sun, the sublimation of ice leads to outgassing and the formation of a coma of gas and dust. Ionisation of the outgassing material then results in mass-loading of the solar wind and magnetic field draping around the cometary nucleus. Here we present three-dimensional fully kinetic simulations of the solar wind interaction with comet 67P/Churyumov- Gerasimenko at a low-activity regime, before collisions have any impact on the plasma dynamics. The interaction scales are well below the relevant ion gyroradii. Non-equilibrium electron distributions develop. To first order, the dynamical interaction is representative of a four-fluid coupled system [Deca et al., PRL 2017, Divin et al., submitted]. Our approach is self-consistent and allows to distill Ohm's law directly from the electron dynamics in the simulation, rather than imposing it beforehand. In the vicinity of the cometary nucleus, the balance changes between the different terms in the equation. Deciphering the relative importance of each term allows to identify the driving physics in the various regions of the cometary plasma environment. For example, we find that close to the outgassing nucleus the electron pressure gradient dominates; that at sub-ion scales, the total electric field is a superposition of the solar wind convective electric field, where electrons are frozen-in, and the ambipolar electric field; that the latter accelerates electrons parallel to the magnetic field and is the source of/provides feedback to the electron pressure gradient that balances the Ohm's law perpendicular to the magnetic field, and that the role of electron inertia is negligible to balance the electric field. In conclusion, Ohm's law shows us what happens to all ion and electron species, and why. Title: Numerical simulation of the solar wind-Moon interaction using 3D Particle-in-Cell (PIC) simulations. Authors: Ahmadi, Tara; Divin, Andrey; Deca, Jan; Lue, Charles; Olshevsky, Vyacheslav; Markidis, Stefano; Semenov, Vladimir Bibcode: 2018EGUGA..2016798A Altcode: We present results of three-dimensional Particle-in-Cell (PIC) simulations of quiet solar wind-Moon interaction using full-particle electromagnetic implicit code iPIC3D. The Moon is taken as a passive absorber of the inflowing particles, without intrinsic magnetic fields or resistivity. We show that (similar to past 1D, 2D PIC and 3D hybrid studies) the large-scale Lunar wake with nearly zero density is formed, which is bounded by strong rarefaction and compression waves attached to the Moon. We investigate in detail velocity distribution functions (VDFs) and ion and electron moments in the wake, including regions with very small macroparticle count. In order to reconstruct the distributions in low-density wake, we sample VDFs using a backward Liouville method by tracing particles back in time in quasisteady electric and magnetic fields taken from original 3D PIC simulation. Obtained VDFs display large degree of anisotropy and nongyrotropy and reveal fine-scale features which can be interpreted as the Moon's shadow in velocity domain. Title: Nonlinear waves and instabilities leading to secondary reconnection in reconnection outflows Authors: Lapenta, Giovanni; Pucci, Francesco; Olshevsky, Vyacheslav; Servidio, Sergio; Sorriso-Valvo, Luca; Newman, David L.; Goldman, Martin V. Bibcode: 2018JPlPh..84a7103L Altcode: 2018arXiv180808612L Reconnection outflows have been under intense recent scrutiny, from in situ observations and from simulations. These regions are host to a variety of instabilities and intense energy exchanges, often even superior to the main reconnection site. We report here a number of results drawn from an investigation of simulations. First, the outflows are observed to become unstable to drift instabilities. Second, these instabilities lead to the formation of secondary reconnection sites. Third, the secondary processes are responsible for large energy exchanges and particle energization. Finally, the particle distribution function are modified to become non-Maxwellian and include multiple interpenetrating populations. Title: Simulating the Solar Wind Interaction with Comet 67P/Churyumov-Gerasimenko: Latest Results Authors: Deca, J.; Divin, A. V.; Henri, P.; Eriksson, A. I.; Markidis, S.; Olshevsky, V.; Goldstein, R.; Myllys, M. E.; Horanyi, M. Bibcode: 2017AGUFM.P51D2628D Altcode: First observed in 1969, comet 67P/Churyumov-Gerasimenko was escorted for almost two years along its 6.45-yr elliptical orbit by ESA's Rosetta orbiter spacecraft. When a comet is sufficiently close to the Sun, the sublimation of ice leads to an outgassing atmosphere and the formation of a coma, and a dust and plasma tail. Comets are critical to decipher the physics of gas release processes in space. The latter result in mass-loaded plasmas, which more than three decades after the Active Magnetospheric Particle Tracer Explorers (AMPTE) space release experiments are still not fully understood. Using a 3D fully kinetic approach, we study the solar wind interaction with comet 67P/Churyumov-Gerasimenko, focusing in particular on the ion-electron dynamics for various outgassing rates. A detailed kinetic treatment of the electron dynamics is critical to fully capture the complex physics of mass-loading plasmas and to describe the strongly inhomogeneous plasma dynamics observed by Rosetta, down to electron kinetic scales. Title: Properties of Turbulence in the Reconnection Exhaust: Numerical Simulations Compared with Observations Authors: Pucci, F.; Servidio, S.; Sorriso-Valvo, L.; Olshevsky, V.; Matthaeus, W. H.; Malara, F.; Goldman, M. V.; Newman, D. L.; Lapenta, G. Bibcode: 2017ApJ...841...60P Altcode: The properties of the turbulence that develops in the outflows of magnetic reconnection have been investigated using self-consistent plasma simulations, in three dimensions. As commonly observed in space plasmas, magnetic reconnection is characterized by the presence of turbulence. Here we provide a direct comparison of our simulations with reported observations of reconnection events in the magnetotail, investigating the properties of the electromagnetic field and the energy conversion mechanisms. In particular, simulations show the development of a turbulent cascade consistent with spacecraft observations, statistics of the dissipation mechanisms in the turbulent outflows similar to the ones observed in reconnection jets in the magnetotail, and that the properties of turbulence vary as a function of the distance from the reconnecting X-line. Title: Electron and Ion Dynamics of the Solar Wind Interaction with a Weakly Outgassing Comet Authors: Deca, Jan; Divin, Andrey; Henri, Pierre; Eriksson, Anders; Markidis, Stefano; Olshevsky, Vyacheslav; Horányi, Mihály