Publications

2014

On the relationship between quadrupolar magnetic field and collisionless reconnection
- Smets Roch
- Aunai Nicolas
- Belmont Gérard
- Boniface C.
- Fuchs J. C.
Physics of Plasmas, American Institute of Physics, 2014, 21 (6), pp.062111. Using hybrid simulations, we investigate the onset of fast reconnection between two cylindrical magnetic shells initially close to each other. This initial state mimics the plasma structure in High Energy Density Plasmas induced by a laser-target interaction and the associated self-generated magnetic field. We clearly observe that the classical quadrupolar structure of the out-of-plane magnetic field appears prior to the reconnection onset. Furthermore, a parametric study reveals that, with a non-coplanar initial magnetic topology, the reconnection onset is delayed and possibly suppressed. The relation between the out-of-plane magnetic field and the out-of-plane electric field is discussed. (10.1063/1.4885097)
DOI : 10.1063/1.4885097
Radiation from Ag high energy density Z-pinch plasmas and applications to lasing
- Weller Michael E.
- Safronova Alla S.
- Kantsyrev Viktor L.
- Esaulov A. A.
- Shrestha Ishor
- Apruzese J. P.
- Giuliani J. L.
- Chuvatin Alexandre S.
- Stafford A.
- Keim S. F.
- Shlyaptseva V. V.
- Osborne Glenn C.
- Petkov E. E.
Physics of Plasmas, American Institute of Physics, 2014, 21 (03), pp.031206. Silver (Ag) wire arrays were recently introduced as efficient x-ray radiators and have been shown to create L-shell plasmas that have the highest electron temperature (>1.8 keV) observed on the Zebra generator so far and upwards of 30 kJ of energy output. In this paper, results of single planar wire arrays and double planar wire arrays of Ag and mixed Ag and Al that were tested on the UNR Zebra generator are presented and compared. To further understand how L-shell Ag plasma evolves in time, a time-gated x-ray spectrometer was designed and fielded, which has a spectral range of approximately 3.55.0 Å. With this, L-shell Ag as well as cold Lα and Lβ Ag lines was captured and analyzed along with photoconducting diode (PCD) signals (>0.8 keV). Along with PCD signals, other signals, such as filtered XRD (>0.2 keV) and Si-diodes (SiD) (>9 keV), are analyzed covering a broad range of energies from a few eV to greater than 53 keV. The observation and analysis of cold Lα and Lβ lines show possible correlations with electron beams and SiD signals. Recently, an interesting issue regarding these Ag plasmas is whether lasing occurs in the Ne-like soft x-ray range, and if so, at what gains? To help answer this question, a non-local thermodynamic equilibrium (LTE) kinetic model was utilized to calculate theoretical lasing gains. It is shown that the Ag L-shell plasma conditions produced on the Zebra generator at 1.7 maximum current may be adequate to produce gains as high as 6 cm−1 for various 3p → 3s transitions. Other potential lasing transitions, including higher Rydberg states, are also included in detail. The overall importance of Ag wire arrays and plasmas is discussed. (10.1063/1.4865368)
DOI : 10.1063/1.4865368
Helicon plasma thruster discharge model
- Lafleur Trevor
Physics of Plasmas, American Institute of Physics, 2014, 21, pp.043507. By considering particle, momentum, and energy balance equations, we develop a semi-empirical quasi one-dimensional analytical discharge model of radio-frequency and helicon plasma thrusters. The model, which includes both the upstream plasma source region as well as the downstream diverging magnetic nozzle region, is compared with experimental measurements and confirms current performance levels. Analysis of the discharge model identifies plasma power losses on the radial and back wall of the thruster as the major performance reduction factors. These losses serve as sinks for the input power which do not contribute to the thrust, and which reduce the maximum plasma density and hence propellant utilization. With significant radial plasma losses eliminated, the discharge model (with argon) predicts specific impulses in excess of 3000 s, propellant utilizations above 90%, and thruster efficiencies of about 30%. (10.1063/1.4871727)
DOI : 10.1063/1.4871727
Experimental validation of the dual positive and negative ion beam acceleration in the plasma propulsion with electronegative gases thruster
- Rafalskyi D.V.
