Publications

2014

• Ambipolar and non-ambipolar diffusion in an rf plasma source containing a magnetic filter
  
  • Lafleur Trevor
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2014, 21, pp.063510. By placing a magnetic filter across a rectangular plasma source (closed at one end with a ceramic plate and an rf antenna, and terminated at the opposite end by a grounded grid), we experimentally investigate the effect of conducting and insulating source walls on the nature of the plasma diffusion phenomena. The use of a magnetic filter creates a unique plasma, characterized by a high upstream electron temperature (Teu∼5 eV) near the rf antenna and a low downstream electron temperature (Ted∼1 eV) near the grid, which more clearly demonstrates the role of the source wall materials. For conducting walls a net ion current to ground is measured on the grid, and the plasma potential is determined by a mean electron temperature within the source. For insulating walls the plasma potential is determined by the downstream electron temperature (i.e., Vp∼5.2Ted in argon), and the net current to the grid is exactly zero. Furthermore, by inserting a small additional upstream conductor (that can be made floating or grounded through an external circuit switch), we demonstrate that the plasma potential can be controlled and set to a low (Vp∼5.2Ted), or high (Vp∼5.2Teu) value. (10.1063/1.4885109)
  DOI : 10.1063/1.4885109
• Surface deactivation of vibrationally excited N<SUB>2</SUB> studied using infrared titration combined with quantum cascade laser absorption spectroscopy
  
  • Marinov Daniil
  • Lopatik D.
  • Guaitella Olivier
  • Ionikh Y.
  • Röpcke J.
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (1), pp.015203. The wall de-excitation probability of vibrationally excited nitrogen molecules was determined using infrared (IR) titration with CO, CO 2 and N 2 O. Gas mixtures of N 2 with 0.05?0.5% of CO (CO 2 or N 2 O) were excited by a pulsed dc discharge at p = 133 Pa in a cylindrical discharge tube. During the afterglow, the vibrational relaxation of titrating molecules was monitored in situ with quantum cascade laser absorption spectroscopy. The value of was deduced from measured vibrational relaxation times using a model of vibrational kinetics in N 2 . It was found that adsorption of IR tracers on the surface may increase the value of by a factor up to two, depending on the molecule and the surface material. It was demonstrated that N 2 O is the most inert and reliable tracer and it was used for the determination of on silica, Pyrex, TiO 2 , Al 2 O 3 and anodized aluminum. Pretreatment of the silica surface by low-pressure plasma was found to have a strong effect on the vibrational de-excitation. Values of measured after O 2 , Ar and N 2 plasma pretreatment of the same silica discharge tube were 5.7 × 10 ?4 , 8.2 × 10 ?4 and 11 × 10 ?4 , respectively. This study clearly demonstrates that the presence of adsorbed atoms and molecules on the surface may significantly alter the value of . (10.1088/0022-3727/47/1/015203)
  DOI : 10.1088/0022-3727/47/1/015203
• Quantified energy dissipation rates in the terrestrial bow shock: 2. Waves and dissipation
  
  • Wilson Iii L. B.
  • Sibeck David G.
  • Breneman A. W.
  • Le Contel Olivier
  • Cully C. M.
  • Turner D. L.
  • Angelopoulos V.
  • Malaspina D. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119 (8), pp.6475-6495. We present the first quantified measure of the energy dissipation rates, due to wave-particle interactions, in the transition region of the Earth's collisionless bow shock using data from the Time History of Events and Macroscale Interactions during Substorms spacecraft. Our results show that wave-particle interactions can regulate the global structure and dominate the energy dissipation of collisionless shocks. In every bow shock crossing examined, we observed both low-frequency (<10 Hz) and high-frequency (&#8819;10 Hz) electromagnetic waves throughout the entire transition region and into the magnetosheath. The low-frequency waves were consistent with magnetosonic-whistler waves. The high-frequency waves were combinations of ion-acoustic waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and whistler mode waves. The high-frequency waves had the following: (1) peak amplitudes exceeding deltaB 10 nT and deltaE 300 mV/m, though more typical values were deltaB 0.1-1.0 nT and deltaE 10-50 mV/m; (2) Poynting fluxes in excess of 2000 muW m<SUP>-2</SUP> (typical values were 1-10 muW m<SUP>-2</SUP>); (3) resistivities > 9000 Omega m; and (4) associated energy dissipation rates >10 muW m<SUP>-3</SUP>. The dissipation rates due to wave-particle interactions exceeded rates necessary to explain the increase in entropy across the shock ramps for 90% of the wave burst durations. For 22% of these times, the wave-particle interactions needed to only be <= 0.1% efficient to balance the nonlinear wave steepening that produced the shock waves. These results show that wave-particle interactions have the capacity to regulate the global structure and dominate the energy dissipation of collisionless shocks. (10.1002/2014JA019930)
  DOI : 10.1002/2014JA019930
• Theory for helical turbulence under fast rotation
  
