Publications

2015

Kinetic simulations of secondary reconnection in the reconnection jet
- Huang S. Y.
- Zhou M.
- Yuan Z. G.
- Fu H.S.
- He J. S.
- Sahraoui Fouad
- Aunai Nicolas
- Deng X. H.
- Fu S. Y.
- Pang Y.
- Wang D. D.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (8), pp.61886198. Magnetic reconnection, as one important energy dissipation process in plasmas, has been extensively studied in the past several decades. Magnetic reconnection occurring in the downstream of a primary X line is referred to as secondary reconnection. In this paper, we used kinetic simulations to investigate the secondary reconnection in detail. We found that secondary reconnection is reversed by the compression caused by the outflowing jet originating from the primary reconnection site, which results in the erosion of the magnetic island between the two X lines within ~3 ωci−1. We show the observational signatures expected in electromagnetic fields and plasma measurements in the Earth's magnetotail, associated with this mechanism. These simulation results could be applied to interpret the signatures associated with the evolution of earthward magnetic islands in the Earth's magnetotail. (10.1002/2014JA020969)
DOI : 10.1002/2014JA020969
Nature of the MHD and Kinetic Scale Turbulence in the Magnetosheath of Saturn: Cassini Observations
- Hadid Lina
- Sahraoui Fouad
- Kiyani K. H.
- Retinò Alessandro
- Modolo Ronan
- Canu Patrick
- Masters Adam
- Dougherty Michele K.
The Astrophysical Journal Letters, Bristol : IOP Publishing, 2015, 813 (2), pp.L29. Low-frequency turbulence in Saturn's magnetosheath is investigated using in situ measurements of the Cassini spacecraft. Focus is put on the magnetic energy spectra. A set of 42 time intervals in the magnetosheath were analyzed, and three main results that contrast with known features of solar wind turbulence are reported. (10.1088/2041-8205/813/2/L29)
DOI : 10.1088/2041-8205/813/2/L29
Negative hydrogen ion production mechanisms
- Bacal M.
- Wada M.
Applied Physics Reviews, AIP Publishing, 2015, 2 (2), pp.021305. Negative hydrogen/deuterium ions can be formed by processes occurring in the plasma volume and on surfaces facing the plasma. The principal mechanisms leading to the formation of these negative ions are dissociative electron attachment to ro-vibrationally excited hydrogen/deuterium molecules when the reaction takes place in the plasma volume, and the direct electron transfer from the low work function metal surface to the hydrogen/deuterium atoms when formation occurs on the surface. The existing theoretical models and reported experimental results on these two mechanisms are summarized. Performance of the negative hydrogen/deuterium ion sources that emerged from studies of these mechanisms is reviewed. Contemporary negative ion sources do not have negative ion production electrodes of original surface type sources but are operated with caesium with their structures nearly identical to volume production type sources. Reasons for enhanced negative ion current due to caesium addition to these sources are discussed. (31 pages, 265 refs) (10.1063/1.4921298)
DOI : 10.1063/1.4921298
Isopropanol removal using MnXOY packed bed non-thermal plasma reactor: Comparison between continuous treatment and sequential sorption/regeneration
- Sivachandiran Loganathan
- Thévenet Frédéric
- Rousseau Antoine
Chemical Engineering Journal, Elsevier, 2015, 270, pp.327-335. MnXOY coated glass beads packed bed non-thermal plasma (NTP) reactor has been designed and operated for isopropanol (IPA) removal close to indoor air conditions. The IPA removal efficiency of continuous NTP treatment is compared with the sequential approach, i.e. adsorption of IPA on MnXOY and subsequent regeneration of the saturated MnXOY surface by non-thermal plasma. The comparison between both approaches has been achieved with the same packed bed reactor and model VOC under equivalent indoor air conditions. Firstly, based on carbon mass balance calculations, the continuous treatment has shown better performances from an IPA abatement point of view, as well as from a mineralization point of view. However, the characterization of ppb level side-products evidenced that the continuous treatment leads to a more significant release of organic side products which may impact indoor air quality. Secondly, both processes have been compared in terms of energetic costs regarding (i) IPA removal, and (ii) CO2 forma- tion. Interestingly, it is evidenced that, to treat the same amount of IPA, the sequential approach requires 14.5 times less energy than the continuous NTP treatment process. Similarly, to produce the same amount of CO2, the sequential approach consumes 10 times less energy. This comparison evidences the interest of adsorption combined with subsequent non-thermal plasma regeneration for indoor air effluent treatment. (10.1016/j.cej.2015.01.055)
DOI : 10.1016/j.cej.2015.01.055
Equatorial noise emissions with quasiperiodic modulation of wave intensity
- Nemec F.