- Popelier Lara
- Aanesland Ane
Journal of Applied Physics, American Institute of Physics, 2014, 115 (5), pp.053301. The PEGASES (Plasma Propulsion with Electronegative Gases) thruster is a gridded ion thruster, where both positive and negative ions are accelerated to generate thrust. In this way, additional downstream neutralization by electrons is redundant. To achieve this, the thruster accelerates alternately positive and negative ions from an ion-ion plasma where the electron density is three orders of magnitude lower than the ion densities. This paper presents a first experimental study of the alternate acceleration in PEGASES, where SF6 is used as the working gas. Various electrostatic probes are used to investigate the source plasma potential and the energy, composition, and current of the extracted beams. We show here that the plasma potential control in such system is key parameter defining success of ion extraction and is sensitive to both parasitic electron current paths in the source region and deposition of sulphur containing dielectric films on the grids. In addition, large oscillations in the ion-ion plasma potential are found in the negative ion extraction phase. The oscillation occurs when the primary plasma approaches the grounded parts of the main core via sub-millimetres technological inputs. By controlling and suppressing the various undesired effects, we achieve perfect ion-ion plasma potential control with stable oscillation-free operation in the range of the available acceleration voltages (±350 V). The measured positive and negative ion currents in the beam are about 10 mA for each component at RF power of 100 W and non-optimized extraction system. Two different energy analyzers with and without magnetic electron suppression system are used to measure and compare the negative and positive ion and electron fluxes formed by the thruster. It is found that at alternate ion-ion extraction the positive and negative ion energy peaks are similar in areas and symmetrical in position with /− ion energy corresponding to the amplitude of the applied acceleration voltage. (10.1063/1.4863876)
DOI : 10.1063/1.4863876
Regeneration of isopropyl alcohol saturated MnXOY surface: Comparison of thermal, ozonolysis and non-thermal plasma treatments
- Sivachandiran Loganathan
- Thévenet Frédéric
- Rousseau Antoine
Chemical Engineering Journal, Elsevier, 2014, 246, pp.184-195. IPA saturated MnXOY surface regeneration has been investigated under dry air. MnXOY coated glass beads packed-bed reactor has been designed and used for IPA storage under gas-flowing condition at 296 K. The coated MnXOY material is characterized by BrunauerEmmettTeller (BET), non destructive Optical Pro- filometer and X-ray diffraction (XRD) techniques. Atmospheric pressure gas phase Fourier Transform Infrared Spectroscopy (FTIR) and online Thermal Desorption coupled with Gas Phase Chromatography and Mass Spectrometry (TDGCMS) have been respectively used to quantify and to identify the gas phase species produced during the regeneration processes. This study mainly aims at investigating three different methods to regenerate the IPA saturated MnXOY surface. In this framework, methods have been investigated for IPA saturated MnXOY surface regeneration namely (i) direct thermal treatment (DTT), (ii) ozonolysis and (iii) In-Situ Non Thermal Plasma Treatment (NTP). Among the employed methods, In-Situ NTP treatment has shown better regeneration efficiency, and twice more CO2 selectivity. Notably, dry air In-Situ NTP treatment prior to thermal treatment has significantly improved the mineralization. The order of mineralization efficiency and/or COx selectivity can be written as follows: In-Situ NTP > dry air ozonolysis > dry air DTT. (10.1016/j.cej.2014.02.058)
DOI : 10.1016/j.cej.2014.02.058
Investigation of capillary nanosecond discharges in air at moderate pressure: comparison of experiments and 2D numerical modeling
- Klochko A.V.
- Starikovskaia Svetlana
- Xiong Z.
- Kushner M.J.
Journal of Physics D: Applied Physics, IOP Publishing, 2014, 47, pp.365202. Nanosecond electrical discharges in the form of ionization waves are of interest for rapidly ionizing and exciting complex gas mixtures to initiate chemical reactions. Operating with a small discharge tube diameter can significantly increase the specific energy deposition and so enable optimization of the initiation process. Analysis of the uniformity of energy release in small diameter capillary tubes will aid in this optimization. In this paper, results for the experimentally derived characteristics of nanosecond capillary discharges in air at moderate pressure are presented and compared with results from a two-dimensional model. The quartz capillary tube, having inner and outer diameters of 1.5 and 3.4 mm, is about 80 mm long and filled with synthetic dry air at 27 mbar. The capillary tube with two electrodes at the ends is inserted into a break of the central wire of a long coaxial cable. A metal screen around the tube is connected to the cable ground shield. The discharge is driven by a 19 kV 35 ns voltage pulse applied to the powered electrode. The experimental measurements are conducted primarily by using a calibrated capacitive probe and back current shunts. The numerical modelling focuses on the fast ionization wave (FIW) and the plasma properties in the immediate afterglow after the conductive plasma channel has been established between the two electrodes. The FIW produces a highly focused region of electric field on the tube axis that sustains the ionization wave that eventually bridges the electrode gap. Results from the model predict FIW propagation speed and current rise time that agree with the experiment. (10.1088/0022-3727/47/36/365202)
DOI : 10.1088/0022-3727/47/36/365202
Whistler mode waves at magnetotail dipolarization fronts
- Viberg H.