  • Galtier Sébastien
  Physical Review E, American Physical Society (APS), 2014, 89, pp.41001. Recent numerical simulations have shown the strong impact of helicity on homogeneous rotating hydrodynamic turbulence. The main effect can be summarized through the law n ñ=-4, where n and ñ are the power law indices of the one-dimensional energy and helicity spectra, respectively. We investigate this rotating turbulence problem in the small Rossby number limit by using the asymptotic weak turbulence theory derived previously. We show that the empirical law is an exact solution of the helicity equation where the power law indices correspond to perpendicular (to the rotation axis) wave number spectra. It is proposed that when the cascade towards small scales tends to be dominated by the helicity flux the solution tends to ñ=-2, whereas it is ñ=-3/2 when the energy flux dominates. The latter is compatible with the solution previously observed numerically and derived theoretically in the weak turbulence regime when only the energy equation is used, whereas the former solution is constrained by a locality condition. (10.1103/PhysRevE.89.041001)
  DOI : 10.1103/PhysRevE.89.041001
• Numerical computation of the modified plasma dispersion function with curvature
  
  • Gürcan Özgür D.
  Journal of Computational Physics, Elsevier, 2014, 269, pp.156-167. A particular generalization of the plasma dispersion function, which is linked to the regular plasma dispersion function via recurrence relations is discussed. The generalization allows a fast numerical implementation of a certain two-dimensional integral that appears in the description of the plasma dispersion in curved geometry, by reducing it to a single integral over a function involving the generalized plasma dispersion function. The local dielectric function of the toroidal ion temperature gradient driven mode can be written in terms of these integral functions. A matrix method is proposed to combine the consecutive integrals as a single 1D integral over a single integrand. The method allows two orders of magnitude speed up over the 2D integral implementation. Using various optimizations and an efficient implementation of the regular plasma dispersion function, further speed up is obtained. (C) 2014 Elsevier Inc. All rights reserved. (10.1016/j.jcp.2014.03.017)
  DOI : 10.1016/j.jcp.2014.03.017
• Finite ballooning angle effects on ion temperature gradient driven mode in gyrokinetic flux tube simulations
  
  • Singh Rameswar
  • Brunner Stephan
  • Ganesh R.
  • Jenko F.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (3), pp.032115. This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch (k&#8869;&#961;i&#8201;>&#8201;1) when growth rates maximized over all ballooning angles are considered. However, the SW branch disappears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremely high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle. (10.1063/1.4868425)
  DOI : 10.1063/1.4868425
• Momentum transport in the vicinity of q<SUB>min</SUB> in reverse shear tokamaks due to ion temperature gradient turbulence
  
  • Singh Rameswar
  • Singh R.
  • Jhang Hogun
  • Diamond P.H.
  Physics of Plasmas, American Institute of Physics, 2014, 21 (1), pp.012302. This paper presents effects of finite ballooning angles on linear ion temperature gradient (ITG) driven mode and associated heat and momentum flux in Gyrokinetic flux tube simulation GENE. It is found that zero ballooning angle is not always the one at which the linear growth rate is maximum. The ITG mode acquires a short wavelength (SW) branch (k&#8869;&#961;i&#8201;>&#8201;1) when growth rates maximized over all ballooning angles are considered. However, the SW branch disappears on reducing temperature gradient showing characteristics of zero ballooning angle SWITG in case of extremely high temperature gradient. Associated heat flux is even with respect to ballooning angle and maximizes at nonzero ballooning angle while the parallel momentum flux is odd with respect to the ballooning angle. (10.1063/1.4861625)
  DOI : 10.1063/1.4861625
• Study of gas heating mechanisms in millisecond pulsed discharges and afterglows in air at low pressures
  
