Publications

Sont listées ci-dessous, par année, les publications figurant dans l'archive ouverte HAL.

2016

Effect of gas properties on the dynamics of the electrical slope asymmetry effect in capacitive plasmas: comparison of Ar, H2 and CF4
- Bruneau Bastien
- Lafleur Trevor
- Gans T.
- O'Connell D.
- Greb Arthur
- Korolov Ihor
- Derzsi A.
- Donkó Z.
- Brandt S.
- Schüngel E.
- Schulze J.
- Diomede P.
- Economou D. J.
- Longo S.
- Johnson E.V.
- Booth Jean-Paul
Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (1), pp.01LT02. Tailored voltage excitation waveforms provide an efficient control of the ion energy (through the electrical asymmetry effect) in capacitive plasmas by varying the ?amplitude? asymmetry of the waveform. In this work, the effect of a ?slope? asymmetry of the waveform is investigated by using sawtooth-like waveforms, through which the sheath dynamic can be manipulated. A remarkably different discharge dynamic is found for Ar, H 2 , and CF 4 gases, which is explained by the different dominant electron heating mechanisms and plasma chemistries. In comparison to Argon we find that the electrical asymmetry can even be reversed by using an electronegative gas such as CF 4 . Phase resolved optical emission spectroscopy measurements, probing the spatiotemporal distribution of the excitation rate show excellent agreement with the results of particle-in-cell simulations, confirming the high degree of correlation between the excitation rates with the dominant heating mechanisms in the various gases. It is shown that, depending on the gas used, sawtooth-like voltage waveforms may cause a strong asymmetry. (10.1088/0963-0252/25/1/01LT02)
DOI : 10.1088/0963-0252/25/1/01LT02
Pressure broadening of atomic oxygen two-photon absorption laser induced fluorescence
- Marinov Daniil
- Drag Cyril
- Blondel Christophe
- Guaitella Olivier
- Golda Judith
- Klarenaar Bart
- Engeln Richard
- Gathen Volker Schulz-von Der
- Booth Jean-Paul
Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (6), pp.06LT03. Atomic oxygen, considered to be a determining reactant in plasma applications at ambient pressure, is routinely detected by two-photon absorption laser induced fluorescence (TALIF). Here, pressure broadening of the (2 p 4 3 P 2 ?????3 p 3 P J =0,1,2 ) two-photon transition in oxygen atoms was investigated using a high-resolution TALIF technique in normal and Doppler-free configurations. The pressure broadening coefficients determined were ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn001.gif] γ_\textO_2 ??=??0.40??±??0.08? cm ?1 /bar for oxygen molecules and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn002.gif] γ_\textHe ??=??0.46??±??0.03?cm ?1 /bar for helium atoms. These correspond to pressure broadening rate constants ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn003.gif] k_\textPB^\textO_2 ??=??9 · 10 ?9 cm 3 s ?1 and ##IMG## [http://ej.iop.org/images/0963-0252/25/6/06LT03/psstaa4481ieqn004.gif] k_\textPB^\textHe ??=??4 · 10 ?9 cm 3 s ?1 , respectively. The well-known quenching rate constants of O(3 p 3 P J ) by O 2 and He are at least one order of magnitude smaller, which signifies that non-quenching collisions constitute the main line-broadening mechanism. In addition to providing new insights into collisional processes of oxygen atoms in electronically excited 3 p 3 P J state, reported pressure broadening parameters are important for quantification of oxygen TALIF line profiles when both collisional and Doppler broadening mechanisms are important. Thus, the Doppler component (and hence the temperature of oxygen atoms) can be accurately determined from high resolution TALIF measurements in a broad range of conditions. (10.1088/0963-0252/25/6/06LT03)
DOI : 10.1088/0963-0252/25/6/06LT03
Controlling the shape of the ion energy distribution at constant ion flux and constant mean ion energy with tailored voltage waveforms
- Bruneau Bastien
- Lafleur Trevor
- Booth Jean-Paul
- Johnson Erik
Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025006. In this paper, we investigate the excitation of a capacitively coupled plasma using a non-sinusoidal voltage waveform whose amplitude- and slope-asymmetry varies continuously with a period which is a multiple of the fundamental RF period. We call this period the ?beating? period. Through particle-in-cell (PIC) simulations, we show that such waveforms cause oscillation of the self-bias at this beating frequency, corresponding to the charging and discharging of the external capacitor. The amplitude of this self-bias oscillation depends on the beating period, the value of the external capacitor, and the ion flux to the electrodes. This self-bias oscillation causes temporal modulation of the ion flux distribution function (IFDF), albeit at a constant ion flux and constant mean ion energy, and allows the energy width of the IFDF (averaged over the beating period) to be varied in a controlled fashion. (10.1088/0963-0252/25/2/025006)
DOI : 10.1088/0963-0252/25/2/025006
Solar quiet current response in the African sector due to a 2009 sudden stratospheric warming event
- Bolaji O. S.
