Publications

2015

• Geodesic acoustic modes in a fluid model of tokamak plasma: the effects of finite beta and collisionality
  
  • Singh Rameswar
  • Storelli A.
  • Gürcan Özgür D.
  • Hennequin Pascale
  • Vermare Laure
  • Morel Pierre
  • Singh R.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2015, 57, pp.125002. Starting from the Braginskii equations, relevant for the tokamak edge region, a complete set of nonlinear equations for the geodesic acoustic modes (GAM) has been derived which includes collisionality, plasma beta and external sources of particle, momentum and heat. Local linear analysis shows that the GAM frequency increases with collisionality at low radial wave number and decreases at high . GAM frequency also decreases with plasma beta. Radial profiles of GAM frequency for two Tore Supra shots, which were part of a collisionality scan, are compared with these calculations. A discrepancy between experiment and theory is observed, which seems to be explained by a finite for the GAM when flux surface averaged density \langle n\rangle and temperature are assumed to vanish. It is shown that this agreement is incidental and self-consistent inclusion of and responses enhances the disagreement more with at high . So the discrepancy between the linear GAM calculation and experiment, (which also persist for more 'complete' linear models such as gyrokinetics) can probably not be resolved by simply adding a finite . (10.1088/0741-3335/57/12/125002)
  DOI : 10.1088/0741-3335/57/12/125002
• Plasma acceleration using a radio frequency self-bias effect
  
  • Rafalskyi D.V.
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2015, 22 (6), pp.063502. In this work plasma acceleration using a RF self-bias effect is experimentally studied. The experiments are conducted using a novel plasma accelerator system, called Neptune, consisting of an inductively coupled plasma source and a RF-biased set of grids. The plasma accelerator can operate in a steady state mode, producing a plasma flow with separately controlled plasma flux and velocity without any magnetic configuration. The operating pressure at the source output is as low as 0.2 mTorr and can further be decreased. The ion and electron flows are investigated by measuring the ion and electron energy distribution functions both space resolved and with different orientations with respect to the flow direction. It is found that the flow of electrons from the source is highly anisotropic and directed along the ion flow and this global flow of accelerated plasma is well localized in the plasma transport chamber. The maximum flux is about 7.5·1015 ions s−1 m−2 (at standard conditions) on the axis and decreasing to almost zero at a radial distances of more than 15 cm from the flow axis. Varying the RF acceleration voltage in the range 20350 V, the plasma flow velocity can be changed between 10 and 35 km/s. The system is prospective for different technology such as space propulsion and surface modification and also interesting for fundamental studies for space-related plasma simulations and investigation of the dynamo effect using accelerated rotating plasmas. I. INTRODUCT (10.1063/1.4922065)
  DOI : 10.1063/1.4922065
• Numerical experiment to estimate the validity of negative ion diagnostic using photo-detachment combined with Langmuir probing
  
  • Oudini N.
  • Sirse Nishant
  • Benallal R.
  • Taccogna F.
  • Aanesland Ane
  • Bendib A.
  • Ellingboe A. R.
  Physics of Plasmas, American Institute of Physics, 2015, 22 (7), pp.073509. This paper presents a critical assessment of the theory of photo-detachment diagnostic method used to probe the negative ion density and electronegativity α = n-/ne. In this method, a laser pulse is used to photo-detach all negative ions located within the electropositive channel (laser spot region). The negative ion density is estimated based on the assumption that the increase of the current collected by an electrostatic probe biased positively to the plasma is a result of only the creation of photo-detached electrons. In parallel, the background electron density and temperature are considered as constants during this diagnostics. While the numerical experiments performed here show that the background electron density and temperature increase due to the formation of an electrostatic potential barrier around the electropositive channel. The time scale of potential barrier rise is about 2 ns, which is comparable to the time required to completely photo-detach the negative ions in the electropositive channel (∼3 ns). We find that neglecting the effect of the potential barrier on the background plasma leads to an erroneous determination of the negative ion density. Moreover, the background electron velocity distribution function within the electropositive channel is not Maxwellian. This is due to the acceleration of these electrons through the electrostatic potential barrier. In this work, the validity of the photo-detachment diagnostic assumptions is questioned and our results illustrate the weakness of these assumptions. (10.1063/1.4926826)
  DOI : 10.1063/1.4926826
• Reply to Comment on "A review on ion-ion plasmas created in weakly magnetized electronegative plasmas
  