Bibcode: 2017PhRvL.118t5101D Altcode: Using a 3D fully kinetic approach, we disentangle and explain the ion and electron dynamics of the solar wind interaction with a weakly outgassing comet. We show that, to first order, the dynamical interaction is representative of a four-fluid coupled system. We self-consistently simulate and identify the origin of the warm and suprathermal electron distributions observed by ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko and conclude that a detailed kinetic treatment of the electron dynamics is critical to fully capture the complex physics of mass-loading plasmas. Title: Three dimensional Particle-in-Cell (PIC) simulations of the 67P environment Authors: Divin, Andrey; Deca, Jan; Henri, Pierre; Horanyi, Mihaly; Markidis, Stefano; Lapenta, Giovanni; Olshevsky, Vyacheslav; Eriksson, Anders Bibcode: 2017EGUGA..19.1556D Altcode: ESA's Rosetta orbiter spacecraft escorted comet 67P/Churyumov-Gerasimenko for two years, carrying 21 scientific instruments. Five of those were dedicated to plasma measurements. The mission revealed for the first time, and in unprecedented detail, the fascinating evolution of a comet and its interaction with our Sun as it races along its 6.45yr elliptical orbit around the Sun. Using a self-consistent 3-D fully kinetic electromagnetic particle-in-cell approach, we focus on the global cometary environment and, in particular, on the collisionless electron-kinetic interaction. We include cometary ions and electrons produced by the ionization of the outgassing cometary atmosphere in addition to the solar wind ion and electron plasma flow. We approximate mass-loading of the cold cometary ion and electron populations using a 1/r relation with distance to the comet with a total neutral production rate of Q = 1026 s-1. Our simulation results disentangle for the first time the kinetic ion and electron dynamics of the solar wind interaction with a weakly outgassing comet. The simulated global structure of the solar wind-comet interaction confirms the results reported in hybrid simulations of the induced cometary magnetosphere. Moreover, we show that cometary and solar wind electrons neutralize the solar wind protons and cometary ions, respectively, in the region of influence around the comet, representing to first order a four-fluid behavior. The electron energy distribution close to the comet is shown to be a mix of cometary and solar wind electrons that appear as, respectively, a thermal and a suprathermal components. Analyzing ion and electron energy distribution functions, and comparing with plasma measurements from ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko, we conclude that a detailed kinetic treatment of the electron dynamics is critical to fully capture the complex physics of mass-loading plasmas. Title: The Role of Electron Dynamics in the Solar Wind Interaction with Comet 67P/Churyumov-Gerasimenko at 3 AU Authors: Deca, J.; Divin, A.; Henri, P.; Eriksson, A.; Markidis, S.; Olshevsky, V.; Horányi, M. Bibcode: 2017LPI....48.1315D Altcode: Using a self-consistent 3D full-kinetic PIC approach, we disentangle the ion and electron dynamics of the solar wind interaction with a weakly outgassing comet. Title: Intermittent energy dissipation by turbulent reconnection Authors: Fu, H. S.; Vaivads, A.; Khotyaintsev, Y. V.; André, M.; Cao, J. B.; Olshevsky, V.; Eastwood, J. P.; Retinò, A. Bibcode: 2017GeoRL..44...37F Altcode: Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the "diffusion region" at the sub-ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E' ṡ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines. Title: A new model for the electron pressure nongyrotropy in the outer electron diffusion region Authors: Divin, A.; Semenov, V.; Korovinskiy, D.; Markidis, S.; Deca, J.; Olshevsky, V.; Lapenta, G. Bibcode: 2016GeoRL..4310565D Altcode: We present a new model to describe the electron pressure nongyrotropy inside the electron diffusion region (EDR) in an antiparallel magnetic reconnection scenario. A combination of particle-in-cell simulations and analytical estimates is used to identify such a component of the electron pressure tensor in the rotated coordinates, which is nearly invariant along the outflow direction between the X line and the electron remagnetization points in the outer EDR. It is shown that the EDR two-scale structure (inner and outer parts) is formed due to superposition of the nongyrotropic meandering electron population and gyrotropic electron population with large anisotropy parallel to the magnetic field upstream of the EDR. Inside the inner EDR the influence of the pressure anisotropy can largely be ignored. In the outer EDR, a thin electron layer with electron flow speed exceeding the E × B drift velocity is supported by large-momentum flux produced by the electron pressure anisotropy upstream of the EDR. We find that this fast electron exhaust flow with |Ve×B|>|E| is in fact a constituent part of the EDR, a finding which will steer the interpretation of the Magnetospheric Multiscale Mission (MMS) data. Title: Magnetic nulls in three-dimensional kinetic simulations of space plasmas Authors: Olshevsky, Vyacheslav; Deca, Jan; Divin, Andrey; Peng, Ivy Bo; Markidis, Stefano; Innocenti, Maria Elena; Cazzola, Emanuele; Lapenta, Giovanni Bibcode: 2016EGUGA..18.4053O Altcode: We present a survey of magnetic nulls and associated energy dissipation in different three-dimensional kinetic particle-in-cell simulations of space plasmas. The configurations under study include: a traditional Harris current sheet and current sheets with asymmetric density distribution, dipolar and quadrupolar planetary magnetospheres, lunar magnetic anomalies, and decaying turbulence. Nulls are detected in the simulation snapshots by the topological degree method. In all runs except the quadrupolar magnetospere the dominating majority of nulls are of spiral topological type. When supported by strong currents, these nulls indicate the regions of strong energy dissipation. Dissipation, often accompanied by the changes in magnetic topology, is caused by plasma instabilities in the current channels or on their interfaces. Radial nulls show less activity, they can be created or destroyed in pairs, via topological bifurcations. Although such events demonstrate energy release, they are rather rare and short-living. An important implication of our study to observations is that magnetic topology should not be considered independently of other plasma properties such as currents. Title: Magnetic Null Points in Kinetic Simulations of Space Plasmas Authors: Olshevsky, Vyacheslav; Deca, Jan; Divin, Andrey; Peng, Ivy Bo; Markidis, Stefano; Innocenti, Maria Elena; Cazzola, Emanuele; Lapenta, Giovanni