- Santolík O.
- Hrbackova Z.
- Pickett J. S.
- Cornilleau-Wehrlin Nicole
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.2649-2661. Equatorial noise (EN) emissions are electromagnetic wave events at frequencies between the proton cyclotron frequency and the lower hybrid frequency observed in the equatorial region of the inner magnetosphere. They propagate nearly perpendicular to the ambient magnetic field, and they exhibit a harmonic line structure characteristic of the proton cyclotron frequency in the source region. However, they were generally believed to be continuous in time. We investigate more than 2000 EN events observed by the Spatio-Temporal Analysis of Field Fluctuations and Wide-Band Data Plasma Wave investigation instruments on board the Cluster spacecraft, and we show that this is not always the case. A clear quasiperiodic (QP) time modulation of the wave intensity is present in more than 5% of events. We perform a systematic analysis of these EN events with QP modulation of the wave intensity. Such events occur usually in the noon-to-dawn magnetic local time sector. Their occurrence seems to be related to the increased geomagnetic activity, and it is associated with the time intervals of enhanced solar wind flow speeds. The modulation period of these events is on the order of minutes. Compressional ULF magnetic field pulsations with periods about double the modulation periods of EN wave intensity and magnitudes on the order of a few tenths of nanotesla were identified in about 46% of events. We suggest that these compressional magnetic field pulsations might be responsible for the observed QP modulation of EN wave intensity, in analogy to formerly reported VLF whistler mode QP events. (10.1002/2014JA020816)
DOI : 10.1002/2014JA020816
Gyrokinetic turbulence cascade via predator-prey interactions between different scales
- Kobayashi Sumire
- Gürcan Özgür D.
Physics of Plasmas, American Institute of Physics, 2015, 22 (5), pp.050702. Gyrokinetic simulations in a closed fieldline geometry are presented to explore the physics of nonlinear transfer in plasma turbulence. As spontaneously formed zonal flows and small-scale turbulence demonstrate predator-prey dynamics, a particular cascade spectrum emerges. The electrostatic potential and the density spectra appear to be in good agreement with the simple theoretical prediction based on Charney-Hasegawa-Mima equation |||ϕ̃k|||2∼||ñk||2∝k−3/(1 k2)2, with the spectra becoming anisotropic at small scales. The results indicate that the disparate scale interactions, in particular, the refraction and shearing of larger scale eddies by the self-consistent zonal flows, dominate over local interactions, and contrary to the common wisdom, the comprehensive scaling relation is created even within the energy injection region. (10.1063/1.4920965)
DOI : 10.1063/1.4920965
Distribution of energetic oxygen and hydrogen in the near-Earth plasma sheet
- Kronberg E. A.
- Grigorenko E. E.
- Haaland S. E.
- Daly P. W.
- Delcourt Dominique C.
- Luo H.
- Kistler L. M.