  • Aanesland Ane
  • Bredin Jérôme
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (3), pp.038002. This is a reply to the comments made by Bogdanov et al on our paper entitled ?A review on ion?ion plasmas created in weakly magnetized electronegative plasmas?. We have clarified some issues regarding the electronegative plasma stratification and the definition of an ion?ion plasma, and we discuss the issue of extracting negative ions from a steady-state ion?ion plasma formed at the periphery or downstream a magnetized electronegative plasma. (10.1088/0963-0252/24/3/038002)
  DOI : 10.1088/0963-0252/24/3/038002
• Magnetic reconnection in th Solar System
  
  • Aunai N.
  , 2015.
• Observations of discrete harmonics emerging from equatorial noise
  
  • Balikhin M. A.
  • Shprits Y. Y.
  • Walker S. N.
  • Chen Lunjin
  • Cornilleau-Wehrlin Nicole
  • Dandouras Iannis
  • Santolík O.
  • Carr Christopher
  • Yearby K. H.
  • Weiss Benjamin
  Nature Communications, Nature Publishing Group, 2015, 6. A number of modes of oscillations of particles and fields can exist in space plasmas. Since the early 1970s, space missions have observed noise-like plasma waves near the geomagnetic equator known as `equatorial noise'. Several theories were suggested, but clear observational evidence supported by realistic modelling has not been provided. Here we report on observations by the Cluster mission that clearly show the highly structured and periodic pattern of these waves. Very narrow-banded emissions at frequencies corresponding to exact multiples of the proton gyrofrequency (frequency of gyration around the field line) from the 17th up to the 30th harmonic are observed, indicating that these waves are generated by the proton distributions. Simultaneously with these coherent periodic structures in waves, the Cluster spacecraft observes `ring' distributions of protons in velocity space that provide the free energy for the waves. Calculated wave growth based on ion distributions shows a very similar pattern to the observations. (10.1038/ncomms8703)
  DOI : 10.1038/ncomms8703
• Finding the Elusive E × B Staircase in Magnetized Plasmas
  
  • Dif-Pradalier Guilhem
  • Hornung G
  • Ghendrih Philippe
  • Sarazin Yanick
  • Clairet F
  • Vermare L
  • Diamond P H
  • Abiteboul J
  • Cartier-Michaud T
  • Ehrlacher C
  • Estève Daniel
  • Garbet Xavier
  • Grandgirard Virginie
  • Gürcan Özgür D.
  • Hennequin P
  • Kosuga Y
  • Latu Guillaume
  • Maget P
  • Morel Pierre
  • Norscini C
  • Sabot R
  • Storelli A.
  Physical Review Letters, American Physical Society, 2015, 114, pp.085004. Turbulence in hot magnetized plasmas is shown to generate permeable localized transport barriers that globally organize into the so-called "ExB staircase" [G. Dif-Pradalier et al., Phys. Rev. E, 82, 025401(R) (2010)]. Its domain of existence and dependence with key plasma parameters is discussed theoretically. Based on these predictions, staircases are observed experimentally in the Tore Supra tokamak by means of high-resolution fast-sweeping X-mode reflectometry. This observation strongly emphasizes the critical role of mesoscale self-organization in plasma turbulence and may have far-reaching consequences for turbulent transport models and their validation. (10.1103/PhysRevLett.114.085004)
  DOI : 10.1103/PhysRevLett.114.085004
• Imprints of Expansion on the Local Anisotropy of Solar Wind Turbulence
  
  • Verdini Andrea
  • Grappin Roland
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2015, 808, pp.L34. We study the anisotropy of II-order structure functions (SFs) defined in a frame attached to the local mean field in three-dimensional (3D) direct numerical simulations of magnetohydrodynamic turbulence, with the solar wind expansion both included and not included. We simulate spacecraft flybys through the numerical domain by taking increments along the radial (wind) direction that form an angle of 45° with the ambient magnetic field. We find that only when expansion is taken into account do the synthetic observations match the 3D anisotropy observed in the solar wind, including the change of anisotropy with scale. Our simulations also show that the anisotropy changes dramatically when considering increments oblique to the radial directions. Both results can be understood by noting that expansion reduces the radial component of the magnetic field at all scales, thus confining fluctuations in the plane perpendicular to the radial. Expansion is thus shown to affect not only the (global) spectral anisotropy, but also the local anisotropy of second-order SF by influencing the distribution of the local mean field, which enters this higher-order statistics. (10.1088/2041-8205/808/2/L34)
  DOI : 10.1088/2041-8205/808/2/L34
• Comprehensive comparisons of geodesic acoustic mode characteristics and dynamics between Tore Supra experiments and gyrokinetic simulations
  
