Paper Submission
ETC2019 17th European Turbulence Conference





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14:00   Turbulence, Waves and Instabilities in Plasmas
14:00
15 mins

#287
SOLAR WIND TURBULENCE
Renaud Ferrand, Nahuel Andrés, Fouad Sahraoui, Sébastien Galtier, Romain Meyrand, Pablo Mininni, Pablo Dmitruk
Abstract: Understanding the turbulent behavior of the solar wind (SW) has been a longstanding problem in astrophysics. In situ measurements made with different spacecraft (like Cluster/ESA or Themis/NASA) revealed magnetic, kinetic and density spectra over several decades in frequency spanning the magnetohydrodynamic (MHD) range down to the electron gyro-scale for the magnetic fluctuations. A slow decrease of the temperature with the heliocentric distance (slower than an adiabatic cooling) is also reported, which requires a local heating to be explained. Turbulence is seen as the main mechanism to provide that heating, however, the SW is a collisionless plasma, which makes the classical viscous dissipation irrelevant. Instead, kinetic effects are thought to play a leading role in that dissipation, in particular at small scales. A way to study space plasma turbulence is to estimate the total energy cascade rate, which is the energy transferred from the largest scales into the dissipative scales of the system. This was recently achieved within the Earth magnetosheath using an exact law for compressible isothermal MHD turbulence, which highlighted the importance of compressibility in this region. However, MHD models of the SW cannot describe the sub-ionic scales, where an important part of the dissipation is thought to happen. It is thus necessary to adapt the widely used ideal MHD models to include (at least) the contribution of the Hall effect at sub-ionic scales. In this presentation we will make a review of previous results on this topic before applying an exact law for compressible Hall-MHD to investigate the influence of the Hall effect on the energy cascade rate. We will show new results from three-dimensional direct numerical simulations of compressible Hall-MHD for sub-sonic turbulence and discuss the implications of the results on the turbulence dynamics and energy dissipation at sub-ion scales.
14:15
15 mins

#525
Shear flow instabilities in asymmetric magnetic reconnection
Dario Borgogno, Anna Perona, Daniela Grasso, Emanuele Tassi
Abstract: Magnetic reconnection is a common instability in space and laboratory plasmas, often invoked to justify explosive phenomena, such as a rapid transfer of magnetic energy into heat and charged particle acceleration. Asymmetric reconnection, for which the peak of the current density is not located at the resonant surface of the equilibrium magnetic field, creates configurations which are quite natural in most of the plasma environments, e. g. in the magnetosheath and in tokamaks, and which have been recently observed by the Nasa's MMS mission [1]. Shear flow instabilities of the plasma current and vorticity, analogous to the Kelvin-Helmoltz mode, have been identified since the early 2000 in numerical fluid simulations of symmetric magnetic reconnection of dissipationless plasmas in low electron temperature regimes [2-4]. We show how these instabilities can be found in both a two- and three-dimensional fluid approach in finite electron temperature regimes provided that the magnetic reconnection configuration is sufficiently asymmetric. Such a fluid approach yields a simpler description of magnetic reconnection with respect to the richer PIC investigations [5-7], as it allows for a clear separation of the space scales characterizing the process. Moreover it provides a possible interface for further test-particle simulations aiming at shedding light on peculiar features of particle transport in complex magnetic configurations. 1) J. L. Burch et al., Science 352, aaf2939 (2016). 2) D. Del Sarto et al., Phys. Rev. Lett. 91, 235001 (2003). 3) D. Grasso at al., Phys. Plasmas 14, 055703 (2007). 4) D. Grasso et al., Nonlin. Processes Geophys. 16, 241 (2009). 5) Price et al., Geophys. Res. Lett., 43, 6020–6027, (2016). 6) J. Egedal et al., Phys. Rev. Lett. 117, 185101 (2016). 7) A. Let et al., Phys Plasmas 25, 062103 (2018).
14:30
15 mins

#106
Macrophysics and microphysics of energy transfer in kinetic plasma
Yan Yang, Minping Wan, William Matthaeus, Luca Sorriso-Valvo, Tulasi Parashar, Quanming Lu, Yipeng Shi, Shiyi Chen
Abstract: There is a vast range of scales in kinetic plasma, spanning from macroscopic fluid scales to sub-electron scales. Magnetohydrodynamic (MHD) model remains a credible approximation for a kinetic plasma at scales large enough to be well separated from kinetic effects, while more refined kinetic description is required at kinetic scales. The turbulence, in particular the classical energy cascade scenario, can tie them all together. That is of great importance in studying energy dissipation mechanism for weakly collisional or collisionless plasma, thus might explaining the heating of collisionless plasmas such as solar corona and solar wind. We will briefly review several surrogates, arising from the energy transfer process, that are used to estimate energy dissipation rate in collisionless plasma, including the Politano-Pouquet law [1] that describes the scaling law of the mixed third-order moments of Elsasser fields increments, the work done by electromagnetic fields on charged particles [2, 3], and the pressure-strain interaction [5, 4]. We investigate in detail the associations and differences that exist among theses energy dissipation surrogates and find that they dominate at different scales, and are spatially located in proximity to each other even if their point-wise cross-correlations may be weak [6].
14:45
15 mins