Bibcode: 2016ApJ...819...52O Altcode: 2015arXiv151202018O We present a systematic attempt to study magnetic null points and the associated magnetic energy conversion in kinetic particle-in-cell simulations of various plasma configurations. We address three-dimensional simulations performed with the semi-implicit kinetic electromagnetic code iPic3D in different setups: variations of a Harris current sheet, dipolar and quadrupolar magnetospheres interacting with the solar wind, and a relaxing turbulent configuration with multiple null points. Spiral nulls are more likely created in space plasmas: in all our simulations except lunar magnetic anomaly (LMA) and quadrupolar mini-magnetosphere the number of spiral nulls prevails over the number of radial nulls by a factor of 3-9. We show that often magnetic nulls do not indicate the regions of intensive energy dissipation. Energy dissipation events caused by topological bifurcations at radial nulls are rather rare and short-lived. The so-called X-lines formed by the radial nulls in the Harris current sheet and LMA simulations are rather stable and do not exhibit any energy dissipation. Energy dissipation is more powerful in the vicinity of spiral nulls enclosed by magnetic flux ropes with strong currents at their axes (their cross sections resemble 2D magnetic islands). These null lines reminiscent of Z-pinches efficiently dissipate magnetic energy due to secondary instabilities such as the two-stream or kinking instability, accompanied by changes in magnetic topology. Current enhancements accompanied by spiral nulls may signal magnetic energy conversion sites in the observational data. Title: How to Find Magnetic Nulls and Reconstruct Field Topology with MMS Data? Authors: Fu, H.; Vaivads, A.; Khotyaintsev, Y. V.; Olshevsky, V.; Andre, M.; Cao, J.; Huang, S.; Retino, A.; Lapenta, G. Bibcode: 2015AGUFMSM51A2517F Altcode: In this study, we apply a new method—the first-order Taylor expansion (FOTE)—to find magnetic nulls and reconstruct magnetic field topology, in order to use it with the data from the forth-coming MMS mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the FOTE method can find a null both inside and outside the tetrahedron and also deduce its drift velocity. In addition, the FOTE method can (1) avoid limitations of the PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2D (X and O); (3) reconstruct the magnetic field topology. We quantitively test the accuracy of FOTE in positioning magnetic nulls and reconstructing field topology, by using the data from 3D kinetic simulations. The influences of SC separation (0.05~1 di) and null-SC distance (0~1 di) on the accuracy are both considered. We find that: (1) for an isolated null, the method is accurate when the SC separation is smaller than 1 di, and the null-SC distance is smaller than 0.25~0.5 di; (2) for a null pair, the accuracy is same as in the isolated-null situation, except at the separator line, where the field is nonlinear. We define a parameter in terms of the eigenvalues of the null to quantify the quality of our method—the smaller this parameter the better the results. Comparing to the previously used one, this parameter is more relevant for null identification. Using the new method, we reconstruct the magnetic field topology around a radial-type null and a spiral-type null, and find that the topologies are well consistent with those predicted in theory. We therefore suggest using this method to find magnetic nulls and reconstruct field topology with four-point measurements, particularly from Cluster and the forth-coming MMS mission. For the MMS mission, this null-finding algorithm can be used to trigger its burst-mode measurements. Title: Null Points in Three-Dimensional Kinetic Simulations of Magnetic Reconnection Authors: Deca, J.; Olshevsky, V.; Divin, A. V.; Innocenti, M. E.; Cazzola, E.; Peng, B.; Markidis, S.; Ormvråk, M.; Lapenta, G. Bibcode: 2015AGUFMSH43A2420D Altcode: Kinetic particle-in-cell simulations are the primary tool for studying magnetic reconnection in space plasmas. Magnetic null points are believed to be the preferred locations in space where magnetic reconnection is luckily to happen, and are in the focus of interest of space missions such as Cluster and MMS. Simulations of magnetic reconnection in various configurations performed with the implicit particle-in-cell code iPic3D revealed that nulls are ubiquitious in these models. We apply the Poincare index technique to locate and identify the topological characteristics of the magnetic null points in different three-dimensional simulations. We investigate the relevance of magnetic nulls to energy dissipation, turbulence and plasma instabilities. In particular, we found out that magnetic nulls of spiral type associated with magnetic islands and flux ropes play more important role in the energy release than the radial nulls. This finding is in accordance with some recent MHD simulations and in situ observations of Cluster spacecraft. Title: Where should MMS look for the electron and ion diffusion regions? Authors: Lapenta, G.; Goldman, M. V.; Newman, D. L.; Olshevsky, V. Bibcode: 2015AGUFMSH54A..06L Altcode: Our message is that if we think of reconnection with the usual cartoon, the MMS mission should follow the advice of Indiana Jones: X never marks the spot. Based on 3D fully kinetic simulations started with a well defined x-line, we observe that reconnection transitions towards a more chaotic regime. Two fronts develop downstream of the x-line where the outflow meets the pre-existing plasma. In the fronts an instability develops caused by the local gradients of the density. The consequence is the break up of the fronts in a fashion similar to the classical fluid Rayleigh-Taylor instability with the formation of "fingers" of plasma and embedded magnetic fields. These fingers interact and produce secondary reconnection sites. We present several different diagnostics that prove the existence of these secondary reconnection sites. Each site is surrounded by its own electron diffusion region.At the fronts the ions are generally not magnetized and considerable ion slippage is present. The discovery we present is that electrons are also slipping, forming localized diffusion regions near secondary reconnection sites [1].The consequence of this discovery is twofold. First, the instability in the fronts has strong energetic implications. We observe that the energy transfer locally is very strong, an order of magnitude stronger than in the "X" line. However, this energy transfer is of both signs as it is natural for a wavy rippling with regions of magnetic to kinetic and regions of kinetic to magnetic energy conversion.Second, and most important