- Dandouras I.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (5), pp.3415-3431. The spatial distributions of different ion species are useful indicators for plasma sheet dynamics. In this statistical study based on 7 years of Cluster observations, we establish the spatial distributions of oxygen ions and protons at energies from 274 to 955 keV, depending on geomagnetic and solar wind (SW) conditions. Compared with protons, the distribution of energetic oxygen has stronger dawn-dusk asymmetry in response to changes in the geomagnetic activity. When the interplanetary magnetic field (IMF) is directed southward, the oxygen ions show significant acceleration in the tail plasma sheet. Changes in the SW dynamic pressure (Pdyn) affect the oxygen and proton intensities in the same way. The energetic protons show significant intensity increases at the near-Earth duskside during disturbed geomagnetic conditions, enhanced SW Pdyn, and southward IMF, implying there location of effective inductive acceleration mechanisms and a strong duskward drift due to the increase of the magnetic field gradient in the near-Earth tail. Higher losses of energetic ions are observed in the dayside plasma sheet under disturbed geomagnetic conditions and enhanced SW Pdyn. These observations are in agreement with theoretical models. (10.1002/2014JA020882)
DOI : 10.1002/2014JA020882
Magnetic noise contribution of the ferromagnetic core of induction magnetometers
- Coillot C
- El Moussalim M
- Brun E
- Rhouni A
- Lebourgeois R
- Sou Gérard
- Mansour Malik
Journal of Sensors and Sensor Systems, Copernicus Publ, 2015, 4, pp.229 - 237. The performance of induction magnetometers, in terms of resolution, depends both on the induction sensor and the electronic circuit. To investigate accurately the sensor noise sources, an induction sensor, made of a ferrite ferromagnetic core, is combined with a dedicated low voltage and current noise preamplifier, designed in CMOS 0.35 µm technology. A modelling of the contribution of the ferromagnetic core to the noise through the complex permeability formalism is performed. Its comparison with experimental measurements highlight another possible source for the dominating noise near the resonance. (10.5194/jsss-4-229-2015)
DOI : 10.5194/jsss-4-229-2015
A Review of General Physical and Chemical Processes Related to Plasma Sources and Losses for Solar System Magnetospheres
- Seki K.
- Nagy A.
- Jackman C. M.
- Crary F.
- Fontaine Dominique
- Zarka P.
- Wurz Peter
- Milillo A.
- Slavin J. A.
- Delcourt Dominique C.
- Wiltberger M.
- Ilie R.
- Jia X.
- Ledvina S. A.
- Liemohn M. W.
- Schunk R. W.
Space Science Reviews, Springer Verlag, 2015, 192 (1-4), pp.27-89. The aim of this paper is to provide a review of general processes related to plasma sources, their transport, energization, and losses in the planetary magnetospheres. We provide background information as well as the most up-to-date knowledge of the comparative studies of planetary magnetospheres, with a focus on the plasma supply to each region of the magnetospheres. This review also includes the basic equations and modeling methods commonly used to simulate the plasma sources of the planetary magnetospheres. In this paper, we will describe basic and common processes related to plasma supply to each region of the planetary magnetospheres in our solar system. First, we will describe source processes in Sect. 1. Then the transport and energization processes to supply those source plasmas to various regions of the magnetosphere are described in Sect. 2. Loss processes are also important to understand the plasma population in the magnetosphere and Sect. 3 is dedicated to the explanation of the loss processes. In Sect. 4, we also briefly summarize the basic equations and modeling methods with a focus on plasma supply processes for planetary magnetospheres. (10.1007/s11214-015-0170-y)
DOI : 10.1007/s11214-015-0170-y
Direct identification of predator-prey dynamics in gyrokinetic simulations
- Kobayashi Sumire
- Gürcan Özgür D.
- Diamond P.H.
Physics of Plasmas, American Institute of Physics, 2015, 22 (9), pp.090702. The interaction between spontaneously formed zonal flows and small-scale turbulence in nonlinear gyrokinetic simulations is explored in a shearless closed field line geometry. It is found that when clear limit cycle oscillations prevail, the observed turbulent dynamics can be quantitatively captured by a simple Lotka-Volterra type predator-prey model. Fitting the time traces of full gyrokinetic simulations by such a reduced model allows extraction of the model coefficients. Scanning physical plasma parameters, such as collisionality and density gradient, it was observed that the effective growth rates of turbulence (i.e., the prey) remain roughly constant, in spite of the higher and varying level of primary mode linear growth rates. The effective growth rate that was extracted corresponds roughly to the zonal-flow-modified primary mode growth rate. It was also observed that the effective damping of zonal flows (i.e., the predator) in the parameter range, where clear predator-prey dynamics is observed, (i.e., near marginal stability) agrees with the collisional damping expected in these simulations. This implies that the Kelvin-Helmholtz-like instability may be negligible in this range. The results imply that when the tertiary instability plays a role, the dynamics becomes more complex than a simple Lotka-Volterra predator prey. (10.1063/1.4930127)
DOI : 10.1063/1.4930127
Predator-prey model for the self-organization of stochastic oscillators in dual populations.
- Moradi S.
- Anderson J.
- Gürcan Özgür D.