  • Pintassilgo C.D.
  • Guerra V.
  • Guaitella Olivier
  • Rousseau Antoine
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23, pp.025006. A self-consistent model is developed to study the temporal variation of the gas temperature in millisecond single dc pulsed discharges and their afterglows in air-like mixtures (N220%O2) at low pressures. The model is based on the solutions to the time-dependent gas thermal balance equation, under the assumption of a parabolic gas temperature profile across the discharge tube, coupled to the electron, vibrational and chemical kinetics. Modelling results provide a satisfactory explanation for recently published time-resolved experimental data for the gas temperature in a 5 ms pulsed air plasma with a current of 150mA and the corresponding afterglow at a pressure of 133 Pa (1 Torr). It is shown that the main heating mechanisms during the first millisecond of the pulse come predominantly from O2 dissociation by electron impact through the pre-dissociative excited state O2(B 3&#8722; u ) and the quenching of nitrogen electronically excited states N2(A 3 u , B 3 g, a 1&#8722; u , a 1 g, w 1u) by O2, agreeing with other studies on fast gas heating in air plasmas. As the pulse duration increases, other gas heating sources become important, namely VT N2O energy exchanges, recombination of oxygen atoms at the wall, N2(A) quenching by O(3P) and reaction N(4S) NO(X) &#8594; N2(X, v &#8764;3) O, contributing altogether to an additional smooth increase in the gas temperature until the end of the pulse. In the first instants of the early afterglow, the gas temperature decreases very rapidly as a consequence of the minor role played by electronic collisions and due to a fast decay of N2 electronic states. For afterglow times up to 10 ms, the gas temperature continues to decrease, following the time-dependent kinetics of [N2(X,v)], [N(4S)], [O(3P)] and [NO(X)]. Sensitivity of the model to different input parameters such as thermal accommodation coefficient and probabilities for atomic recombination at the wall are reported. (10.1088/0963-0252/23/2/025006)
  DOI : 10.1088/0963-0252/23/2/025006
• Investigation of Switch Designs for the Dynamic Load Current Multiplier Scheme on the SPHYNX Microsecond Linear Transformer Driver
  
  • Maysonnave Thomas
  • Bayol Frédéric
  • Demol Gauthier
  • d'Almeida Thierry
  • Lassalle Francis
  • Morell Alain
  • Grunenwald Julien
  • Chuvatin Alexandre S.
  • Pecastaing Laurent
  • de Ferron Antoine Silvestre
  IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2014, 42 (10), pp.2974-2980. SPHINX is a microsecond linear transformer driver LTD, used essentially for implosion of Z-pinch loads in direct drive mode. It can deliver a 6-MA current pulse within 800 ns into a Z-pinch load. The dynamic load current multiplier concept enables the current pulse to be modified by increasing its amplitude while reducing its rise time before being delivered to the load. This compact system is made up of concentric electrodes (autotransformer), a dynamic flux extruder (cylindrical wire array), a vacuum convolute (eight postholes), and a vacuum closing switch, which is the key component of the system. Several different schemes are investigated for designing a vacuum switch suitable for operating the dynamic load current multiplier on the SPHINX generator for various applications, including isentropic compression experiments and Z-pinch radiation effects studies. In particular, the design of a compact vacuum surface switch and a multichannel vacuum switch, located upstream of the load are studied. Electrostatic simulations supporting the switch designs are presented along with test bed experiments. Initial results from shots on the SPHINX driver are also presented. (10.1109/TPS.2014.2313372)
  DOI : 10.1109/TPS.2014.2313372
• Plasma composition and ion acceleration in the PEGASES thruster
  
  • Renaud D.
  • Mazouffre S.
  • Aanesland Ane
  Space Propulsion, 2014 (2969109).
• Pulsed discharges in a wide density range: plasma development and media excitation
  
  • Starikovskaia Svetlana
  • Starikovskii A.Yu.
  , 2014. The Chapter demonstrates the peculiarities of pulsed nonequilibrium plasma generation in different media by nano- and picosecond high-voltage discharges. Fast ionization waves in long tubes at low pressures, streamers and dielectric barrier discharges at moderate and atmospheric pressures, surface dielectric barrier discharges at elevated pressures are considered. Special attention is given to start of picoseconds and nanosecond discharges in liquid media. The data used were consciously restricted to ICCD fast imaging of picoseconds and nanosecond discharges, to underline the dynamics of pulsed discharge development and the importance of spatial distribution of the discharge energy.
• Fourier spectrum and phases for a signal in a finite interval
  
