Publications

2015

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
The impingement of a kHz helium atmospheric pressure plasma jet on a dielectric surface
- Guaitella Olivier
- Sobota Ana
Journal of Physics D: Applied Physics, IOP Publishing, 2015, 48 (25), pp.255202. A parametric study of the impingement of a helium kHz atmospheric pressure plasma jet on a flat glass surface was performed by means of time-resolved intensified charge-coupled device imaging. The development of the plasma on the target is linked to the plasma evolution in the source and governed by the power supply. The glass surface takes part in the elongation of the plasma jet by the virtue of two mechanisms: the local enhancement of the electric field and the supply of pre-deposited charge. The evidence for the pre-deposited charge is the formation of a sheath on the glass surface, and the faint discharge formed on the glass surface during the negative voltage slope starting at the maximum of the negative current peak. The influence of the gas flow dynamics taking into account various gas flows, incident angles and distances is more important for the behaviour of the discharge on the surface than the voltage amplitude or the geometry of the source. The capacitance of the target strongly modifies the interaction with the plasma jet and increases the deposited surface charge density, featuring a streamer-like propagation mechanism in the case of high electric field enhancement at the surface. (10.1088/0022-3727/48/25/255202)
DOI : 10.1088/0022-3727/48/25/255202
Matched dipole probe for magnetized low electron density laboratory plasma diagnostics
- Rafalskyi D.V.
- Aanesland Ane
Physics of Plasmas, American Institute of Physics, 2015, 22 (7), pp.073504. In this paper, a diagnostic method for magnetized and unmagnetized laboratory plasma is proposed, based on impedance measurements of a short matched dipole. The range of the measured electron densities is limited to low density plasmas (10121015 m−3), where other diagnostic methods have strong limitations on the magnetic field strength and topology, plasma dimensions, and boundary conditions. The method is designed for use in both large- and small-dimension plasma (<10 cm) without or with strong non-homogeneous magnetic field, which can be undefined within the probe size. The design of a matched dipole probe allows to suppress the sheath resonance effects and to reach high sensitivity at relatively small probe dimensions. Validation experiments are conducted in both magnetized (B ∼ 170 G) and unmagnetized (B = 0) low density (7 × 1012 m−37 × 1013 m−3) low pressure (1 mTorr) 10 cm scale plasmas. The experimentally measured data show very good agreement with an analytical theory both for a non-magnetized and a magnetized case. The electron density measured by the matched dipole and Langmuir probes in the range of 7 × 1012 m−37 × 1013 m−3 show less than 30% difference. An experimentally measured tolerance/uncertainty of the dipole probe method is estimated to ±1% for plasma densities above 2 × 1013 m−3. A spatial resolution is estimated from the experiments to be about 3d, where d is the dipole diameter. The diagnostic method is also validated by comparing the measured plasma impedance curves with results of analytical modelling. (10.1063/1.4926447)
DOI : 10.1063/1.4926447
Kinetic scale turbulence and dissipation in the solar wind: key observational results and future outlook
- Goldstein M. L.
- Wicks R. T.
- Perri S.
- Sahraoui Fouad
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Royal Society, The, 2015, 373, pp.20140147. Turbulence is ubiquitous in the solar wind. Turbulence causes kinetic and magnetic energy to cascade to small scales where they are eventually dissipated, adding heat to the plasma. The details of how this occurs are not well understood. This article reviews the evidence for turbulent dissipation and examines various diagnostics for identifying solar wind regions where dissipation is occurring. We also discuss how future missions will further enhance our understanding of the importance of turbulence to solar wind dynamics. (10.1098/rsta.2014.0147)
DOI : 10.1098/rsta.2014.0147
3D hybrid simulations of the interaction of a magnetic cloud with a bow shock
- Turc Lucile
- Fontaine Dominique
- Savoini Philippe
- Modolo Ronan
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (8), pp.6133-6151. In this paper, we investigate the interaction of a magnetic cloud (MC) with a planetary bow shock using hybrid simulations. It is the first time to our knowledge that this interaction is studied using kinetic simulations which include self-consistently both the ion foreshock and the shock wave dynamics. We show that when the shock is in a quasi-perpendicular configuration, the MC's magnetic structure in the magnetosheath remains similar to that in the solar wind, whereas it is strongly altered downstream of a quasi-parallel shock. The latter can result in a reversal of the magnetic field north-south component in some parts of the magnetosheath. We also investigate how the MC affects in turn the outer parts of the planetary environment, i.e., from the foreshock to the magnetopause. We find the following: (i) The decrease of the Alfvén Mach number at the MC's arrival causes an attenuation of the foreshock region because of the weakening of the bow shock. (ii) The foreshock moves along the bow shock's surface, following the rotation of the MC's magnetic field. (iii) Owing to the low plasma beta, asymmetric flows arise inside the magnetosheath, due to the magnetic tension force which accelerates the particles in some parts of the magnetosheath and slows them down in others. (iv) The quasi-parallel region forms a depression in the shock's surface. Other deformations of the magnetopause and the bow shock are also highlighted. All these effects can contribute to significantly modify the solar wind/magnetosphere coupling during MC events. (10.1002/2015JA021318)
DOI : 10.1002/2015JA021318
Polarization analysis of CuXX-lines emitted from X-pinch
- Baronova E O
- Larour Jean
- Rosmej F B
- Khattak F y