Physical Review E, American Physical Society (APS), 2015, 92, pp.06293. A predator-prey model of dual populations with stochastic oscillators is presented. A linear cross-coupling between the two populations is introduced following the coupling between the motions of a Wilberforce pendulum in two dimensions: one in the longitudinal and the other in torsional plain. Within each population a Kuramoto-type competition between the phases is assumed. Thus, the synchronization state of the whole system is controlled by these two types of competitions. The results of the numerical simulations show that by adding the linear cross-coupling interactions predator-prey oscillations between the two populations appear, which results in self-regulation of the system by a transfer of synchrony between the two populations. The model represents several important features of the dynamical interplay between the drift wave and zonal flow turbulence in magnetically confined plasmas, and a novel interpretation of the coupled dynamics of drift wave-zonal flow turbulence using synchronization of stochastic oscillator is discussed. (10.1103/PhysRevE.92.062930)
DOI : 10.1103/PhysRevE.92.062930
Turbulence elasticity: a key concept to a unified paradigm of L -> I -> H transition
- Guo Z. B.
- Diamond P.H.
- Kosuga Y.
- Gürcan Özgür D.
Nuclear Fusion, IOP Publishing, 2015, 55 (4), pp.043022. We present a theory of turbulence elasticity, which follows from delayed response of drift waves (DWs) to zonal flow (ZF) shears. It is shown that when |〈V〉'ZF|/Δωk ≥ 1, with |〈V〉'ZF| the ZF shearing rate and Δωk the local turbulence decorrelation rate, the ZF evolution equation is converted from a diffusion equation to a telegraph equation. This insight provides a natural framework for understanding temporally periodic ZF structures, e.g., propagation of the ZF/turbulence intensity fronts. Furthermore, by incorporating the elastic property of the DWZF turbulence, we propose a unified paradigm of low-confinement-mode to intermediate-confinement-mode to high-confinement-mode (L → I → H) transitions. In particular, we predict the onset and termination conditions of the limit cycle oscillations, i.e. the I-mode. The transition from an unstable L-mode to I-mode is predicted to occur when Δωk < |〈V〉'ZF|<〈V〉'cr, where 〈V〉'cr is a critical flow shearing rate and is derived explicitly. If |〈V〉'E×B| > 〈V〉'cr(〈V〉E×B is mean E × B shear flow driven by edge radial electrostatic field), the I-mode will terminate and spiral into the H-mode. (10.1088/0029-5515/55/4/043022)
DOI : 10.1088/0029-5515/55/4/043022
Asymmetric kinetic equilibria: Generalization of the BAS model for rotating magnetic profile and non-zero electric field
- Dorville Nicolas
- Belmont Gérard
- Aunai Nicolas
- Dargent Jérémy
- Rezeau Laurence
Physics of Plasmas, American Institute of Physics, 2015, 22 (9), pp.092904. Finding kinetic equilibria for non-collisional/collisionless tangential current layers is a key issue as well for their theoretical modeling as for our understanding of the processes that disturb them, such as tearing or Kelvin Helmholtz instabilities. The famous Harris equilibrium [E. Harris, Il Nuovo Cimento Ser. 10 23, 115121 (1962)] assumes drifting Maxwellian distributions for ions and electrons, with constant temperatures and flow velocities; these assumptions lead to symmetric layers surrounded by vacuum. This strongly particular kind of layer is not suited for the general case: asymmetric boundaries between two media with different plasmas and different magnetic fields. The standard method for constructing more general kinetic equilibria consists in using Jeans theorem, which says that any function depending only on the Hamiltonian constants of motion is a solution to the steady Vlasov equation [P. J. Channell, Phys. Fluids (19581988) 19, 1541 (1976); M. Roth et al., Space Sci. Rev. 76, 251317 (1996); and F. Mottez, Phys. Plasmas 10, 15411545 (2003)]. The inverse implication is however not true: when using the motion invariants as variables instead of the velocity components, the general stationary particle distributions keep on depending explicitly of the position, in addition to the implicit dependence introduced by these invariants. The standard approach therefore strongly restricts the class of solutions to the problem and probably does not select the most physically reasonable. The BAS (Belmont-Aunai-Smets) model [G. Belmont et al., Phys. Plasmas 19, 022108 (2012)] used for the first time the concept of particle accessibility to find new solutions: considering the case of a coplanar-antiparallel magnetic field configuration without electric field, asymmetric solutions could be found while the standard method can only lead to symmetric ones. These solutions were validated in a hybrid simulation [N. Aunai et al., Phys. Plasmas (1994-present) 20, 110702 (2013)], and more recently in a fully kinetic simulation as well [J. Dargent and N. Aunai, Phys. Plasmas (submitted)]. Nevertheless, in most asymmetric layers like the terrestrial magnetopause, one would indeed expect a magnetic field rotation from one direction to another without going through zero [J. Berchem and C. T. Russell, J. Geophys. Res. 87, 81398148 (1982)], and a non-zero normal electric field. In this paper, we propose the corresponding generalization: in the model presented, the profiles can be freely imposed for the magnetic field rotation (although restricted to a 180 rotation hitherto) and for the normal electric field. As it was done previously, the equilibrium is tested with a hybrid simulation. (10.1063/1.4930210)
DOI : 10.1063/1.4930210
Strong Ionization Asymmetry in a Geometrically Symmetric Radio Frequency Capacitively Coupled Plasma Induced by Sawtooth Voltage Waveforms
- Bruneau Bastien
- Gans T.