for this session, is that MMS should not limit the search for electron diffusion regions to the location marked with X in all reconnection cartoons. Our simulations predict more numerous and perhaps more easily measurable electron diffusion regions in the fronts. [1] Lapenta, G et al., Nature Physics 11, 690-695 (2015) Title: Energy Dissipation in Magnetic Null Points at Kinetic Scales Authors: Olshevsky, Vyacheslav; Divin, Andrey; Eriksson, Elin; Markidis, Stefano; Lapenta, Giovanni Bibcode: 2015ApJ...807..155O Altcode: 2015arXiv150907961O We use kinetic particle-in-cell and MHD simulations supported by an observational data set to investigate magnetic reconnection in clusters of null points in space plasma. The magnetic configuration under investigation is driven by fast adiabatic flux rope compression that dissipates almost half of the initial magnetic field energy. In this phase powerful currents are excited producing secondary instabilities, and the system is brought into a state of “intermittent turbulence” within a few ion gyro-periods. Reconnection events are distributed all over the simulation domain and energy dissipation is rather volume-filling. Numerous spiral null points interconnected via their spines form null lines embedded into magnetic flux ropes; null point pairs demonstrate the signatures of torsional spine reconnection. However, energy dissipation mainly happens in the shear layers formed by adjacent flux ropes with oppositely directed currents. In these regions radial null pairs are spontaneously emerging and vanishing, associated with electron streams and small-scale current sheets. The number of spiral nulls in the simulation outweighs the number of radial nulls by a factor of 5-10, in accordance with Cluster observations in the Earth's magnetosheath. Twisted magnetic fields with embedded spiral null points might indicate the regions of major energy dissipation for future space missions such as the Magnetospheric Multiscale Mission. Title: How to find magnetic nulls and reconstruct field topology with MMS data? Authors: Fu, H. S.; Vaivads, A.; Khotyaintsev, Y. V.; Olshevsky, V.; André, M.; Cao, J. B.; Huang, S. Y.; Retinò, A.; Lapenta, G. Bibcode: 2015JGRA..120.3758F Altcode: In this study, we apply a new method—the first-order Taylor expansion (FOTE)—to find magnetic nulls and reconstruct magnetic field topology, in order to use it with the data from the forthcoming MMS mission. We compare this method with the previously used Poincare index (PI), and find that they are generally consistent, except that the PI method can only find a null inside the spacecraft (SC) tetrahedron, while the FOTE method can find a null both inside and outside the tetrahedron and also deduce its drift velocity. In addition, the FOTE method can (1) avoid limitations of the PI method such as data resolution, instrument uncertainty (Bz offset), and SC separation; (2) identify 3-D null types (A, B, As, and Bs) and determine whether these types can degenerate into 2-D (X and O); (3) reconstruct the magnetic field topology. We quantitatively test the accuracy of FOTE in positioning magnetic nulls and reconstructing field topology by using the data from 3-D kinetic simulations. The influences of SC separation (0.05~1 di) and null-SC distance (0~1 di) on the accuracy are both considered. We find that (1) for an isolated null, the method is accurate when the SC separation is smaller than 1 di, and the null-SC distance is smaller than 0.25~0.5 di; (2) for a null pair, the accuracy is same as in the isolated-null situation, except at the separator line, where the field is nonlinear. We define a parameter ξ ≡ |( λ1 + λ2 + λ3 )|/|λ|max in terms of the eigenvalues (λi) of the null to quantify the quality of our method—the smaller this parameter the better the results. Comparing to the previously used parameter (η≡|∇ ṡ B|/|∇ × B|), ξ is more relevant for null identification. Using the new method, we reconstruct the magnetic field topology around a radial-type null and a spiral-type null, and find that the topologies are well consistent with those predicted in theory. We therefore suggest using this method to find magnetic nulls and reconstruct field topology with four-point measurements, particularly from Cluster and the forthcoming MMS mission. For the MMS mission, this null-finding algorithm can be used to trigger its burst-mode measurements. Title: Oscillatory patterns in three-dimensional kinetic simulations of space plasma Authors: Olshevsky, Vyacheslav; Deca, Jan; Divin, Andrey; Lapenta, Giovanni; Markidis, Stefano Bibcode: 2015EGUGA..17.9026O Altcode: We analyse kinetic simulations of the relaxation of a magnetic field configuration with multiple null-points. The power spectral density of the magnetic field is dissipative and exhibits two breaks: at ion-inertial and at electron-gyration scales; the slopes are steeper than observed in solar wind. Although different simulations in the same configuration show similar energetics, the local evolution pattern is rather chaotic. Most of the null-points in the simulations are of the spiral type, they are surrounded by twisted field lines, and powerful currents establish through them forming Z-pinches. Various instabilities are associated with the current channels, especially prominent is the kinking which drives secondary magnetic reconnection that dissipates the magnetic energy. In some regions the current channels produce thin secondary threads that show lower hybrid drift-like oscillatory characteristics. Oscillatory patterns are also detected at the halo boundary above dipolar lunar anomalies in 3-D kinetic simulations. It is found that they are (at least partially) in relation to the position of the B=0 line across the halo formed due to the opposing directions of the dipolar and interplanetary magnetic field in the simulation set-up, as well is to the strength of both fields and the solar wind parameters. We investigate and compare the detailed characteristics of small-scale wave patterns in both 3D simulations of null points and lunar magnetic anomalies. Title: Role of Z-pinches in magnetic reconnection in space plasmas Authors: Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano; Divin, Andrey Bibcode: 2015JPlPh..81a3205O Altcode: 2015arXiv150907962O A widely accepted scenario of magnetic reconnection in collisionless space plasmas is the breakage of magnetic field lines in X-points. In laboratory, reconnection is commonly studied in pinches, current channels embedded into twisted magnetic fields. No model of magnetic reconnection in space plasmas considers both null-points and pinches as peers. We have performed a particle-in-cell simulation of magnetic reconnection in a three-dimensional