- Oyeyemi E. O.
- Owolabi O. P.
- Yamazaki Y.
- Rabiu A. B.
- Okoh D.
- Fujimoto A.
- Amory-Mazaudier Christine
- Seemala G. K.
- Yoshikawa A.
- Onanuga O. K.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016. We present solar quiet (S q) variation of the horizontal (H) magnetic field intensity deduced from Magnetic Data Acquisition System (MAGDAS) records over Africa during an unusual strong and prolonged 2009 sudden stratospheric warming (SSW) event. A reduction in the S q H magnitude that enveloped the geomagnetic latitudes between 21.13°N (Fayum FYM) in Egypt and 39.51°S (Durban DRB) in South Africa was observed, while the stratospheric polar temperature was increasing and got strengthened when the stratospheric temperature reached its maximum. Another novel feature associated with the hemispheric reduction is the reversal in the north-south asymmetry of the S q H, which is indicative of higher S q H magnitude in the Northern Hemisphere compared to the Southern Hemisphere during SSW peak phase. The reversal of the equatorial electrojet (EEJ) or the counter electrojet (CEJ) was observed after the polar stratospheric temperature reached its maximum. The effect of additional currents associated with CEJ was observed in the Southern Hemisphere at middle latitude. Similar changes were observed in the EEJ at the South America, Pacific Ocean, and Central Asia sectors. The effect of the SSW is largest in the South American sector and smallest in the Central Asian sector. (10.1002/2016JA022857)
DOI : 10.1002/2016JA022857
The 2π charged particles analyzer: All-sky camera concept and development for space missions
- Vaisberg Oleg
- Berthelier Jean-Jacques
- Moore T.
- Avanov L.
- Leblanc François
- Leblanc Frédéric
- Moiseev Pavel P.
- Moiseenko D.
- Becker Joël
- Collier Michael R.
- Laky G.
- Keller J.
- Koynash G.
- Lichtenneger H.
- Leibov A. W.
- Zhuravlev R.
- Shestakov A.
- Burch J.
- Mccomas D.
- Shuvalov S.
- Chornay D.
- Torkar K.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (12), pp.11,750–11,765. Increasing the temporal resolution and instant coverage of velocity space of space plasma measurements is one of the key issues for experimentalists. Today the top-hat plasma analyzer appears to be the favorite solution due to its relative simplicity and the possibility to extend its application by adding a mass-analysis section and an electrostatic angular scanner. Similarly, great success has been achieved in MMS mission using such multiple top-hat analyzers to achieve unprecedented temporal resolution. An instantaneous angular coverage of charged particles measurements is an alternative approach to pursuing the goal of high time resolution. This was done with FONEMA 4-D and, to a lesser extent, by DYMIO instruments for Mars-96 and with the FIPS instrument for MESSENGER mission. In this paper we describe, along with precursors, a plasma analyzer with a 2π electrostatic mirror that was developed originally for the Phobos-Soil mission with a follow-up in the frame of the BepiColombo mission, and is under development for future Russian missions. Different versions of instrument are discussed along with their advantages and drawbacks. (10.1002/2016JA022568)
DOI : 10.1002/2016JA022568
Observations of turbulence in a Kelvin-Helmholtz event on 8 September 2015 by the Magnetospheric Multiscale mission
- Stawarz J. E.