- O'Connell D.
- Greb Arthur
- Johnson E.V.
- Booth Jean-Paul
Physical Review Letters, American Physical Society, 2015, 114, pp.125002. The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode. (10.1103/PhysRevLett.114.125002)
DOI : 10.1103/PhysRevLett.114.125002
Wide-banded NTC radiation: local to remote observations by the four Cluster satellites
- Décréau Pierrette
- Aoutou S.
- Denazelle A.
- Galkina I.
- Rauch Jean-Louis
- Vallières Xavier
- Canu Patrick
- Rochel Grimald S.
- El-Lemdani Mazouz Farida
- Darrouzet F.
Annales Geophysicae, European Geosciences Union, 2015, 33 (10), pp.1285-1300. The Cluster multi-point mission offers a unique collection of non-thermal continuum (NTC) radio waves observed in the 2-80 kHz frequency range over almost 15 years, from various view points over the radiating plasmasphere. Here we present rather infrequent case events, such as when primary electrostatic sources of such waves are embedded within the plasmapause boundary far from the magnetic equatorial plane. The spectral signature of the emitted electromagnetic waves is structured as a series of wide harmonic bands within the range covered by the step in plasma frequency encountered at the boundary. Developing the concept that the frequency distance df between harmonic bands measures the magnetic field magnitude B at the source (df = Fce, electron gyrofrequency), we analyse three selected events. The first one (studied in Grimald et al., 2008) presents electric field signatures observed by a Cluster constellation of small size (~ 200 to 1000 km spacecraft separation) placed in the vicinity of sources. The electric field frequency spectra display frequency peaks placed at frequencies fs = n df (n being an integer), with df of the order of Fce values encountered at the plasmapause by the spacecraft. The second event, taken from the Cluster tilt campaign, leads to a 3-D view of NTC waves ray path orientations and to a localization of a global source region at several Earth radii (RE) from Cluster (Décréau et al., 2013). The measured spectra present successive peaks placed at fs ~ (n 1/2) df. Next, considering if both situations might be two facets of the same phenomenon, we analyze a third event. The Cluster fleet, configured into a constellation of large size (~ 8000 to 25 000 km spacecraft separation), allows us to observe wide-banded NTC waves at different distances from their sources. Two new findings can be derived from our analysis. First, we point out that a large portion of the plasmasphere boundary layer, covering a large range of magnetic latitudes, is radiating radio waves. The radio waves are issued from multiple sources of small size, each related to a given fs series and radiating inside a beam of narrow cone angle, referred to as a beamlet. The beamlets illuminate different satellites simultaneously, at different characteristic fs values, according to the latitude at which the satellite is placed. Second, when an observing satellite moves away from its assumed source region (the plasmapause surface), it is illuminated by several beamlets, issued from nearby sources with characteristic fs values close to each other. The addition of radio waves blurs the spectra of the overall received electric field. It can move the signal peaks such that their position fs satisfiesfs = (n alpha) df, with 0 < alpha < 1. These findings open new perspectives for the interpretation of NTC events displaying harmonic signatures. (10.5194/angeo-33-1285-2015)
DOI : 10.5194/angeo-33-1285-2015
Ignition of methane and n-butane containing mixtures at high pressures by pulsed nanosecond discharge
- Boumehdi M.A.