configuration where null-points are present initially, and Z-pinches are formed during the simulation along the lines of spiral null-points. The non-spiral null-points are more stable than spiral ones, and no substantial energy dissipation is associated with them. On the contrary, turbulent magnetic reconnection in the pinches causes the magnetic energy to decay at a rate of ~1.5% per ion gyro period. Dissipation in similar structures is a likely scenario in space plasmas with large fraction of spiral null-points. Title: Turbulent convection in the Sun: modeling in unstructured meshes Authors: Olshevsky, Vyacheslav; Liang, Chunlei; Ham, Frank Bibcode: 2014arXiv1412.7318O Altcode: We adopted an unstructured hydrodynamical solver CharLES to the problem of global convection in the Sun. With the aim to investigate the properties of solar turbulent convection and reproduce differential rotation pattern. We performed simulations in two spherical shells, with 1.3 and 10 million cells. In the first, coarse mesh, the solution does not reproduce realistic convection, and is dominated by numerical effects. In the second mesh, thermal conduction leads to cooling of bottom layers, that could not be compensated by solar irradiance. More simulations in the 10M cells mesh should be performed to investigate the influence of transport coefficients and numerical effects. Our estimate of the code performance suggests, that realistic simulations in even finer grids could be performed for reasonable computational cost. Title: Role of Z-pinches in magnetic reconnection in space plasmas Authors: Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano; Divin, Andrey Bibcode: 2014AAS...22440902O Altcode: A generally accepted scenario of magnetic reconnection in space plasmas is the breakage of magnetic field lines in X-points. In laboratory, reconnection is widely studied in pinches, current channels embedded into twisted magnetic fields. No model of magnetic reconnection in space plasmas considers both null-points and pinches as peers. We have performed a particle-in-cell simulation of magnetic reconnection in a three-dimensional configuration where null-points are present initially, and Z-pinches are formed during the simulation. The X-points are relatively stable, and no substantial energy dissipation is associated with them. On the contrary, turbulent magnetic reconnection driven by kinking of the pinches causes the magnetic energy to decay at a rate of 1.5% per ion gyro period. Current channels and twisted magnetic fields are ubiquitous in turbulent space plasmas, so pinches can be responsible for the observed high magnetic reconnection rates. Title: Influence of pinches on magnetic reconnection in turbulent space plasmas Authors: Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano; Divin, Andrey Bibcode: 2014cosp...40E2370O Altcode: A generally accepted scenario of magnetic reconnection in space plasmas is the breakage of magnetic field lines in X-points. In laboratory, reconnection is widely studied in pinches, current channels embedded into twisted magnetic fields. No model of magnetic reconnection in space plasmas considers both null-points and pinches as peers. We have performed a particle-in-cell simulation of magnetic reconnection in a three-dimensional configuration where null-points are present nitially, and Z-pinches are formed during the simulation. The X-points are relatively stable, and no substantial energy dissipation is associated with them. On contrary, turbulent magnetic reconnection in the pinches causes the magnetic energy to decay at a rate of approximately 1.5 percent per ion gyro period. Current channels and twisted magnetic fields are ubiquitous in turbulent space plasmas, so pinches can be responsible for the observed high magnetic reconnection rates. Title: 2D collisionless magnetic reconnection: background density dependence. Authors: Divin, Andrey; Lapenta, Giovanni; Markidis, Stefano; André, Mats; Khotyaintsev, Yuri; Olshevsky, Vyacheslav; Vaivads, Andris Bibcode: 2014cosp...40E.715D Altcode: Even the simplest 2D configuration susceptible to magnetic reconnection (namely, Harris current sheet), possesses a number of "free parameters" that determine the dynamics and energetics of the process. Among such parameters are T_i/T_e ratio, guide field value, current sheet thickness, etc. In this report we systematically study the effect of changing the background density (from n_b/n_0=0.5 to n_b/n_0=0.003), which covers the range of lobe density values observed in the Earth's magnetotail. We performed two-dimensional Particle-in-Cell (PIC) simulations using implicit parallel code iPIC3D with double-periodic configuration. Increase of the jet front magnetic field (B_z) with n_b decrease is in agreement with 2D simulations reported previously. The elevated B_z region (magnetic field component normal to the current sheet) is several times larger than the initial current layer thickness, whereas large normal electric field (E_x) area is focused between the front and current sheet edge. Normal electric field has a bipolar profile (in the X direction), intensity scales roughly as (n_b/n_0)(-1/2) with changing n_b. In the low density case intense waves are generated near magnetic reconnection separatrices, what can be explained either by the separatrix electron flow disruption or by electron holes propagation. Title: Energetics of Kinetic Reconnection in a Three-Dimensional Null-Point Cluster Authors: Olshevsky, V.; Lapenta, G.; Markidis, S. Bibcode: 2013PhRvL.111d5002O Altcode: 2015arXiv150907969O We perform three-dimensional particle-in-cell simulations of magnetic reconnection with multiple magnetic null points. Magnetic field energy conversion into kinetic energy is about five times higher than in traditional Harris sheet configuration. More than 85% of initial magnetic field energy is transferred to particle energy during 25 reversed ion cyclofrequencies. Magnetic reconnection in the cluster of null points evolves in three phases. During the first phase, ion beams are excited, then give part of their energy back to the magnetic field in the second phase. In the third phase, magnetic reconnection occurs in many small patches around the current channels formed along the stripes of a low magnetic field. Magnetic reconnection in null points essentially presents three-dimensional features, with no two-dimensional symmetries or current sheets. Title: SWIFF: Space weather integrated forecasting framework Authors: Lapenta, Giovanni; Pierrard, Viviane; Keppens, Rony; Markidis, Stefano; Poedts, Stefaan; Šebek, Ondřej; Trávníček, Pavel M.; Henri, Pierre; Califano, Francesco; Pegoraro, Francesco; Faganello, Matteo; Olshevsky, Vyacheslav; Restante, Anna