- Eriksson S.
- Wilder F. D.
- Ergun R. E.
- Schwartz S. J.
- Pouquet A.
- Burch J. L.
- Giles B. L.
- Khotyaintsev Y. V.
- Le Contel Olivier
- Lindqvist P.-A.
- Magnes W.
- Pollock C. J.
- Russell C. T.
- Strangeway R. J.
- Torbert R. B.
- Avanov L. A.
- Dorelli J. C.
- Eastwood Jonathan P.
- Gershman D. J.
- Goodrich K. A.
- Malaspina D. M.
- Marklund G. T.
- Mirioni Laurent
- Sturner A. P.
Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (11), pp.11,021-11,034. Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI. (10.1002/2016JA023458)
DOI : 10.1002/2016JA023458
Electron Acceleration by Langmuir Waves Produced by a Decay Cascade
- Krafft C.
- Volokitin A. S.
The Astrophysical Journal, American Astronomical Society, 2016, 821 (2), pp.99. It was recently reported that a significant part of the Langmuir waveforms observed by the STEREO satellite during type III solar radio bursts are likely consistent with the occurrence of electrostatic decay instabilities, when a Langmuir wave L resonantly interacts with another Langmuir wave L \prime and an ion sound wave S \prime through the decay channel L \to L \prime S \prime . Usually such wave-wave interactions occur in regions of the solar wind where the presence of electron beams can drive Langmuir turbulence to levels allowing waves L to decay. Moreover, such solar wind plasmas can present long-wavelength, randomly fluctuating density inhomogeneities or monotonic density gradients which can significantly modify the development of such resonant instabilities. If some conditions are met, the waves can encounter a second decay cascade (SDC) according to L \prime \to L \prime\prime S \prime\prime . Analytical estimates and observations based on numerical simulations show that the Langmuir waves L \prime\prime produced by this SDC can accelerate beam particles up to velocities and kinetic energies exceeding two times the beam drift velocity vb and half the initial beam energy, respectively. Moreover, this process can be particularly efficient if the scattering effects of waves on the background plasma inhomogeneities have already accelerated a sufficient amount of beam electrons up to the velocity range where the phase velocities of the L \prime\prime waves are lying. The paper shows that the conditions necessary for such process to occur can be easily met in solar wind plasmas if the beam velocities do not exceed around 35 times the plasma thermal velocity. (10.3847/0004-637X/821/2/99)
DOI : 10.3847/0004-637X/821/2/99
Diffusion of Energetic Electrons in Turbulent Plasmas of the Solar Wind
- Volokitin A. S.
- Krafft C.