  • Storelli A.
  • Vermare Laure
  • Hennequin Pascale
  • Gürcan Özgür D.
  • Dif-Pradalier Guilhem
  • Sarazin Y.
  • Garbet X.
  • Görler T.
  • Singh Rameswar
  • Morel Pierre
  • Grandgirard Virginie
  • Ghendrih Philippe
  • Tore Supra Team
  Physics of Plasmas, American Institute of Physics, 2015, 22 (6). In a dedicated collisionality scan in Tore Supra, the geodesic acoustic mode (GAM) is detected and identified with the Doppler backscattering technique. Observations are compared to the results of a simulation with the gyrokinetic code GYSELA. We found that the GAM frequency in experiments is lower than predicted by simulation and theory. Moreover, the disagreement is higher in the low collisionality scenario. Bursts of non harmonic GAM oscillations have been characterized with filtering techniques, such as the Hilbert-Huang transform. When comparing this dynamical behaviour between experiments and simulation, the probability density function of GAM amplitude and the burst autocorrelation time are found to be remarkably similar. In the simulation, where the radial profile of GAM frequency is continuous, we observed a phenomenon of radial phase mixing of the GAM oscillations, which could influence the burst autocorrelation time. (10.1063/1.4922845)
  DOI : 10.1063/1.4922845
• Experimental turbulence studies for gyro-kinetic code validation using advanced microwave diagnostics
  
  • Stroth U.
  • Bañón Navarro A.
  • Conway G. D.
  • Görler T.
  • Happel T.
  • Hennequin Pascale
  • Lechte C.
  • Manz P.
  • Simon P.
  • Biancalani A.
  • Blanco E.
  • Bottereau C.
  • Clairet F.
  • Coda S.
  • Eibert T.
  • Estrada T.
  • Fasoli A.
  • Guimarais L.
  • Gürcan Özgür D.
  • Huang Z.
  • Jenko F.
  • Kasparek W.
  • Koenen C.
  • Krämer-Flecken A.
  • Manso M.-E.
  • Medvedeva A.
  • Molina D.
  • Nikolaeva V.
  • Plaum B.
  • Porte L.
  • Prisiazhniuk D.
  • Ribeiro T.
  • Scott B.D.
  • Siart U.
  • Storelli A.
  • Vermare Laure
  • Wolf S.
  Nuclear Fusion, IOP Publishing, 2015, 55 (8), pp.083027. For a comprehensive comparison with theoretical models and advanced numerical turbulence simulations, a large spectrum of fluctuation parameters was measured on the devices ASDEX Upgrade, TCV, and Tore-Supra. Radial profiles of scale-resolved turbulence levels in H-mode discharges are measured and compared with GENE simulations in the transition range from ion-temperature-gradient to trapped-electron-mode turbulence. Correlation reflectometry is used to study the microscopic structure of turbulence and GAMs in discharges where poloidal flow damping was varied by means of variations of the shape of the poloidal plasma cross-section and collisionality. Full-wave codes and synthetic diagnostics are applied for the interpretation of the data. (10.1088/0029-5515/55/8/083027)
  DOI : 10.1088/0029-5515/55/8/083027
• Some statistical equilibrium mechanics and stability properties of a class of two-dimensional Hamiltonian mean-field models
  
  • Maciel J. M.
  • Firpo Marie-Christine
  • Amato M. A.
  Physica A: Statistical Mechanics and its Applications, Elsevier, 2015, 424, pp.34-43. Abstract A two-dimensional class of mean-field models that may serve as a minimal model to study the properties of long-range systems in two space dimensions is considered. The statistical equilibrium mechanics is derived in the microcanonical ensemble using Monte Carlo simulations for different combinations of the coupling constants in the potential leading to fully repulsive, fully attractive and mixed attractive?repulsive potential along the Cartesian axis and diagonals. Then, having in mind potential realizations of long-range systems using cold atoms, the linear theory of this two-dimensional mean-field Hamiltonian models is derived in the low temperature limit. (10.1016/j.physa.2014.12.030)
  DOI : 10.1016/j.physa.2014.12.030
• Towards an emerging understanding of non-locality phenomena and non-local transport
  