Lisa; Nordlund, Åke; Trier Frederiksen, Jacob; Mackay, Duncan H.; Parnell, Clare E.; Bemporad, Alessandro; Susino, Roberto; Borremans, Kris Bibcode: 2013JSWSC...3A..05L Altcode: SWIFF is a project funded by the Seventh Framework Programme of the European Commission to study the mathematical-physics models that form the basis for space weather forecasting. The phenomena of space weather span a tremendous scale of densities and temperature with scales ranging 10 orders of magnitude in space and time. Additionally even in local regions there are concurrent processes developing at the electron, ion and global scales strongly interacting with each other. The fundamental challenge in modelling space weather is the need to address multiple physics and multiple scales. Here we present our approach to take existing expertise in fluid and kinetic models to produce an integrated mathematical approach and software infrastructure that allows fluid and kinetic processes to be modelled together. SWIFF aims also at using this new infrastructure to model specific coupled processes at the Solar Corona, in the interplanetary space and in the interaction at the Earth magnetosphere. Title: Homologous CME: a multispacecraft approach supported by simulations Authors: Sanna, L.; Lapenta, G.; Steed, K.; Olshevsky, V.; Restante, A. Bibcode: 2012AGUFMSH51A2212S Altcode: Using in situ and remote observations from multiple space crafts (STEREO, SDO and Venus Express) provides the opportunity to to study homologous CMEs. For example, on 7 August 2010, a halo CME originating from NOAA AR11093 was observed remotely by STEREO B. Seven days later this active region erupted again, and a halo CME was observed remotely by STEREO A on 14 August 2010. In this and in similar other examples, we show that multiple eruptions are associated with reverse S-shaped flux rope structures and display a number of typical large-scale features related to CMEs, including coronal dimmings and EUV waves. By combining remote sensing and in situ observations of the ejecta, we consider the structure and heliospheric evolution of these CMEs and their interplanetary counterparts. The work is complemented by a theoretical investigation where observed features are replicated and clarified by simulation. This work is part of the eHeroes project (www.eheroes.eu), funded by the European Commission, under the grant agreement eHeroes (project n° 284461) Title: Kinetic structure of collisionless reconnection: hybrid simulations Authors: Šebek, O.; Trávníček, P. M.; Hellinger, P.; Lapenta, G.; Keppens, R.; Olshevsky, V.; Restante, A. L.; Hendrix, T. Bibcode: 2012EGUGA..14.8382S Altcode: Magnetic reconnection is a fundamental process observed in various space plasma systems, such as, for example, interface between planetary magnetosphere and solar wind at the dayside magnetopause. We study magnetic reconnection by means of two-dimensional hybrid approach (kinetic ions and fluid electrons). Our initial configuration consists of Harris equilibrium layer with small amplitude perturbation of magnetic field. These perturbations are origins of the formation of magnetic islands. In this study we focus on the role of ionic kinetic effects during the reconnection process, we examine the temperature anisotropy and gyrotropy of the ion velocity distribution functions. We discuss the importance of these kinetic effects by comparing the results from hybrid simulations with the results from magneto-hydrodynamic (MHD) simulations results. Title: Numerical simulation of propagation of the MHD waves in sunspots Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky, V.; Collados, M. Bibcode: 2010HiA....15..354P Altcode: We present results of numerical 3D simulation of propagation of MHD waves in sunspots. We used two self consistent magnetohydrostatic background models of sunspots. There are two main differences between these models: (i) the topology of the magnetic field and (ii) dependence of the horizontal profile of the sound speed on depth. The model with convex shape of the magnetic field lines near the photosphere has non-zero horizorntal perturbations of the sound speed up to the depth of 7.5 Mm (deep model). In the model with concave shape of the magnetic field lines near the photosphere Δ c/c is close to zero everywhere below 2 Mm (shallow model). Strong Alfven wave is generated at the wave source location in the deep model. This wave is almost unnoticeable in the shallow model. Using filtering technique we separated magnetoacoustic and magnetogravity waves. It is shown, that inside the sunspot magnetoacoustic and magnetogravity waves are not spatially separated unlike the case of the horizontally uniform background model. The sunspot causes anisotropy of the amplitude distribution along the wavefront and changes the shape of the wavefront. The amplitude of the waves is reduced inside the sunspot. This effect is stronger for the magnetogravity waves than for magnetoacoustic waves. The shape of the wavefront of the magnetogravity waves is distorted stronger as well. The deep model causes bigger anisotropy for both mgnetoacoustic and magneto gravity waves than the shallow model. Title: The Irkutsk Barium filter for narrow-band wide-field high-resolution solar images at the Dutch Open Telescope Authors: Hammerschlag, Robert H.; Skomorovsky, Valery I.; Bettonvil, Felix C. M.; Kushtal, Galina I.; Olshevsky, Vyacheslav L.; Rutten, Robert J.; Jägers, Aswin P. L.; Sliepen, Guus; Snik, Frans Bibcode: 2010SPIE.7735E..85H Altcode: 2010SPIE.7735E.265H A wide-field birefringent filter for the barium II line at 455.4nm is developed in Irkutsk. The Barium line is excellent for Doppler-shift measurements because of low thermal line-broadening and steep flanks of the line profile. The filter width is 0.008nm and the filter is tunable over 0.4nm through the whole line and far enough in the neighboring regions. A fast tuning system with servomotor is developed at the Dutch Open Telescope (DOT). Observations are done in speckle mode with 10 images per second and Keller-VonDerLühe reconstruction using synchronous images of a nearby bluecontinuum channel at 450.5nm. Simultaneous observation of several line positions, typically 3 or 5, are made with this combination of fast tuning and speckle. All polarizers are birefringent prisms which largely reduced the light loss compared to polarizing sheets. The advantage of this filter over Fabry-Perot filters is its wide field due to a large permitted entrance angle and no need of polishing extremely precise surfaces. The BaII observations at the DOT occur simultaneously with those of a fast-tunable birefringent H-alpha filter. This gives the unique possibility of simultaneous speckle-reconstructed observations