- Stepanyan S.A.
- Vanhove Guillaume
- Desgroux Pascale
- Starikovskaia Svetlana
Combustion and Flame, Elsevier, 2015, 162, pp.1336-1349. A novel experimental scheme to study the ignition of combustible mixtures at high pressures under the action of a high-voltage nanosecond discharge has been developed. The experiments were performed in the combustion chamber of a Rapid Compression Machine (RCM) with a specially designed system of electrodes. A nanosecond surface dielectric barrier discharge (SDBD) provided two-dimensional low-temperature non-equilibrium plasma in the vicinity of the end plate of the combustion chamber. Radially symmetric plasma channels triggered multi-point ignition of gas mixtures at controlled pressure and temperature. Ignition delay times and energies deposited in the gaseous mixtures by the discharge were measured for different parameters of high voltage pulse, for positive or negative high-voltage pulses. The propagation of the subsequent flame in the combustion chamber was recorded with the help of high repetition rate imaging. Preliminary numerical analysis of the ignition under the action of a pulsed nanosecond discharge has been made; it was shown that production of atomic oxygen by the discharge, will modify the ignition chemistry by perturbation of the radical pool. Experiments and calculations were performed in methaneoxygen and n-butaneoxygen mixtures with equivalence ratios between 0.3 and 1 diluted by 7076% of Ar or nitrogen for temperatures between 600 and 1000 K and pressures between 6 and 16 bar. (10.1016/j.combustflame.2014.11.006)
DOI : 10.1016/j.combustflame.2014.11.006
Electron-less negative ion extraction from ion-ion plasmas
- Rafalskyi D.V.
- Aanesland Ane
Applied Physics Letters, American Institute of Physics, 2015, 106 (10), pp.104101. This paper presents experimental results showing that continuous negative ion extraction, without co-extracted electrons, is possible from highly electronegative SF6 ion-ion plasma at low gas pressure (1 mTorr). The ratio between the negative ion and electron densities is more than 3000 in the vicinity of the two-grid extraction and acceleration system. The measurements are conducted by both magnetized and non-magnetized energy analyzers attached to the external grid. With these two analyzers, we show that the extracted negative ion flux is almost electron-free and has the same magnitude as the positive ion flux extracted and accelerated when the grids are biased oppositely. The results presented here can be used for validation of numerical and analytical models of ion extraction from ion-ion plasma. (10.1063/1.4914507)
DOI : 10.1063/1.4914507
Magnetic Reconnection in Different Environments
- Jackman C. M.
- Delamere Peter A.
- Hesse Michael
- Aunai N.
- Kuznetsova M. M.
- Zenitani Seiji
- Birn Joachim
, 2015, 207, pp.259-267. This chapter addresses two important aspects of magnetic reconnection: time-dependent rates affected by island formation and the preferred direction of the reconnection line if merging magnetic field components are not anti-parallel. While it is widely known that magnetic reconnection facilitates mass, momentum, and energy transport in plasmas, it is illustrative to consider an analytical approach to describing the efficacy of reconnection as a transport mechanism. The chapter performs a detailed analysis of the correlation between diffusion region dimensions and reconnection rate. For this purpose, it employs open boundary condition calculations of a continuously driven reconnecting system. In summary, results indicate that the magnetic reconnection line in asymmetric systems is preferentially oriented in such a way that it bisects the direction of the asymptotic magnetic field direction on both inflow sides. This orientation is identical to the one for which the product of available magnetic energy is maximized.
Kinetic scale solar wind turbulence: Landau-fluid simulations and spacecraft observations
- Sahraoui Fouad
, 2015.
Multipoint observations of plasma phenomena made in space by Cluster
- Goldstein M. L.
- Escoubet P.
- Hwang K.-J.
- Wendel D. E.
- Viñas A.-F.
- Fung S. F.
- Perri S.
- Servidio S.
- Pickett J. S.
- Parks G. K.
- Sahraoui Fouad
- Gurgiolo C.
- Matthaeus W.
- Weygand J. M.