- Khotyaintsev Y. V.
- Vaivads A.
- André M.
- Fu H.S.
- Cornilleau-Wehrlin Nicole
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.2605-2611. We report the statistics of whistler mode waves observed in relation to dipolarization fronts (DFs) in Earth's magnetotail using data from the four Cluster spacecraft spanning a period of 9 years, 20012009. We show that whistler mode waves are common in a vicinity of DFs: between 30 and 60% of all DFs are associated with whistlers. Whistlers are about 7 times more likely to be observed near a DF than at any random location in the magnetotail. Therefore, whistlers are a characteristic signature of DFs. We find that whistlers are most often detected in the flux pileup region (FPR) following the DF, close to the center of the current sheet (Bx ∼ 0) and in association with anisotropic electron distributions (Tperp>T∥). This suggests that we typically observe emissions in the source region where they are generated by the anisotropic electrons produced by the betatron process inside the FPR. (10.1002/2014JA019892)
DOI : 10.1002/2014JA019892
Pure airplasma bullets propagating inside microcapillaries and in ambient air
- Lacoste D.A.
- Bourdon Anne
- Kuribara Koichi
- Urabe Keiichiro
- Stauss Sven
- Terashima Kazuo
Plasma Sources Science and Technology, IOP Publishing, 2014, 23, pp.062006. This paper reports on the characterization of airplasma bullets in microcapillary tubes and in ambient air, obtained without the use of inert or noble gases. The bullets were produced by nanosecond repetitively pulsed discharges, applied in a dielectric barrier discharge configuration. The anode was a tungsten wire with a diameter of 50μm, centered in the microcapillary, while the cathode was a silver ring, fixed on the outer surface of the fused silica tube. The effects of the applied voltage and the inner diameter of the microcapillary tube on the plasma behavior were investigated. Inside the tubes, while the topology of the bullets seems to be strongly dependent on the diameter, their velocity is only a function of the amplitude of the applied voltage. In ambient air, the propagation of air bullets with a velocity of about 1.25 × 105 ms−1 is observed. (10.1088/0963-0252/23/6/062006)
DOI : 10.1088/0963-0252/23/6/062006
Simulation of long term variation of the F2-layer critical frequency f0F2 at the northern tropical crest of ionization at Phu Thuy, Vietnam with the thermosphere-ionosphere-electrodynamics general circulation model (TIE-GCM)
- Pham Thi Thu Hong
- Amory-Mazaudier Christine
- Le Huy Minh
Vietnam Journal of Earth Sciences, Vietnam Academy of Science and Technology (VAST), 2014 (36), pp.470-479. In this work, the long-term variations of the simulated f0F2 by the NCAR thermosphere ionosphere-electrodynamics general circulation model (TIE-GCM) at the northern tropical crest of ionization at Phu Thuy-Vietnam (geographic latitudes 21.030N and longitude: 105.950E) during the period from 1962 to 2002 are examined to evaluate the ability of this model to reproduce the major features of the f0F2 as observed. The TIE-GCM simulates the influences of migrating and non-migrating diurnal and semidiurnal tides at the lower thermosphere, and changes of geomagnetic activity on the long-term variation of the f0F2. It reproduces well the diurnal and seasonal variations. We analyze the diurnal and seasonal variations of the observed f0F2 at Phu Thuy in approximately the same solar activity condition as in 1964, 1996 for the March and September equinoxes and June and December solstices. The local time and seasonal structures of these simulated results correspond well to those that are observed in 1964. On the contrary, the TIE-GCM model does not reproduce the amplitude observed at Thuy Phu in 1996. The TIE-GCM model with the chosen inputs does not yet allow us to explain well the long-term variations observed at Phu Thuy. We also try the different numerical simulations to understand how the long-term variation of the f0F2 is formed, how it relates to the current global system and its relationship with the thermosphere wind. The simulations show that the calculated NmF2 values are lower than the observed values. We find that the modeled contributions of the migrating and non-migrating diurnal and semidiurnal tides may cause them to play a major role in reducing the amplitude of the NmF2. The contributions of the integrated hemispheric power of auroral electrons and the cross polar cap potential seem to play an important role in increasing the amplitude of the NmF2. Keywords: F2 layer; Long-term trends; Ionosphere equatorial ionization anomaly; Ionosphere (ionospheric conductivities, ionospheric currents and electric field), thermospheric tides, Electrodynamics of the ionosphere (ionospheric dynamo).