The Astrophysical Journal, American Astronomical Society, 2016, 833 (2), pp.166. A method of calculation of the diffusion coefficients D (v) of particles in velocity space, based on the statistical analysis of the motion of a great number of test electrons, is proposed. In the case of Langmuir turbulence developing in plasmas with fluctuating density inhomogeneities such as the solar wind, simulations provide coefficients D (v) which mainly depend on the Langmuir wave spectra and agree well with the analytical predictions D th(v) of the quasilinear theory of weak turbulence. Nevertheless, some noticeable differences exist with this theory: in the range of phase velocity of the short waves where the main part of the wave energy is concentrated, D (v) is noticeably smaller than D th(v), due to the scattering, the reflection, and the focusing effects encountered by the Langmuir waves when they interact with the plasma density inhomogeneities. Moreover, the probability of large velocity jumps in the particles' trajectories essentially exceeds the probability of a Gaussian distribution. These large jumps, which are connected with the waves' transformation processes, modify the nature of the particle diffusion, which is no more classical. These higher order effects cause the discrepancies observed with the quasilinear theory, which does not take them into account in its perturbative approach. The solar wind plasmas, which present fluctuating density inhomogeneities of noticeable average levels, are a very good laboratory to study such diffusion processes, which can eventually influence significantly on the development of essential physical phenomena, as electromagnetic radio emissions by type III solar radio bursts, for example. (10.3847/1538-4357/833/2/166)
DOI : 10.3847/1538-4357/833/2/166
Improved Design of a Multistage Axial Vircator With Reflectors for Enhanced Performances
- Champeaux Stéphanie
- Gouard Philippe
- Cousin Richard
- Larour Jean
IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (1), pp.31-38. The basic design of an axial virtual cathode oscillator (vircator) with axial extraction operating in TM01 mode is modified by introducing thin conducting disks, also called reflectors, into the cylindrical waveguide. The operation principal of this novel type of device relies on the formation of a series of virtual cathodes, located at the center of adjacent quasi-cavities. The behavior of this new type of multistage vircator is numerically investigated using CST Particle Studio 3-D particle-in-cell code. Progressively decreasing the radii of the reflectors installed upstream in the tube allows the mitigation of spurious modes. Tapering the radii of the reflectors turns out to be crucial in focusing the electron beam on axis in the downstream region and maximizing the TM01 power conversion efficiency. This novel architecture enables a five-reflector vircator operating with an injected electron beam of 508-kV mean voltage and 19-kA mean current to deliver up to 2-GW mean power sustained only by the TM01 mode in the S-band with a power conversion efficiency close to 21%. (10.1109/TPS.2015.2502432)
DOI : 10.1109/TPS.2015.2502432
Turbulence Heating ObserveR - satellite mission proposal
- Vaivads A.
- Retinò Alessandro
- Soucek J.
- Khotyaintsev Yu. V. V
- Valentini F.
- Escoubet C. Philippe
- Alexandrova Olga
- André M.
- Bale S. D.
- Balikhin M. A.
- Burgess D.
- Camporeale E.
- Caprioli D.
- Chen C. H. K.
- Clacey E.
- Cully C. M.
- de Keyser J.
- Eastwood Jonathan P.
- Fazakerley A. N.
- Eriksson S.
- Goldstein M. L.
- Graham D. B.
- Haaland S.
- Hoshino M.
- Ji H.
- Karimabadi H.
- Kucharek H.
- Lavraud B.
- Marcucci F.
- Matthaeus W. H.
- Moore T. E.
- Nakamura R.
- Narita Y.
- Nemecek Z.
- Norgren C.
- Opgenoorth H.
- Palmroth M.
- Perrone D.
- Pinçon Jean-Louis
- Rathsman P.
- Rothkaehl H.
- Sahraoui Fouad
- Servidio S.
- Sorriso-Valvo L.
- Vainio R.
- Vörös Z.
- Wimmer-Schweingruber R. F.
Journal of Plasma Physics, Cambridge University Press (CUP), 2016, 82 (5), pp.905820501 (16 pages). The Universe is permeated by hot, turbulent, magnetized plasmas. Turbulent plasma is a major constituent of active galactic nuclei, supernova remnants, the intergalactic and interstellar medium, the solar corona, the solar wind and the Earth's magnetosphere, just to mention a few examples. Energy dissipation of turbulent fluctuations plays a key role in plasma heating and energization, yet we still do not understand the underlying physical mechanisms involved. THOR is a mission designed to answer the questions of how turbulent plasma is heated and particles accelerated, how the dissipated energy is partitioned and how dissipation operates in different regimes of turbulence. THOR is a single-spacecraft mission with an orbit tuned to maximize data return from regions in near-Earth space - magnetosheath, shock, foreshock and pristine solar wind - featuring different kinds of turbulence. Here we summarize the THOR proposal submitted on 15 January 2015 to the `Call for a Medium-size mission opportunity in ESAs Science Programme for a launch in 2025 (M4)'. THOR has been selected by European Space Agency (ESA) for the study phase. (10.1017/S0022377816000775)
DOI : 10.1017/S0022377816000775
Transport of Solar Wind H+ and He++ Ions across Earth's Bow Shock
- Parks G. K.