Journal of Physics: Conference Series, IOP Science, 2015, 653, pp.012145. Soft x-ray emission from CuXX L-shell lines emitted by a dense X-pinch plasma have been investigated with high-resolution curved Bragg crystals at different angles of orientation. Single shot time integrated spectra show clear evidences of polarization for the Ne-like spectral lines 2s22p6 1S0 → 2s22p53s 1P1 (λ = 12.570 Å), 2s22p6 1S0 → 2s22p53s 3P1 (λ = 12.8277 Å). The variation of the intensity ratio of these two well-separated L-shell lines is discussed in view of its application for suprathermal electron characterization under real experimental conditions of pinch plasmas. We demonstrated that the simultaneous use of two different polarization spectrometers (means 4 Bragg crystals) permitted a high level of confidence for the analysis of the variation of the line ratios due to polarization. (10.1088/1742-6596/653/1/012145)
DOI : 10.1088/1742-6596/653/1/012145
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
The Plasma E × B Staircase: Turbulence Self-Regulation through Spontaneous Flow Patterning
- Dif-Pradalier Guilhem
- Hornung G
- Ghendrih Philippe
- Clairet R
- Diamond P H
- Sarazin Yanick
- Vermare L
- Abiteboul J
- Cartier-Michaud T
- Ehrlacher C
- Estève D
- Garbet Xavier
- Grandgirard Virginie
- Gürcan Özgür D.
- Hennequin P
- Kosuga Y
- Latu Guillaume
- Morel Pierre
- Norscini C
- Sabot R
, 2015. The E × B staircase [1, 2] is a spontaneously formed, turbulence-driven self-organising pattern of quasi-regular, long-lived and localised shear flow [3] and stress layers. These layers coincide with long-lived pressure corrugations and are interspersed between regions of turbulent avalanching. The typical spacing between these layers is mesoscale, noted ∆ ∼ 25 − 30ρ i [1, 4] —in-between the turbulence auto-correlation length c ∼ 5ρ i at micro scales and the profile macroscale L 100ρ i , see e.g. Fig.2 in [1]— and sets the outer scale of the turbulent avalanching. Here ρ i is ion Larmor radius. Whilst arresting, statistically, to mesoscales the detrimental avalanching these layers, located at the " steps of the staircase " are beneficial to confinement. The E×B staircase is thus best understood as a self-organising and dynamical set of weak or permeable transport barriers. Strong mean zonal flows are generated and endure at the steps of the staircase, resulting in localised deviations of the poloidal flow from its oft-assumed neoclassical prediction [5].
Further details on the plasma E × B staircase
- Dif-Pradalier Guilhem
- Hornung G
- Ghendrih Philippe
- Sarazin Yanick
- Clairet F
- Vermare L
- Diamond P H
- Abiteboul J
- Cartier-Michaud T
- Ehrlacher C
- Esteve D
- Garbet Xavier
- Grandgirard Virginie
- Gürcan Özgür D.
- Hennequin P
- Kosuga A
- Latu Guillaume
- Morel Pierre
- Norscini C
- Sabot R
- Storelli A.
, 2015. Turbulence in hot magnetised plasmas spontaneously has been shown to spontaneously organise on global scales into the so-called " E × B staircase ". Further characterisation of this structure is detailed below.
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
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
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
What is the nature of magnetosheath FTEs?
- Roux A.
- Robert Patrick
- Fontaine Dominique
- Le Contel Olivier
- Canu Patrick
- Louarn P.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2015, 120 (6), pp.4576-4595. Cluster multipoint measurements are used to study two successive magnetosheath flux transfer events (FTEs). Magnetic field lines in the leading region are found to be closed magnetospheric field lines. For event 1 these field lines are wounded up by a large current structure oriented eastward and moving poleward. Conversely, the trailing region corresponds to opened magnetic field lines. For both events the leading edge of the FTEs is a tangential discontinuity separating the magnetosheath from closed field lines. In the case of event 1 magnetosheath ions are accelerated through the FTE trailing edge via a rotational discontinuity and penetrate on closed field lines through a second discontinuity. Thus, the ion jet is accelerated equatorward of the spacecraft but the backtracking of the discontinuities and the lack of dispersion show that ion acceleration occurs at less than 2 RE from Cluster. On the other hand the extrapolation forward indicates that the FTE bulge steepens as in simulations of Dorelli and Bhattacharjee). Evidence is given for the penetration of magnetosheath ions inside the core of the FTE, on closed field lines. Magnetosheath electrons are accelerated in parallel and antiparallel directions on open and on closed field lines, thus breaking the frozen-in condition. Event 2 is also split in two distinct regions but no evidence is found for accelerated bidirectional magnetosheath electrons. For event 2 the two discontinuities at the trailing region are stacked together when they are crossed by the spacecraft, suggesting that the current splitting is a reconnection signature. (10.1002/2015JA020983)
DOI : 10.1002/2015JA020983
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
Wave-number spectrum of dissipative drift waves and a transition scale
- Ghantous K.
- Gürcan Özgür D.
Physical Review E, American Physical Society (APS), 2015, 92 (3), pp.033107. We study the steady state spectrum of the Hasegawa-Wakatani (HW) equations that describe drift wave turbulence. Beyond a critical scale kc, which appears as a balance between the nonlinear time and the parallel conduction time, the adiabatic electron response breaks down nonlinearly and an internal energy density spectrum of the form F(k⊥)∝k−3⊥, associated with the background gradient, is established. More generally a dual power law spectrum, approximately of the form F(k⊥)∝k−3⊥(k−2c k−2⊥) is obtained, which captures this transition. Using dimensional analysis, an expression of the form kc∝C/κ is derived for the transition scale, where C and κ are normalized parameters of the HW equations signifying the electron adiabaticity and the density gradient, respectively. The results are numerically confirmed using a shell model developed and used for the Hasegawa-Wakatani system. (10.1103/PhysRevE.92.033107)
DOI : 10.1103/PhysRevE.92.033107
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
Kinetic scale solar wind turbulence: Landau-fluid simulations and spacecraft observations
- Sahraoui Fouad
, 2015.
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
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.
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
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
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
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

Retour aux années