Radiation in the neighbourhood of a double layer
- Pottelette Raymond
- Berthomier Matthieu
- Pickett J. S.
Annales Geophysicae, European Geosciences Union, 2014, 32, pp.677-687. In the auroral kilometric radiation (AKR) source region, acceleration layers narrow in altitude and associated with parallel field-aligned potential drops of several kV can be identified by using both particles and wave-field high time-resolution measurements from the Fast Auroral SnapshoT explorer spacecraft (FAST). These so-called double layers (DLs) are recorded around density enhancements in the auroral cavity, where the enhancement can be at the edge of the cavity or even within the cavity at a small scale. Once immersed in the plasma, DLs necessarily accelerate particles along the magnetic field lines, thereby generating locally strong turbulent processes leading to the formation of nonlinear phase space holes. The FAST data reveal the asymmetric character of the turbulence: the regions located on the high-potential side of the DLs are characterized by the presence of electron holes, while on the low-potential side, ion holes are recorded. The existence of these nonlinear phase space holes may affect the AKR radiation pattern in the neighbourhood of a DL where the electron distribution function is drastically different from a horseshoe shape. We present some observations which illustrate the systematic generation of elementary radiation events occurring significantly above the local electron gyrofrequency in the presence of electron holes. These fine-scale AKR radiators are associated with a local electron distribution which presents a pronounced beam-like shape. (10.5194/angeo-32-677-2014)
DOI : 10.5194/angeo-32-677-2014
Waveforms of Langmuir turbulence in inhomogeneous solar wind plasmas
- Krafft C
- Volokitin A.
- Krasnoselskikh V.V.
- Dudok de Wit Thierry
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2014, 119, pp.9369–9382. Modulated Langmuir waveforms have been observed by several spacecraft in various regions of the heliosphere, such as the solar wind, the electron foreshock, the magnetotail, or the auroral ionosphere. Many observations revealed the bursty nature of these waves, which appear to be highly modulated, localized, and clumped into spikes with peak amplitudes typically 3 orders of magnitude above the mean. The paper presents Langmuir waveforms calculated using a Hamiltonian model describing self-consistently the resonant interaction of an electron beam with Langmuir wave packets in a plasma with random density fluctuations. These waveforms, obtained for different profiles of density fluctuations and ranges of parameters relevant to solar type III electron beams and plasmas measured at 1 AU, are presented in the form they would appear if recorded by a satellite moving in the solar wind. Comparison with recent measurements by the STEREO and WIND satellites shows that their characteristic features are very similar to the observations. (10.1002/2014JA020329)
DOI : 10.1002/2014JA020329
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 km at altitudes of 40007000 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 s after Cluster 4 at 13002000 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 SCW. (10.1002/2013JA019302)
DOI : 10.1002/2013JA019302
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
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
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
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−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 α line with a strong red shift, with an estimate of the initial plasma density of 1.3 × 1026 m−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
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≈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.
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.
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 (θk) 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-θk histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/fc,e≈0.5 (f is the chorus frequency, fc,e is the local electron cyclotron frequency) and at θk∼40°. Below θk∼40°, the average value for θk is predominantly field aligned, but slightly increasing with frequency toward half of fc,e (θk up to 20°). Above half of fc,e, the average θk is again decreasing with frequency. Above θk∼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 θk>40° and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at θk>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
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
Phase space structures in gyrokinetic simulations of plasma turbulence
- Ghendrih Philippe
- Norscini C.
- Cartier-Michaud T.