- Lee E.
- Fu S. Y.
- Kim H. E.
- Ma Y. Q.
- Yang Z. W.
- Liu Y.
- Lin N.
- Hong J.
- Canu Patrick
- Dandouras I.
- Rème H.
- Goldstein M. L.
The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 825 (2), pp.L27. We have investigated the dependence of mass, energy, and charge of solar wind (SW) transport across Earth?s bow shock. An examination of 111 crossings during quiet SW in both quasi-perpendicular and quasi-parallel shock regions shows that 64 crossings had various degrees of heating and thermalization of SW. We found 22 crossings where the SW speed was <400 km s ?1 . The shock potential of a typical supercritical quasi-perpendicular shock estimated from deceleration of the SW and cutoff energy of electron flat top distribution is ?50 Volts. We find that the temperatures of H and He beams that penetrate the shock can sometimes be nearly the same in the upstream and downstream regions, indicating little or no heating had occurred crossing the bow shock. None of the models predict that the SW can cross the bow shock without heating. Our observations are important constraints for new models of collisionless shocks. (10.3847/2041-8205/825/2/L27)
DOI : 10.3847/2041-8205/825/2/L27
Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause
- Lavraud B.
- Zhang Y. C.
- Vernisse Y.
- Gershman D. J.
- Dorelli J. C.
- Cassak P. A.
- Dargent Jérémy
- Pollock C.
- Giles B. L.
- Aunai Nicolas
- Argall M.
- Avanov L.
- Barrie A.
- Burch J. L.
- Chandler Michael O.
- Chen L.-J.
- Clark G.
- Cohen I.
- Coffey Victoria
- Eastwood Jonathan P.
- Egedal J.
- Eriksson S.
- Ergun R.
- Farrugia C. J.
- Fuselier S. A.
- Génot V.
- Graham Daniel B.
- Grigorenko E. E.
- Hasegawa H.
- Jacquey C.
- Kacem I.
- Khotyaintsev Y. V.
- Macdonald E.
- Magnes W.
- Marchaudon A.
- Mauk B.
- Moore T. E.
- Mukai Toshifumi
- Nakamura R.
- Paterson W. R.
- Penou E.
- Phan T. D.
- Rager A.
- Retinò Alessandro
- Rong Z. J.
- Russell C. T.
- Saito Y.
- Sauvaud J.-A.
- Schwartz S. J.
- Shen C.
- Smith S.
- Strangeway R. J.
- Toledo-Redondo Sergio
- Torbert R.
- Turner D. L.
- Wang S.
- Yokota S.
Geophysical Research Letters, American Geophysical Union, 2016, 43 (7), pp.3042-3050. Based on high-resolution measurements from NASA's Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earth's magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials). (10.1002/2016GL068359)
DOI : 10.1002/2016GL068359
Cold ion demagnetization near the X-line of magnetic reconnection
- Toledo-Redondo Sergio
- André M.
- Khotyaintsev Y. V.
- Vaivads A.
- Walsh Andrew P.
- Li Wenya
- Graham Daniel B.
- Lavraud Benoit
- Masson A.
- Aunai Nicolas
- Divin A. V.
- Dargent Jérémy
- Fuselier Stephen
- Gershman D. J.
- Dorelli J. C.
- Giles B. L.
- Avanov L.
- Pollock Craig
- Saito Y.
- Moore T. E.
- Coffey Victoria
- Chandler Michael O.
- Lindqvist Per-Arne
- Torbert Roy
- Russell Christopher T.