  • Dorville Nicolas
  • Belmont Gérard
  • Sahraoui Fouad
  • Rezeau Laurence
  , 2014, 13, pp.SH13B-4086. When investigating the physics of turbulent media, as the solar wind or the magnetosheath plasmas, obtaining accurate Fourier spectra and phases is a crucial issue. For the different fields, the spectra allow in particular verifying whether one or several power laws can be determined in different frequency ranges. Accurate phases are necessary as well for all the "higher order statistics" studies in Fourier space, the coherence ones and for the polarization studies. Unfortunately, the Fourier analysis is not unique for a finite time interval of duration T: the frequencies lower than 1/T have a large influence on the result, which can hardly be controlled. This unknown "trend" has in particular the effect of superposing jumps at the edges of the interval, for the function under study itself, as well as for all its derivatives. The Fourier transform obtained directly by FFT (Fast Fourier Transform) is generally much influenced by these effects and cannot be used without care for wide band signals. The interferences between the jumps and the signal itself also provide "hairs" on the spectrum, which are clearly visible fluctuations with df&#8776;1/T. These fluctuations are usually eliminated by smoothing the spectrum, or by averaging several successive spectra. Nevertheless, such smoothing introduces uncertainties on the spectral laws and it makes the phases lost. Windowing is also a method currently used to suppress the jumps, but it modifies the signal (the windowed trend has a spectrum, which is convolved with the searched one) and the phases are also lost to a large extent. Here, we present a new data processing technique to circumvent these difficulties. It takes advantage of the fact that the signal is generally not unknown out of the interval under study: the complete signal is tapered to this interval of interest thanks to a new kind of window, sharp but not square. This kind of window is such that the spectrum obtained can then be deconvolved almost exactly, through a minimization procedure based on the weak- hypothesis that it is smooth at the scale of a few successive spectral points. Then, a later step allows reconstructing the phases. Tests with synthetic data are presented, that demonstrate the efficiency of the method, and first results from Cluster data are also shown.
• Equivalence of the hard-wall and kinetic-fluid models of collisionless electron heating in capacitively coupled discharges
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner Miles
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (1), pp.015016. By re-evaluating the hard-wall collisionless electron heating model from first principles, we show that despite previous criticisms (Gozadinos et al 2001 Phys. Rev. Lett. 87 [http://dx.doi.org/10.1103/PhysRevLett.87.135004] 135004 ), this model can in general be made consistent with the requirement of radio frequency (rf) current continuity at the sheath edge, while still producing a net heating effect. In addition, we demonstrate that the hard-wall and kinetic-fluid heating models stem from the same basic physical mechanism, and are in many ways the same theory; they differ only in the spatial region where electron heating is assumed to occur, and the way in which the effective electron distribution function is determined. Fundamentally, both models predict that collisionless heating occurs because of a non-isothermal compression and expansion of the plasma electrons by an oscillating rf sheath. (10.1088/0963-0252/23/1/015016)
  DOI : 10.1088/0963-0252/23/1/015016
• Dynamics of plasma evolution in a nanosecond underwater discharge
  
  • Marinov Ilya
  • Starikovskaia Svetlana
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (22), pp.224017. A positive discharge in water is generated by applying a 30 ns high-voltage (HV) pulse on a micrometre scale electrode. The applied voltage ranges from 6 to 15 kV and a fast plasma propagating mode is launched with a velocity of up to 60 km s&#8722;1 . Time-resolved shadowgraphy and spectroscopy are performed to monitor the time evolution of the discharge structure and of the plasma emission spectra. By analysing the dynamics of the shock front velocity and the lateral expansion of the plasma channel, it is possible to estimate the pressure at the ignition of the plasma by two independent methods: very good agreement is found at 6 kV giving initial pressures of 0.4 GPa and 0.3 GPa, respectively. At 15 kV, only the shock front velocity method is applicable under our experimental conditions, giving an estimate of the initial pressure of 5.8 GPa. Such high initial pressures show that, under a nanosecond HV pulse, the plasma is ignited directly in the dense phase. Emission spectra show a strong continuum emission as well as a broad Balmer &#945; line with a strong red shift, with an estimate of the initial plasma density of 1.3 × 1026 m&#8722;3. The relaxation of discharge pressure and plasma density is studied under a series of six successive pulses. (10.1088/0022-3727/47/22/224017)
  DOI : 10.1088/0022-3727/47/22/224017
• Time-resolved imaging of nanosecond-pulsed micro-discharges in heptane
  