of velocities in photosphere (BaII) and chromosphere (H-alpha). Title: Numerical Simulation of Excitation and Propagation of Helioseismic MHD Waves in Magnetostatic Models of Sunspots Authors: Parchevsky, K.; Kosovichev, A.; Khomenko, E.; Olshevsky, V.; Collados, M. Bibcode: 2010arXiv1002.1117P Altcode: We present comparison of numerical simulations of propagation of MHD waves,excited by subphotospheric perturbations, in two different ("deep" and "shallow") magnetostatic models of the sunspots. The "deep" sunspot model distorts both the shape of the wavefront and its amplitude stronger than the "shallow" model. For both sunspot models, the surface gravity waves (f-mode) are affected by the sunspots stronger than the acoustic p-modes. The wave amplitude inside the sunspot depends on the photospheric strength of the magnetic field and the distance of the source from the sunspot axis. For the source located at 9 Mm from the center of the sunspot, the wave amplitude increases when the wavefront passes through the central part of the sunspot. For the source distance of 12 Mm, the wave amplitude inside the sunspot is always smaller than outside. For the same source distance from the sunspot center but for the models with different strength of the magnetic field, the wave amplitude inside the sunspot increases with the strength of the magnetic field. The simulations show that unlike the case of the uniform inclined background magnetic field, the p- and f-mode waves are not spatially separated inside the sunspot where the magnetic field is strongly non-uniform. These properties have to be taken into account for interpretation of observations of MHD waves traveling through sunspot regions. Title: The solar Ba{II} 4554 Å line as a Doppler diagnostic: NLTE analysis in 3D hydrodynamical model Authors: Shchukina, N. G.; Olshevsky, V. L.; Khomenko, E. V. Bibcode: 2009A&A...506.1393S Altcode: 2009arXiv0905.0985S Aims: The aim of this paper is to analyse the validity of the Dopplergram and λ-meter techniques for the Doppler diagnostics of solar photospheric velocities using the Ba II 4554 Å line.
Methods: Both techniques are evaluated by means of NLTE radiative transfer calculations of the Ba II 4554 Å line in a three-dimensional hydrodynamical model of solar convection. We consider the cases of spatially unsmeared profiles and the profiles smeared to the resolution of ground-based observations.
Results: We find that: (i) speckle-reconstructed Dopplergram velocities reproduce the “true” velocities well at heights around 300 km, except for intergranular lanes with strong downflows where the velocity can be overestimated; (ii) the λ-meter velocities give a good representation of the “true” velocities through the whole photosphere, both under the original and reduced spatial resolutions. The velocities derived from the inner wing of smeared Ba II 4554 Å line profiles are more reliable than those for the outer wing. Only under high spatial resolution does the inner wing velocities calculated in intergranular regions give an underestimate (or even a sign reversal) compared with the model velocities; (iii) NLTE effects should be taken into account in modelling the Ba II 4554 Å line profiles. Such effects are more pronounced in intergranular regions.
Conclusions: Our analysis supports the opinion that the Dopplergram technique applied to the Ba II 4554 Å line is a valuable tool for the Doppler diagnostics of the middle photosphere around 300 km. The λ-meter technique applied to this line gives us a good opportunity to “trace” the non-thermal motions along the whole photosphere up to the temperature minimum and lower chromosphere.

Appendix is only available in electronic form at http://www.aanda.org Title: Theoretical Modeling of Propagation of Magnetoacoustic Waves in Magnetic Regions Below Sunspots Authors: Khomenko, E.; Kosovichev, A.; Collados, M.; Parchevsky, K.; Olshevsky, V. Bibcode: 2009ApJ...694..411K Altcode: 2008arXiv0809.0278K We use two-dimensional numerical simulations and eikonal approximation to study properties of magnetohydrodynamic (MHD) waves traveling below the solar surface through the magnetic structure of sunspots. We consider a series of magnetostatic models of sunspots of different magnetic field strengths, from 10 Mm below the photosphere to the low chromosphere. The purpose of these studies is to quantify the effect of the magnetic field on local helioseismology measurements by modeling waves excited by subphotospheric sources. Time-distance propagation diagrams and wave travel times are calculated for models of various field strengths and compared to the nonmagnetic case. The results clearly indicate that the observed time-distance helioseismology signals in sunspot regions correspond to fast MHD waves. The slow MHD waves form a distinctly different pattern in the time-distance diagram, which has not been detected in observations. The numerical results are in good agreement with the solution in the short-wavelength (eikonal) approximation, providing its validation. The frequency dependence of the travel times is in good qualitative agreement with observations. Title: Seismology of Sunspots: An Interplay between Temperature and Magnetic Field Structures Authors: Olshevsky, V.; Khomenko, E.; Collados, M. Bibcode: 2008ESPM...12..3.2O Altcode: Using a numerical three-dimensional MHD modelling of magneto-acoustic wave propagation in a realistic magnetostatic sunspot model we investigate the influence of the magnetic field on the parameters measured by local helioseismology. We find that the variations of temperature as well as the presence of the magnetic field cause important changes to the wave travel times. Magnetic field speeds up the waves to considerable amount, while the temperature depression within a sunspot causes the opposite action. The calculated travel time differences between the unmagnetized and magnetized atmospheres lie in the range typically obtained from local helioseismology correlation analysis. Our numerical results are also in agreement with the analytical calculations of the travel times applying WKB technique. Title: The Ba II [lambda]4554 resonance line and solar granulation Authors: Olshevsky, V. L.; Shchukina, N. G. Bibcode: 2007msfa.conf..307O Altcode: We present the results of an investigation of the impact of NLTE effects and of granulation inhomogeneities on the solar Ba II [lambda]4554 Å line. Our analysis is based on both the classical one-dimensional (1D) solar atmosphere models and on the new generation of three-dimensional (3D) hydrodynamical models. We show that NLTE and 3D effects have to be taken into account for reliable diagnostics of the solar atmosphere using this line. We analyse the influence of