Journal of Plasma Physics, Cambridge University Press (CUP), 2015, 81 (3), pp.325810301. Plasmas are ubiquitous in nature, surround our local geospace environment, and permeate the universe. Plasma phenomena in space give rise to energetic particles, the aurora, solar flares and coronal mass ejections, as well as many energetic phenomena in interstellar space. Although plasmas can be studied in laboratory settings, it is often difficult, if not impossible, to replicate the conditions (density, temperature, magnetic and electric fields, etc.) of space. Single-point space missions too numerous to list have described many properties of near-Earth and heliospheric plasmas as measured both in situ and remotely (see http://www.nasa.gov/missions/#.U1mcVmeweRY for a list of NASA-related missions). However, a full description of our plasma environment requires three-dimensional spatial measurements. Cluster is the first, and until data begin flowing from the Magnetospheric Multiscale Mission (MMS), the only mission designed to describe the three-dimensional spatial structure of plasma phenomena in geospace. In this paper, we concentrate on some of the many plasma phenomena that have been studied using data from Cluster. To date, there have been more than 2000 refereed papers published using Cluster data but in this paper we will, of necessity, refer to only a small fraction of the published work. We have focused on a few basic plasma phenomena, but, for example, have not dealt with most of the vast body of work describing dynamical phenomena in Earth's magnetosphere, including the dynamics of current sheets in Earth's magnetotail and the morphology of the dayside high latitude cusp. Several review articles and special publications are available that describe aspects of that research in detail and interested readers are referred to them (see for example, Escoubet et al. 2005Multiscale Coupling of Sun-Earth Processes, p. 459, Keith et al. 2005Sur. Geophys.26, 307339, Paschmann et al. 2005Outer Magnetospheric Boundaries: Cluster Results, Space Sciences Series of ISSI. Berlin: Springer, Goldstein et al. 2006Adv. Space Res.38, 2136, Taylor et al. 2010The Cluster Mission: Space Plasma in Three Dimensions, Springer, pp. 309330 and Escoubet et al. 2013Ann. Geophys.31, 10451059). (10.1017/S0022377815000185)
DOI : 10.1017/S0022377815000185
Intensities and spatiotemporal variability of equatorial noise emissions observed by the Cluster spacecraft
- Nemec F.
- Santolík O.
- Hrbackova Z.
- Cornilleau-Wehrlin Nicole
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.1620-1632. Equatorial noise (EN) emissions are electromagnetic waves observed in the equatorial region of the inner magnetosphere at frequencies between the proton cyclotron frequency and the lower hybrid frequency. We present the analysis of 2229 EN events identified in the Spatio-Temporal Analysis of Field Fluctuations (STAFF) experiment data of the Cluster spacecraft during the years 2001-2010. EN emissions are distinguished using the polarization analysis, and their intensity is determined based on the evaluation of the Poynting flux rather than on the evaluation of only the electric/magnetic field intensity. The intensity of EN events is analyzed as a function of the frequency, the position of the spacecraft inside/outside the plasmasphere, magnetic local time, and the geomagnetic activity. The emissions have higher frequencies and are more intense in the plasma trough than in the plasmasphere. EN events observed in the plasma trough are most intense close to the local noon, while EN events observed in the plasmasphere are nearly independent on magnetic local time (MLT). The intensity of EN events is enhanced during disturbed periods, both inside the plasmasphere and in the plasma trough. Observations of the same events by several Cluster spacecraft allow us to estimate their spatiotemporal variability. EN emissions observed in the plasmasphere do not change on the analyzed spatial scales (DeltaMLT<0.2h, Deltar<0.2 RE), but they change significantly on time scales of about an hour. The same appears to be the case also for EN events observed in the plasma trough, although the plasma trough dependencies are less clear. (10.1002/2014JA020814)
DOI : 10.1002/2014JA020814
Experimental determination of the dispersion relation of magnetosonic waves
- Balikhin M. A.
- Shklyar D. R.
- Yearby K. H.
- Canu Patrick
- Carr C. M.
- Dandouras I.