  • Hamdan A.
  • Marinov Ilya
  • Belmonte Thierry
  • Rousseau A.
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (5). Nanosecond-pulsed micro-discharges in heptane are studied by time-resolved imaging in pin-to-plate configuration. When a voltage of +5 kV is applied to the pin electrode, the discharge exhibits one maximum in light intensity. At +15 kV, filtered images show that up to three maxima can be identified. These maxima are associated with local electron-ion recombination and bremsstrahlung emission and attributed to the development of a complex space-charge field. In the post-discharge, the dynamics of the gas bubble can be simulated by the Gilmore model, and the pressure evolution in this bubble is predicted. From our results, it seems reasonable to think that the gas bubble develops from the post-discharge of the spark. Results obtained by using the double-pulse technique show that light emission during the post-discharge of the second discharge lasts 10 times longer than the post-discharge of the first spark. The pressure drop in the gas bubble, predicted by the Gilmore model, is used to explain this result and it provides a control method by optical diagnostics in liquids. (10.1088/0022-3727/47/5/055203)
  DOI : 10.1088/0022-3727/47/5/055203
• Weak turbulence theory for rotating magnetohydrodynamics and planetary flows
  
  • Galtier Sébastien
  Journal of Fluid Mechanics, Cambridge University Press (CUP), 2014, 757, pp.114-154. A weak turbulence theory is derived for magnetohydrodynamics (MHD) under rapid rotation and in the presence of a uniform large-scale magnetic field which is associated with a constant Alfvén velocity . The angular velocity is assumed to be uniform and parallel to . Such a system exhibits left and right circularly polarized waves which can be obtained by introducing the magneto-inertial length . In the large-scale limit ( , with being the wavenumber) the left- and right-handed waves tend to the inertial and magnetostrophic waves, respectively, whereas in the small-scale limit ( ) pure Alfvén waves are recovered. By using a complex helicity decomposition, the asymptotic weak turbulence equations are derived which describe the long-time behaviour of weakly dispersive interacting waves via three-wave interaction processes. It is shown that the nonlinear dynamics is mainly anisotropic, with a stronger transfer perpendicular than parallel to the rotation axis. The general theory may converge to pure weak inertial/magnetostrophic or Alfvén wave turbulence when the large- or small-scale limits are taken, respectively. Inertial wave turbulence is asymptotically dominated by the kinetic energy/helicity, whereas the magnetostrophic wave turbulence is dominated by the magnetic energy/helicity. For both regimes, families of exact solutions are found for the spectra, which do not correspond necessarily to a maximal helicity state. It is shown that the hybrid helicity exhibits a cascade whose direction may vary according to the scale at which the helicity flux is injected, with an inverse cascade if and a direct cascade otherwise. The theory is relevant to the magnetostrophic dynamo, whose main applications are the Earth and the giant planets, such as Jupiter and Saturn, for which a small ( ) Rossby number is expected. (10.1017/jfm.2014.490)
  DOI : 10.1017/jfm.2014.490
• Wave normal angles of whistler-mode chorus rising and falling tones
  
  • Taubenschuss U.
  • Khotyaintsev Y. V.
  • Santolík O.
  • Vaivads A.
  • Cully C. M.
  • Le Contel Olivier
  • Angelopoulos V.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014. We present a study of wave normal angles (&#952;k) of whistler mode chorus emission as observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the year 2008. The three inner THEMIS satellites THA, THD, and THE usually orbit Earth close to the dipole magnetic equator (±20°), covering a large range of L shells from the plasmasphere out to the magnetopause. Waveform measurements of electric and magnetic fields enable a detailed polarization analysis of chorus below 4 kHz. When displayed in a frequency-&#952;k histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/fc,e&#8776;0.5 (f is the chorus frequency, fc,e is the local electron cyclotron frequency) and at &#952;k&#8764;40°. Below &#952;k&#8764;40°, the average value for &#952;k is predominantly field aligned, but slightly increasing with frequency toward half of fc,e (&#952;k up to 20°). Above half of fc,e, the average &#952;k is again decreasing with frequency. Above &#952;k&#8764;40°, wave normal angles are usually close to the resonance cone angle. Furthermore, we present a detailed comparison of electric and magnetic fields of chorus rising and falling tones. Falling tones exhibit peaks in occurrence solely for &#952;k>40° and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at &#952;k>40°, the ratio of magnetic to electric field shows no significant differences. Thus, we conclude that falling tones are generated under similar conditions as rising tones, with common source regions close to the magnetic equatorial plane. (10.1002/2014JA020575)
  DOI : 10.1002/2014JA020575
• In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge
  