#512
CURVATURE OF LAGRANGIAN TRAJECTORIES IN TURBULENCE WITH ZONAL FLOWS
Benjamin KADOCH, Wouter J. T. BOS, Kai Schneider
Abstract: In the present work we will consider a toy-model from the confined fusion community, derived by Hasegawa and Wakatani, which is known to generate turbulence, and for certain choices of the parameters, zonal flows. Direct numerical simulation are carried out and details of the model and method can be found in [5, 1]. The multiscale curvature, which we introduced into turbulence research in references [2, 4, 3], is considered. A control-parameter in the model measures how strongly the vorticity dynamics are tied to the density fluctuations. It is observed that changing the value of this parameter does not drastically change the curvature. However, the modified model shows the formation of strong zonal flows and for this case the multi-scale curvature is radically diminished at all time-lags, reflecting the reduced radial movement of the fluid particles and the anisotropy of the flow structure. References [1] W. J. T. Bos, B. Kadoch, S. Neffaa, and K. Schneider. Lagrangian dynamics of drift-wave turbulence. Physica D: Nonlinear Phenomena, 239(14):1269 – 1277, 2010. [2] W. J. T. Bos, B. Kadoch, and K. Schneider. Angular statistics of Lagrangian trajectories in turbulence. Phys. Rev. Lett., 114:214502, 05 2015. [3] Xiaoliang He, Sourabh Apte, Kai Schneider, and Benjamin Kadoch. Angular multiscale statistics of turbulence in a porous bed. Phys. Rev. Fluids, 3:084501, Aug 2018. [4] Benjamin Kadoch, Wouter J.T. Bos, and Kai Schneider. Directional change of fluid particles in two-dimensional turbulence and of football players. Phys. Rev. Fluids, 2:064604, 2017. [5] A. V. Pushkarev, W. J. T. Bos, and S. V. Nazarenko. Zonal flow generation and its feedback on turbulence production in drift wave turbulence. Phys. Plasmas, 20(4):042304, 2013.
15:00
15 mins

#24
SIMULATION STUDY OF HIGH MAGNETIC PRANDTL NUMBER MAGNETOHYDRODYNAMIC TURBULENCE UNDER HALL EFFECTS
Hideaki Miura, Jingyuan Yang, Toshiyuki Gotoh
Abstract: Homogeneous and isotropic magnetohydorydynamic (MHD) turbulence with the magnetic Prandtl number larger than unity under influences of the Hall effects is studied numerically. The Hall term is proportional to the ratio of the ion skin depth to the system size, and represents ion-electron-separation in the magnetic field equations. Direct numerical simulations of forced Hall MHD turbulence are carried out for the Hall parameter 0.5 and the magnetic Prandtl number, the ratio of the viscosity to the magnetic diffusivity, 100. Our forced Hall MHD turbulence simulations show that tornado-like structures of strong current are erupted in the course of simulations . Because of the Hall term, the magnetic field (current field) is not frozen to the velocity (vorticity) field. Furthermore, because of the magnetic Prandtl number being much larger than unity, the current field has a degree of freedom higher than the vorticity field. As the consequence, the tornado-like structures travel along the magnetic field lines. The strong current regions are strongly correlated with regions of the strong palinstrophy density where the palinstrophy is the rotation of the vorticity (equivalent to the Laplacian of the velocity field). This indicates a strong relation of the current eruption with the dissipation structure in the velocity field. In the DNS, a new power-law regime of k^{-17/3} is formed in the kinetic energy spectrum . We show that the formation of the tornado-like structures is closely related with this new power-law regime, and that the power-law can be explained by the energy budget analysis. These numerical simulations can give hints to understand a high-magnetic-Prandtl-number plasma turbulence in which the ion skin depth is larger than the collisional dissipation scale. We discuss basic mechanism of tornado-like structures in the course of time evolution, including the current field and vorticity field separation which can be observed when the magnetic Prandtl number is unity . We will also discuss the structure formation further in the context of the generalized vorticity and generalized helicity in the context of the conservation law and/or double Beltrami flows.
15:15
15 mins