- Walker S. N.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120, pp.9632-9650. Magnetosonic waves are commonly observed in the vicinity of the terrestrial magnetic equator. It has been proposed that within this region they may interact with radiation belt electrons, accelerating some to high energies. These wave-particle interactions depend upon the characteristic properties of the wave mode. Hence, determination of the wave properties is a fundamental part of understanding these interaction processes. Using data collected during the Cluster Inner Magnetosphere Campaign, this paper identifies an occurrence of magnetosonic waves, discusses their generation and propagation properties from a theoretical perspective, and utilizes multispacecraft measurements to experimentally determine their dispersion relation. Their experimental dispersion is found to be in accordance with that based on cold plasma theory. (10.1002/2015JA021746)
DOI : 10.1002/2015JA021746
TALIF measurements of oxygen atom density in the afterglow of a capillary nanosecond discharge
- Klochko A.V.
- Lemainque J.
- Booth Jean-Paul
- Starikovskaia Svetlana
Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (2), pp.025010. The atomic oxygen density has been measured in the afterglow of a capillary nanosecond discharge in 2430 mbar synthetic air (N 2  : O 2 = 4 : 1) by the two-photon absorption laser-induced fluorescence (TALIF) technique, combined with absolute calibration by comparison with xenon TALIF. The discharge was initiated by a train of 30 ns FWHM pulses of alternating positivenegativepositive polarity, separated by 250 ns, with a train repetition frequency of 10 Hz. The amplitude of the first pulse was 10 kV in the cable. A flow of synthetic air through the tube provided complete gas renewal between pulse trains. The O-atom density measurements were made over the time interval200 ns2 µ s after the initial pulse. The gas temperature was determined by analysis of the molecular nitrogen second positive system optical emission spectrum. The influence of the gas temperature on the atom density measurements, and the reactions producing O atoms, are discussed. (10.1088/0963-0252/24/2/025010)
DOI : 10.1088/0963-0252/24/2/025010
Control and optimization of the slope asymmetry effect in tailored voltage waveforms for capacitively coupled plasmas
- Bruneau Bastien
- Novikova T.
- Lafleur Trevor
- Booth Jean-Paul
- Johnson E.V.
Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (1), pp.015021. Through the use of particle-in-cell simulations, we study the ion flux asymmetry in an argon discharge that is induced by a ?sawtooth-like? excitation voltage waveform. In a previous article we have shown that, due to their differing rising and falling slopes, these waveforms can create a plasma with a significantly higher ion flux to one electrode in a geometrically symmetric reactor. Furthermore, they have the unique property of providing a lower ion energy at the electrode with a higher ion flux. In the present work, we show that a refined waveform allows the ion flux asymmetry to be increased for a given number of harmonics by reducing the ionization rate in front of the low-flux electrode. The flux asymmetry is found to disappear at low pressure due to the increased electron energy transport, which causes a transition from sheath edge ionization to bulk ionization. Changing the fundamental frequency is shown to have two counterbalancing effects: reducing the ionization on the low ion-flux electrode and shifting the maximum ionization to the center of the discharge. Under the representative conditions that we have studied, a maximum asymmetry is found for a base frequency of 3.4 MHz. Finally, it is shown that, by adjusting the rise- to fall-time ratio of the refined waveforms, the ion-flux asymmetry can be continuously shifted from one electrode to the other. (10.1088/0963-0252/24/1/015021)
DOI : 10.1088/0963-0252/24/1/015021
Weak magnetohydrodynamic turbulence and intermittency
- Meyrand Romain
- Kiyani K. H.
- Galtier Sébastien
Journal of Fluid Mechanics, Cambridge University Press (CUP), 2015, 770. Three-dimensional numerical simulation is used to investigate intermittency in incompressible weak magnetohydrodynamic turbulence with a strong uniform magnetic field and zero cross-helicity. At leading order, this asymptotic regime is achieved via three-wave resonant interactions with the scattering of a wave on a 2D mode for which . When the interactions with the 2D modes are artificially reduced, we show numerically that the system exhibits an energy spectrum with , whereas the expected exact solution with is recovered with the full nonlinear system. In the latter case, strong intermittency is found when the vector separation of structure functions is taken transverse to . This result may be explained by the influence of the 2D modes whose regime belongs to strong turbulence. In addition to shedding light on the origin of this intermittency, we derive a log-Poisson law, , which fits the data perfectly and highlights the important role of parallel current sheets. (10.1017/jfm.2015.141)
DOI : 10.1017/jfm.2015.141

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