  • Forsyth C.
  • Fazakerley A.
  • Rae I. J.
  • Watt C. E. J.
  • Murphy K.
  • Wild James A.
  • Karlsson T.
  • Mutel R. L.
  • Owen C. J.
  • Ergun R.
  • Masson A.
  • Berthomier Matthieu
  • Donovan E.
  • Frey H.~u.
  • Matzka J.
  • Stolle C.
  • Zhang Y.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119 (2), pp.927-946. The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100&#8201;km at altitudes of 40007000&#8201;km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120240&#8201;s after Cluster 4 at 13002000&#8201;km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven wedgelets. Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the&#8201;SCW. (10.1002/2013JA019302)
  DOI : 10.1002/2013JA019302
• Kelvin-Helmholtz Vortices and Double Mid-Latitude Reconnection at the Earth's Magnetopause: comparison between observations and simulations
  
  • Faganello Matteo
  • Califano F.
  • Pegoraro F.
  • Retinò Alessandro
  EPL - Europhysics Letters, European Physical Society / EDP Sciences / Società Italiana di Fisica / IOP Publishing, 2014. Observational signatures of Kelvin-Helmholtz (K-H) vortices and of double mid-latitude reconnection are highlighted in satellite data of the THEMIS mission. It is shown that the plasma fluid quantities at the low-latitude flank of the Earth's magnetosphere are compatible with K-H vortices, as described by three-dimensional simulations. At the same time it is shown that the particle fluxes are compatible with the presence of magnetic field lines, embedded in the K-H vortices, that close on Earth but are connected to the solar wind at low-latitude. These field lines are generated during the K-H evolution by magnetic reconnection proceeding spontaneously in both hemispheres at mid-latitudes, allowing the solar wind plasma to enter the Earth's magnetosphere directly.
• Ground state bromine atom density measurements by two-photon absorption laser-induced fluorescence
  
  • Sirse Nishant
  • Foucher Mickaël
  • Chabert Pascal
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2014, 23 (6), pp.062003. Ground state bromine atom detection by two-photon absorption laser-induced fluorescence (TALIF) is demonstrated. The bromine atoms are excited by two-photon absorption at 252.594 nm to the state and detected by 635.25 nm fluorescence to the (5s) 4 P 5/2 state. The atoms are generated in a radio-frequency inductively-coupled plasma in pure HBr. The excitation laser also causes some photodissociation of HBr molecules, but this can be minimized by not focussing the laser beam, still giving adequate signal levels. We determined the natural lifetime of the emitting state, and the rate constant for quenching of this state by collision with HBr molecules, ... (10.1088/0963-0252/23/6/062003)
  DOI : 10.1088/0963-0252/23/6/062003
• Rotational/compressional nature of the magnetopause: Application of the BV technique on a magnetopause case study
  
  • Dorville Nicolas
  • Belmont Gérard
  • Rezeau Laurence
  • Grappin Roland
  • Retinò Alessandro
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.1898-1908. The magnetopause boundary implies two main kinds of variations: a density/temperature gradient and a magnetic field rotation. Other variations are associated with these two, concerning in particular the flow velocity, the electric field, and the plasma composition. Compressional and rotational variations are always observed in a close vicinity of each other, if not inseparably mixed. We present a case study from the Cluster data where the two are clearly separated and investigate the natures of both layers, using the new BV method for discontinuity analysis. We evidence that the first one is a slow shock while the second is a rotational discontinuity. The interaction between these two kinds of discontinuities is then studied with the help of 1.5-D magnetohydrodynamics simulations. The comparison with the data is quite positive and, associated with general theoretical arguments, leads to think that most of the generic properties of the magnetopause may be interpreted in this sense. Our results suggest that a shaken magnetopause is made of the sum of several discontinuities: slow shocks and rotational discontinuities. A statistical study on a larger set of data will be necessary to check this conjecture. (10.1002/2013JA018927)
  DOI : 10.1002/2013JA018927
• Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure
  