  • Ida K.
  • Shi Z.
  • Sun H. J.
  • Inagaki S.
  • Kamiya K.
  • Rice J.E.
  • Tamura N.
  • Diamond P.H.
  • Dif-Pradalier Guilhem
  • Zou X. L.
  • Itoh K.
  • Sugita S.
  • Gürcan Özgür D.
  • Estrada T.
  • Hidalgo C.
  • Hahm T.S.
  • Field A.
  • Ding X. T.
  • Sakamoto Y.
  • Oldenburger S.
  • Yoshinuma M.
  • Kobayashi T.
  • Jiang M.
  • Hahn S. -H.
  • Jeon Y. M.
  • Hong S. H.
  • Kosuga Y.
  • Dong J.
  • Itoh S. I.
  Nuclear Fusion, IOP Publishing, 2015, 55 (1), pp.013022. In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of 'fast front' and 'intrinsically non-local' models, and their success in comparisons with experimental data, are discussed (10.1088/0029-5515/55/1/013022)
  DOI : 10.1088/0029-5515/55/1/013022
• Different types of whistler mode chorus in the equatorial source region
  
  • Taubenschuss U.
  • Santolík O.
  • Graham Daniel B.
  • Fu H.S.
  • Khotyaintsev Y. V.
  • Le Contel Olivier
  Geophysical Research Letters, American Geophysical Union, 2015, 42 (20), pp.8271-8279. The Time History of Events and Macroscale Interactions during Substorms-D spacecraft crossed an active equatorial source region of whistler mode chorus rising tones on 23 October 2008. Rising tones are analyzed in terms of spectral and polarization characteristics, with special emphasis on wave normal angles. The latter exhibit large variations from quasi-parallel to oblique, even within single bursts, but seem to follow a definite pattern, which enables an unambiguous classification into five different groups. Furthermore, we discuss the frequently observed splitting of chorus bursts into a lower and an upper band around one half of the local electron cyclotron frequency. At chorus frequencies close to the gap, we find significantly lowered wave planarities and a tendency of wave normal angles to approach the Gendrin angle. (10.1002/2015GL066004)
  DOI : 10.1002/2015GL066004
• Entanglement of helicity and energy in kinetic Alfvén wave/whistler turbulence
  
  • Galtier Sébastien
  • Meyrand Romain
  Journal of Plasma Physics, Cambridge University Press (CUP), 2015, 81 (1), pp.325810106. The role of magnetic helicity is investigated in kinetic Alfvén wave and oblique whistler turbulence in presence of a relatively intense external magnetic field b <SUB>0</SUB> e <SUB>||</SUB>. In this situation, turbulence is strongly anisotropic and the fluid equations describing both regimes are the reduced electron magnetohydrodynamics (REMHD) whose derivation, originally made from the gyrokinetic theory, is also obtained here from compressible Hall magnetohydrodynamics (MHD). We use the asymptotic equations derived by Galtier and Bhattacharjee (2003 Phys. Plasmas 10, 3065-3076) to study the REMHD dynamics in the weak turbulence regime. The analysis is focused on the magnetic helicity equation for which we obtain the exact solutions: they correspond to the entanglement relation, n ñ = -6, where n and ñ are the power law indices of the perpendicular (to b <SUB>0</SUB>) wave number magnetic energy and helicity spectra, respectively. Therefore, the spectra derived in the past from the energy equation only, namely n = -2.5 and ñ = -3.5, are not the unique solutions to this problem but rather characterize the direct energy cascade. The solution ñ = -3 is a limit imposed by the locality condition; it is also the constant helicity flux solution obtained heuristically. The results obtained offer a new paradigm to understand solar wind turbulence at sub-ion scales where it is often observed that -3 < n < -2.5. (10.1017/S0022377814000774)
  DOI : 10.1017/S0022377814000774
• Thin Current Sheets and Associated Electron Heating in Turbulent Space Plasma
  
  • Chasapis A.
  • Retinò Alessandro
  • Sahraoui Fouad
  • Vaivads A.
  • Khotyaintsev Y. V.
  • Sundkvist D.
  • Greco A.
  • Sorriso-Valvo L.
  • Canu Patrick
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2015, 804 (1). Intermittent structures, such as thin current sheets, are abundant in turbulent plasmas. Numerical simulations indicate that such current sheets are important sites of energy dissipation and particle heating occurring at kinetic scales. However, direct evidence of dissipation and associated heating within current sheets is scarce. Here, we show a new statistical study of local electron heating within proton-scale current sheets by using high-resolution spacecraft data. Current sheets are detected using the Partial Variance of Increments (PVI) method which identifies regions of strong intermittency. We find that strong electron heating occurs in high PVI (>3) current sheets while no significant heating occurs in low PVI cases (<3), indicating that the former are dominant for energy dissipation. Current sheets corresponding to very high PVI (>5) show the strongest heating and most of the time are consistent with ongoing magnetic reconnection. This suggests that reconnection is important for electron heating and dissipation at kinetic scales in turbulent plasmas. (10.1088/2041-8205/804/1/L1)
  DOI : 10.1088/2041-8205/804/1/L1
• 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 R<SUB>E</SUB>), 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
• 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.
• Chorus intensity modulation driven by time-varying field-aligned low-energy plasma
  