Geophysical Research Letters, American Geophysical Union, 2016, 43 (13), pp.6759-6767. Although the effects of magnetic reconnection in magnetospheres can be observed at planetary scales, reconnection is initiated at electron scales in a plasma. Surrounding the electron diffusion region, there is an Ion-Decoupling Region (IDR) of the size of the ion length scales (inertial length and gyroradius). Reconnection at the Earth's magnetopause often includes cold magnetospheric (few tens of eV), hot magnetospheric (10 keV), and magnetosheath (1 keV) ions, with different gyroradius length scales. We report observations of a subregion inside the IDR of the size of the cold ion population gyroradius (15 km) where the cold ions are demagnetized and accelerated parallel to the Hall electric field. Outside the subregion, cold ions follow the E × B motion together with electrons, while hot ions are demagnetized. We observe a sharp cold ion density gradient separating the two regions, which we identify as the cold and hot IDRs. (10.1002/2016GL069877)
DOI : 10.1002/2016GL069877
Introduction to Modern Magnetohydrodynamics
- Galtier Sébastien
, 2016. Preface; Table of physical quantities; Part I. Foundations: 1. Introduction; 2. Magnetohydrodynamics; 3. Conservation laws; Part II. Fundamental Processes: 4. Magnetohydrodynamic waves; 5. Dynamo; 6. Discontinuities and shocks; 7. Magnetic reconnection; Part III. Instabilities and Magnetic Confinement: 8. Static equilibrium; 9. Linear perturbation theory; 10. Study of MHD instabilities; Part IV. Turbulence: 11. Hydrodynamic turbulence; 12. MHD turbulence; 13. Advanced MHD turbulence; Appendix 1. Solutions to the exercises; Appendix 2. Formulary; References; Index.
On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence
- Huang S. Y.
- Sahraoui Fouad
- Hadid L. Z.
- Yuan Z. G.
, 2016, 18, pp.EPSC2016-6911. In solar wind, it is well established that at MHD scales the magnetic energy spectra generally follow the so-called Kolmogorov inertial range spectrum f-5/3 in solar wind. In this study, we used three years data from the Cluster mission to statistically investigate the existence of the Kolmogorov inertial range in the whole magnetosheath, including flanks and subsolar regions. Statistical results show that Kolmogorov inertial range is not ubiquitous in the magnetosheath. Indeed, most spectra were found to be shallower than the Kolmogorov one and have a scaling f-1recalling the energy containing scales of solarwind turbulence. The Kolmogorov scaling is observed only faraway from the bow shock and in the flanks region. These results suggest that random-like fluctuations are generated behind the shock, which reach a fully developed turbulence state only after some time corresponding to their propagation (or advection) away from the shock. These fluctuations are shown to be essentially compressible and not Alfvenic as in the case of the solar wind. Implications on the new theoretical modeling of space plasma turbulence will be discussed.
Observations of cold ion heating inside the magnetospheric separatrix region
- Toledo-Redondo Sergio
- Andre M.
- Vaivads A.
- Khotyaintsev Y. V.
- Lavraud Benoit
- Graham Daniel
- Divin A. V.
- Aunai N.
, 2016, 18, pp.EPSC2016-14007. Several studies have shown that cold ions (energies up to tens of eV) of ionospheric origin can be found in different regions of the magnetosphere, including the dayside magnetopause. They can be very abundant, up to ~100 cc, e.g. plasmaspheric plumes, and become the dominant population of the magnetosphere. Cold ions, when present, participate in magnetic reconnection at the dayside magnetopause, mass loading the magnetospheric side and adding a new length-scale into the system owing to their smaller gyroradius. At the same time, reconnection accelerates and heats the cold ions. Based on multi-spacecraft observations, we report observations of cold ion heating inside the separatrix region when reconnection is ongoing and study the mechanisms that energize the cold ions. The heating is not always observed and our observations indicate that cold ion heating is more effective next to the X-line. We find that large electric field gradients and wave-particle interactions are consistent with the heating observed.
Impact of the Eulerian chaos of magnetic field lines in magnetic reconnection
- Firpo Marie-Christine
- Ettoumi Wahb
- Lifschitz A. F.