- Dif-Pradalier Guilhem
- Abiteboul J.
- Dong Yue
- Garbet X.
- Gürcan Özgür D.
- Hennequin Pascale
- Grandgirard Virginie
- Latu G.
- Morel Pierre
- Sarazin Y.
- Storelli A.
- Vermare Laure
The European Physical Journal D : Atomic, molecular, optical and plasma physics, EDP Sciences, 2014, 68 (10), pp.303. Gyrokinetic simulations of fusion plasmas give extensive information in 5D on turbulence and transport. This paper highlights a few of these challenging physics in global, flux driven simulations using experimental inputs from Tore Supra shot TS45511. The electrostatic gyrokinetic code GYSELA is used for these simulations. The 3D structure of avalanches indicates that these structures propagate radially at localised toroidal angles and then expand along the field line at sound speed to form the filaments. Analysing the poloidal mode structure of the potential fluctuations (at a given toroidal location), one finds that the low modes m = 0 and m = 1 exhibit a global structure; the magnitude of the m = 0 mode is much larger than that of the m = 1 mode. The shear layers of the corrugation structures are thus found to be dominated by the m = 0 contribution, that are comparable to that of the zonal flows. This global mode seems to localise the m = 2 mode but has little effect on the localisation of the higher mode numbers. However when analysing the pulsation of the latter modes one finds that all modes exhibit a similar phase velocity, comparable to the local zonal flow velocity. The consequent dispersion like relation between the modes pulsation and the mode numbers provides a means to measure the zonal flow. Temperature fluctuations and the turbulent heat flux are localised between the corrugation structures. Temperature fluctuations are found to exhibit two scales, small fluctuations that are localised by the corrugation shear layers, and appear to bounce back and forth radially, and large fluctuations, also readily observed on the flux, which are associated to the disruption of the corrugations. The radial ballistic velocity of both avalanche events if of the order of 0.5ρ∗c0 where ρ∗ = ρ0/a, a being the tokamak minor radius and ρ0 being the characteristic Larmor radius, ρ0 = c0/Ω0. c0 is the reference ion thermal velocity and Ω0 = qiB0/mi the reference ion Larmor frequency for the characteristic amplitude of the magnetic field B0, qi and mi being, respectively, the ion charge and mass. The electric drift velocity is also found to exhibit a poloidal pattern, with maximum amplitude of the fluctuations either in the top or in the bottom regions of the machine depending on the sign of the zonal flow shear. This effect is found to be correlated to the stopping capability of the corrugation structures. The neoclassical properties stemming from the trapped particle drifts lead to large distortion of the distribution function. As expected, these prevail at the outer part of the simulation region despite the large collisionality. The distribution function fluctuations appear to be aligned along the v∥ = const. lines at constant poloidal angle. A specific symmetry is observed regarding the interplay of turbulence with the trapped-passing region. (10.1140/epjd/e2014-50210-8)
DOI : 10.1140/epjd/e2014-50210-8
Kinetic Turbulence in the Terrestrial Magnetosheath: Cluster Observations
- Huang S. Y.
- Sahraoui Fouad
- Deng X. H.
- He J. S.
- Yuan Z. G.
- Zhou M.
- Pang Y.
- Fu H.S.
The Astrophysical Journal Letters, Bristol : IOP Publishing, 2014, 789, pp.L28. We present a first statistical study of subproton- and electron-scale turbulence in the terrestrial magnetosheath using waveform data measured by the Cluster/STAFF search coil magnetometer in the frequency range [1, 180] Hz. It is found that clear spectral breaks exist near the electron scale, which separate two power-law-like frequency bands referred to as the dispersive and the electron dissipation ranges. The frequencies of the breaks fb are shown to be well correlated with the electron gyroscale rho e rather than with the electron inertial length de . The distribution of the slopes below fb is found to be narrow and peaks near -2.9, while that of the slopes above fb is found to be broader, peaking near -5.2, with values as low as -7.5. This is the first time that such steep power-law spectra are reported in space plasma turbulence. These observations provide new constraints on theoretical modeling of kinetic turbulence and dissipation in collisionless magnetized plasmas. (10.1088/2041-8205/789/2/L28)
DOI : 10.1088/2041-8205/789/2/L28
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
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 km, with an median thickness of around 1400 km at dawn and around 1150 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

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