different parameters on the line shape. It turns out to be most sensitive to collisional broadening and barium abundance. Uncertainties in the oscillator strength, micro- and macroturbulence (in 1D-case) have a secondary importance. We have derived the barium abundance assuming NLTE. We find ABa = 2.16 in good agreement with the recent result of Asplund et al. (2005). Title: Numerical modeling of MHD wave propagation in sunspots: a 3D case Authors: Olshevsky, V.; Khomenko, E.; Collados, M. Bibcode: 2007msfa.conf..347O Altcode: We present the first results of a 3D numerical modeling of linear MHD wave propagation in a realistic sunspot model. In our simulations, a piston located at the base of the photosphere generates waves with a certain period. The ratio between the acoustic and the Alfven speed, cS /vA, decreases from much larger than one at the photosphere to much lower than one in the chromosphere in our simulation domain. Waves propagate through the region where cS << vA, where mode transformation is observed. At a somewhat higher region, where cS = vA, the fast (magnetic) mode reflects back to the photosphere due to the vertical and horizontal gradients of vA. The slow (acoustic) mode propagates to the upper layers and increases its velocity amplitude. Unlike the 2D simulations, the Alfven mode is also generated by the piston and experiences transformations at the cS = vA layer. The behaviour of this mode requires further study. Title: Line-of-sight velocity distributions of elliptical galaxies from collisionless mergers Authors: González-García, A. C.; Balcells, M.; Olshevsky, V. S. Bibcode: 2006MNRAS.372L..78G Altcode: 2006astro.ph..9365G; 2006MNRAS.tmpL..95G We analyse the skewness of the line-of-sight velocity distributions in model elliptical galaxies built through collisionless galaxy mergers. We build the models using large N-body simulations of mergers between either two spiral or two elliptical galaxies. Our aim is to investigate whether the observed ranges of skewness coefficient (h3) and the rotational support (V/σ), as well as the anticorrelation between h3 and V, may be reproduced through collisionless mergers. Previous attempts using N-body simulations failed to reach V/σ ~ 1-2 and corresponding high h3 values, which suggested that gas dynamics and ensuing star formation might be needed in order explain the skewness properties of ellipticals through mergers. Here we show that high V/σ and high h3 are reproduced in collisionless spiral-spiral mergers whenever a central bulge allows the discs to retain some of their original angular momentum during the merger. We also show that elliptical-elliptical mergers, unless merging from a high-angular momentum orbit, reproduce the strong skewness observed in non-rotating, giant, boxy ellipticals. The behaviour of the h3 coefficient therefore associates rapidly-rotating discy ellipticals to disc-disc mergers, and associates boxy, slowly rotating giant ellipticals to elliptical-elliptical mergers, a framework generally consistent with the expectations of hierarchical galaxy formation. Title: Non-LTE Formation of Ba II Resonance Lines Authors: Olshevsky, V. L.; Shchukina, N. G. Bibcode: 2006IAUJD...3E..16O Altcode: We investigate the formation of the resonance lines of ionized Barium in the Solar atmosphere. We have constructed atomic model of Ba II, which includes 40 energy levels, 99 bound-bound and 39 bound-free transitions. Using the numerical solution of radiative transfer equation together with the statistical equilibrium equations we calculate emergent profiles of the Ba II 4554 A resonance line under different conditions. Line profiles are calculated in both standard 1D and 3D atmospheric models in LTE and non-LTE approximations. We found that non-LTE effects are very important for this line. Main NLTE mechanisms are the resonance line scattering and photon losses. In 1D the line shape is most sensitive to the following parameters: Barium abundance, collisional strengths and macroturbulent velocity. Sensitivity to the uncertainties in oscillator strengths is not so high. Taking precise theoretical values of collisional strengths by Barklem & O'Mara (1998) we found the best agreement with observations for the abundance close to the "classical" value of 2.13. In 3D calculated profiles are in a good agreement with the observations. In 1D the agreement is worse. Thus it is critical for the modelling of Ba II lines to use the 3D non-LTE radiative transfer. Title: Granular and Intergranular Oscillations from the Observations of BA II Resonance Line Authors: Olshevsky, V. L. Bibcode: 2005ESASP.596E..79O Altcode: 2005ccmf.confE..79O No abstract at ADS Title: Ba II 4554A Resonance Line Formation in the Solar Atmosphere Authors: Olshevsky, V.; Shchukina, N. Bibcode: 2005ysc..conf...71O Altcode: 2005yosc.conf...71O We investigate non-LTE radiative transfer in the the Ba II 4554 A line in the Solar atmosphere. This line is particularly suited to measure non thermal motions in the solar photosphere. It is also used in Stokes vector spectropolarimetry. NLTE spectral formation modeling is a necessary step in the formulation of reliable quantitative data interpretation through numerical inversion. The first phase of the work includes creation of a reliable model of the Ba II atom. It includes: energy levels, oscillator strengths for the selected bb-transitions and bf-transitions probabilities. We also take into account the hyper fine structure and isotopic splitting. Next step is to calculate departure coefficients for Ba II through numerical solution of the radiative transfer equations. When these coefficients are derived it is possible to apply inversion techniques to spectral observations in this line. Title: Temperature oscillations in solar photosphere caused by propagation of acoustic-gravity waves of small amplitudes Authors: Olshevsky, V. L. Bibcode: 2004KFNT...20..396O Altcode: We analysed temperature and velocity oscillations in the solar photosphere which are caused by propagation of acoustic-gravity waves. We calculate the oscillations using analytical solution of the system of HD equations for isothermally stratified one-dimensional atmosphere model. We investigated how well temperature oscillations in the photosphere can be described by propagation of the small-amplitude acoustic-gravity waves and how well the inversion methods can reproduce small-amplitude temperature oscillations. It is shown that the calculated velocity oscillations are in good agreement with the observed ones in most of the photosphere for frequencies of 2.5 to 4.5 mHz. Calculated temperature oscillations are in agreement with observed oscillations only in the medium photosphere, at heights from 300 to 400 km.