#616
Magnetic turbulence anisotropy and cascade rates in the heliosheath and local interstellar medium as seen by the Voyagers
Federico Fraternale, Nikolai V Pogorelov, John D Richardson, Daniela Tordella
Abstract: It is currently believed that turbulent fluctuations pervade the inner heliosheath (IHS) and the local interstellar medium (LISM). In these regions of space, turbulence, magnetic reconnection, and their interrelationship are likely the major players in the processes responsible for particle transport and magnetic energy conversion into kinetic energy and heat. State-of-art numerical models take into account for the solar-cycle variations and make it possible to reproduce many of the observed large-scale features of the IHS and LISM plasma and magnetic fields [1,2,3]. However, the dynamics of fluctuations on scales smaller than about the sector spacing (2 AU after the termination shock and less in proximity of the heliopause) remains critically understudied due to the high computational load and the lack of in situ measurements. This makes our understanding of the physical processes in the IHS and LISM a formidable challenge. Our recent research [4,5] showed that the unique, in situ, 48 s magnetic-field data from the Voyager Interstellar Mission (V1 and V2 spacecraft) are appropriate for a spectral analysis of spacecraft-frame frequencies in the range 5x10^{-8} [0.2-1.3] AU at V2) and of the inertial-cascade regime of magnetic field fluctuations. Intermittency analysis shows that the latter is compatible with a turbulent description - anisotropic and dominated by compressible modes, especially in the unipolar regions sampled by V1. Here, we attempt the first analysis of the magnetic-energy cascade rates computed from both the third-order statistics of magnetic field increments and from proxies based on the power spectrum. In Figure 1 below, we take as example the IHS interval sampled by V2 in the sector region from day-of-year 300 of 2013 to 2016, at 106 AU from the Sun. Here, our analysis yields values of magnetic energy fluxes between 60 and 600 J/Kg/s. This research is framed within the project ``Turbulence as Indicator of Physical Processes at the Heliospheric Interface'', NASA's H-GI Open Program, 18-HGIO18\_2-0029. [1] J. D. Richardson and L. F. Burlaga. The solar wind in the outer heliosphere and heliosheath. Space Sci. Rev., 176(1-4):217–235, 2013. [2] N. V. Pogorelov, H. Fichtner, A. Czechowski, A. Lazarian, B. Lembege, J. A. Ie Roux, M. S. Potgieter, K. Scherer, E. C. Stone, R. D. Strauss, T. Wiengarten, P. Wurz, G. P. Zank, and M. Zhang. Heliosheath processes and the structure of the heliopause: modeling energetic particles,cosmic rays, and magnetic fields. Space Sci. Rev., 212(1-2):193–248, 2017. [3] N. V. Pogorelov, J. Heerikhuisen, V. Roytershteyn, L. F. Burlaga, D. A. Gurnett, and W. S. Kurth. Three-dimensional features of the outer heliosphere due to coupling between the interstellar and heliospheric magnetic field. v. the bow wave, heliospheric boundary layer, instabilities, and magnetic reconnection. Astrophys. J., 845(1), 2017. [4] L. Gallana, F. Fraternale, M. Iovieno, S. M. Fosson, E. Magli, M. Opher, J. D. Richardson, and D. Tordella. Voyager 2 solar plasma and magnetic field spectral analysis for intermediate data sparsity. J. Geophys. Res. - Space Physics, 121(5):3905–3919, 2016. [5] F. Fraternale, N. V. Pogorelov, J. D. Richardson, and D. Tordella. Magnetic turbulence spectra and intermittency in the heliosheath and in the local interstellar medium. Astrophys. J., 872(40), 2019.
15:30
15 mins

#409
Solar Wind and Magnetosheath Observations of Energy Transfer, Intermittency and Dissipation
William Matthaeus, Alex Chasapis, Riddhi Bandyopadhyay, Rohit Chhiber, Tulasi Parashar, Manuel Cuesta, Yan Yang, Minping Wan, Luca Sorriso-Valvo
Abstract: Turbulence cascade transfers energy from large scale to small scale but what happens once kinetic scales are reached? In a collisional medium, viscosity and resistivity remove fluctuation energy in favor of heat. In the weakly collisional space plasma (solar wind, corona, magnetosheath, etc), the sequence of events must be different. Heating occurs, but through what mechanisms? In standard approaches, dissipation occurs through linear wave modes or instabilities, or through processes related to reconnection, and one seeks to identify these specific mechanisms. A complementary view is that cascade leads to several channels of energy conversion, interchange and spatial rearrangement that collectively lead to production of internal energy [1,2]. Channels can be described using compressible MHD & multispecies Vlasov-Maxwell formulations. Key steps are: Conservative rearrangement of energy in space; Parallel incompressible and compressible cascades – conservative rearrangement in scale; electromagnetic work on particles that drives flows, both macroscopic and microscopic; and pressure-strain interactions, both compressive and shear-like, that produce internal energy. In contrast to the collisional case, in which viscous and resistive energy conversion are sign-definite and dissipative, for Vlasov-Maxwell turbulence, none of the main steps are of single sign, and each step involves bidirectional transfer, with a slight bias towards the direction leading to dissipation, reminiscent of the unaveraged third-order contributions (“LET”) to the Kolmogorov-Yaglom law. Ultimately a velocity space cascade leads to enhanced collisions. Here, examples of key stapes in the energy transfer and conversion processes are shown, emphasizing results from spacecraft observations such as the Magnetospheric Multiscale Mission [3-6]