  • Marinov Daniil
  • Guaitella Olivier
  • Arcos T. de Los
  • von Keudell A.
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47 (47), pp.475204. The kinetics of adsorption, desorption and recombination of nitrogen atoms on a silica surface is investigated. Stable nitrogen atoms are grafted to the inner surface of a fused silica discharge tube by a discharge in N 2 at 0.53?mbar. After the pre-treatment, the surface is analysed using x-ray photoelectron spectroscopy and an isotopic exchange technique. The latter consists of the exposure of the pre-treated surface with a discharge in the heavy nitrogen isotope 30 N 2 . Nitrogen isotopologues 29 N 2 and 28 N 2 produced on the surface are detected using a mass spectrometer and provide information about the coverage and reactivity of adsorbed 14 N atoms. It is found that during the pre-treatment, a silicon oxynitride (SiO x N y ) layer is formed on the initially clean SiO 2 surface. The coverage of N on the surface increases from 5? × ?10 13 to 5? × ?10 15? cm ?2 for a pre-treatment duration in the range of 10 ?2 ? 10 4 ?s. Atoms on the surface demonstrate a distribution of reactivity, which is attributed to a distribution of their binding energies and configurations on the surface. We demonstrate that stable chemisorbed N ads are not the main recombination sites for N atoms on the surface contrary to previous studies. We conclude that recombination takes place mainly on weakly bonding active sites with the binding energy smaller than 1?eV. (10.1088/0022-3727/47/47/475204)
  DOI : 10.1088/0022-3727/47/47/475204
• 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, 2014, 23 (4), pp.044003. IonIon plasmas are electronegative plasmas where the electron density is several orders of magnitude lower than the negative ion density. These plasmas have been scarcely observed and investigated since the 1960s and are formed as a transient state of pulsed plasmas or in separate regions in magnetized plasmas. In this review we focus on the latter case of continuous formation of ionion plasmas created at the periphery of magnetized plasma columns or downstream localized magnetic barriers. We bring together and review experimental results already published elsewhere and complement them with new results to illustrate the physics important in ionion plasma formation and highlight in particular unanswered questions. We show that with a good design the density in the ionion region is dropping only by a factor of 23 from the initial plasma density. These plasmas can therefore be well suited for various ion source applications when both fluxes or beams of positive and negative ions are desired, and when electrons can cause harmful effects. (10.1088/0963-0252/23/4/044003)
  DOI : 10.1088/0963-0252/23/4/044003
• Neutralizer-free gridded ion thruster
  
  • Rafalskyi D.V.
  • Aanesland Ane
  American Institute of Aeronautics and Astronautics paper, 2014, Propulsion and Energy Forum. (10.2514/6.2014-3423)
  DOI : 10.2514/6.2014-3423
• Characteristics of the flank magnetopause: Cluster observations
  
  • Haaland S.
  • Reistad J.
  • Tenfjord P.
  • Gjerloev J.
  • Maes Lukas
  • de Keyser J.
  • Maggiolo R.
  • Anekallu C.
  • Dorville Nicolas
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.9019-9037. The magnetopause is a current sheet forming the boundary between the geomagnetic field on one side and the shocked solar wind on the other side. This paper discusses properties of the low-latitude dawn and dusk flanks of the magnetopause. The reported results are based on a large number of measurements obtained by the Cluster satellites during magnetopause traversals. Using a combination of single-spacecraft and multispacecraft techniques, we calculated macroscopic features such as thickness, location, and motion of the magnetopause. The results show that the typical flank magnetopause is significantly thicker than the dayside magnetopause and also possesses a pronounced and persistent dawn-dusk asymmetry. Thicknesses vary from 150 to 5000&#8201;km, with an median thickness of around 1400&#8201;km at dawn and around 1150&#8201;km at dusk. Current densities are on average higher on dusk, suggesting that the total current at dawn and dusk are similar. Solar wind conditions and the interplanetary magnetic field cannot fully explain the observed dawn-dusk asymmetry. For a number of crossings we were also able to derive detailed current density profiles. The profiles show that the magnetopause often consists of two or more adjacent current sheets, each current sheet typically several ion gyroradii thick and often with different current direction. This demonstrates that the flank magnetopause has a structure that is more complex than the thin, one-dimensional current sheet described by a Chapman-Ferraro layer. (10.1002/2014JA020539)
  DOI : 10.1002/2014JA020539