  • Nishimura Y.
  • Bortnik J.
  • Li W.
  • Liang Jun
  • Thorne R. M.
  • Angelopoulos V.
  • Le Contel Olivier
  • Auster U.
  • Bonnell J. W.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (9), pp.7433-7446. Recent studies have shown that chorus waves are responsible for scattering and precipitating the energetic electrons that drive the pulsating aurora. While some of the chorus intensity modulation events are correlated with <~100 eV electron density modulation, most of the chorus intensity modulation events in the postmidnight sector occur without apparent density changes. Although it is generally difficult to measure evolution of low-energy (<~20 eV) electron fluxes due to constraints imposed by the spacecraft potential and electrostatic analyzer (ESA) energy range limit, we identified using Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite data that low-energy ions of ~100 eV show density modulation that is correlated with chorus intensity modulation. Those low-energy ions and electrons are field-aligned with major peaks in 0° (for northern hemisphere winter event) and 180° (for northern hemisphere summer event) pitch angle, indicating that outflowing plasma from the sunlit hemisphere is the source of the low-energy plasma density modulation near the equator. Plasma sheet plasma density, and ambient electric and magnetic fields do not show modulations that are correlated with the chorus intensity modulation. Assuming charge neutrality, the low-energy ions can be used to represent cold plasma density in wave growth rate calculations, and the enhancements of the low-energy plasma density are found to contribute most effectively to chorus linear growth rates. These results suggest that chorus intensity modulation is driven by a feedback process where outflowing plasma due to energetic electron precipitation increases the equatorial density that drives further electron precipitation. (10.1002/2015JA021330)
  DOI : 10.1002/2015JA021330
• 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
• 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
• Investigation of drift velocity effects on the EDGE and SOL transport
  
  • Leybros R.
  • Bufferand Hugo
  • Ciraolo Guido
  • Fedorczak Nicolas
  • Ghendrih Philippe
  • Hennequin Pascale
  • Marandet Yannick
  • Serre Eric
  • Schwander Frédéric
  • Tamain Patrick
  Journal of Nuclear Materials, Elsevier, 2015, 463, pp.489-492. To understand the mechanisms behind poloidal asymmetries of the transport in the edge and SOL plasma, it is important to take into account drift velocity in the transport model. We investigate the effects of an imposed radial electric field on the plasma equilibrium in the transport code SOLEDGE2D. In the edge, we show an important modification of the flow pattern due to poloidal E × B drift velocity. The drift velocity generates asymmetry of the density through the Pfirsch–Schluter flows which creates an important parallel rotation through the viscous balance. In comparison to heat load imbalance studies in the SOL of divertor tokamak, a strong link between the amplitude of the radial electric field and the heat load imbalance in the SOL of limiter tokamak has been highlighted using different amplitude of the imposed radial electric field. (10.1016/j.jnucmat.2014.10.079)
  DOI : 10.1016/j.jnucmat.2014.10.079
• 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
• Highly vibrationally excited O<SUB>2</SUB> molecules in low-pressure inductively-coupled plasmas detected by high sensitivity ultra-broad-band optical absorption spectroscopy
  
  • Foucher Mickaël
  • Marinov Daniil
  • Carbone Emile
  • Chabert Pascal
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2015, 24 (4), pp.042001. Inductively-coupled plasmas in pure O 2 (at pressures of 5?80?mTorr and radiofrequency power up to 500?W) were studied by optical absorption spectroscopy over the spectral range 200?450?nm, showing the presence of highly vibrationally excited O 2 molecules (up to v? = 18) by Schumann?Runge band absorption. Analysis of the relative band intensities indicates a vibrational temperature up to 10,000?K, but these hot molecules only represent a fraction of the total O 2 density. By analysing the (11-0) band at higher spectral resolution the O 2 rotational temperature was also determined, and was found to increase with both pressure and power, reaching 900?K at 80?mTorr 500?W. These measurements were achieved using a new high-sensitivity ultra-broad-band absorption spectroscopy setup, based on a laser-plasma light source, achromatic optics and an aberration-corrected spectrograph. This setup allows the measurement of weak broadband absorbances due to a baseline variability lower than 2???×???10 ?5 across a spectral range of 250?nm. (10.1088/0963-0252/24/4/042001)
  DOI : 10.1088/0963-0252/24/4/042001
• 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
• Kinetic scale solar wind turbulence: Landau-fluid simulations and spacecraft observations
  
  • Sahraoui Fouad
  , 2015.