- Retinò Alessandro
- Farengo R F
- Ferrari H E
- García-Martínez P L
Physics of Plasmas, American Institute of Physics, 2016, 23 (12). Stochasticity is an ingredient that may allow the breaking of the frozen-in law in the reconnec-tion process. It will first be argued that non-ideal effects may be considered as an implicit way to introduce stochasticity. Yet there also exists an explicit stochasticity that does not require the invocation of non-ideal effects. This comes from the spatial (or Eulerian) chaos of magnetic field lines that can show up only in a truly three-dimensional description of magnetic reconnection since two-dimensional models impose the integrability of the magnetic field lines. Some implications of this magnetic braiding, such as the increased particle finite-time Lyapunov exponents and increased acceleration of charged particles, are discussed in the frame of tokamak sawteeth that form a laboratory prototype of spontaneous magnetic reconnection. A justification for an increased reconnection rate with chaotic vs integrable magnetic field lines is proposed. Moreover, in 3D, the Eulerian chaos of magnetic field lines may coexist with the Eulerian chaos of velocity field lines, that is more commonly named turbulence. (10.1063/1.4972544)
DOI : 10.1063/1.4972544
Emission spectroscopy of partial discharges in air-filled voids in Unfilled epoxy
- Shcherbanev S.A.
- Nadinov I.U.
- Auvray Philippe
- Starikovskaia Svetlana
- Pancheshnyi Sergey V.
- Herrmann L.G.
IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (7), pp.1219-1227. Emission optical spectroscopy is used as a technique to study partial discharges (PDs) in four unfilled epoxy samples encompassing an artificial air-filled cavity. It is shown that emission spectroscopy can be used to estimate the density and the chemical composition of a gas from the spectrally resolved emission and time-resolved pulse shape of the PDs at any time during the aging process. Two scenarios are observed: either PD continues until the sample breaks down (observed in 1 out of 4 samples) or PD stops at a certain point without sample breakdown (3 out of 4 samples). For both the scenarios, a stable initial phase with a gradual decrease of emission intensity from the discharge is typical for a few hundreds of hours of continuous discharge operation. At this stage, the spectrum of the second positive system of molecular nitrogen dominates in the entire spectral range of 350500 nm studied in this work. Furthermore, time-resolved measurements indicate two types of discharges of very different frequencies and magnitudes as well as a decrease of the pressure in the voids as a function of aging time. Then, a sharp 500% increase of the N2 emission is observed 2 days before the breakdown; during the last day a spectrum of CO and some other C-N-O-H containing molecules is observed instead of the spectrum of molecular nitrogen. This allows predicting a breakdown at least a few hours before it happens by analyzing the broad emission spectra behavior. Additionally, the possible role of surface conductivity increase during aging on PD inhibition is discussed. (10.1109/TPS.2016.2576560)
DOI : 10.1109/TPS.2016.2576560
A computational analysis of the vibrational levels of molecular oxygen in low-pressure stationary and transient radio-frequency oxygen plasma
- Kemaneci Efe
- Booth Jean-Paul
- Chabert Pascal
- van Dijk Jan
- Mussenbrock Thomas
- Brinkmann Ralf Peter
Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025025. Vibrational levels of molecular oxygen, O 2 ( v   <  42), are investigated in continuous and pulse-modulated low-pressure radio-frequency oxygen plasma with a global modelling approach. The model is benchmarked against a variety of pressure-, power- and time-resolved measurements of several inductive and asymmetric capacitive discharges available in the literature, and a good agreement is obtained. The sensitivity of the model with respect to the vibrational kinetics, the wall reactions and the spatial inhomogeneity of the charged particles are presented. The simulations without the vibrational levels are also shown for the sake of comparison. (10.1088/0963-0252/25/2/025025)
DOI : 10.1088/0963-0252/25/2/025025
Influence of the angular scattering of electrons on the runaway threshold in air
- Chanrion Olivier
- Bonaventura Z.
- Bourdon Anne
- Neubert Torsten
Plasma Physics and Controlled Fusion, IOP Publishing, 2016, 58, pp.044001. The runaway electron mechanism is of great importance for the understanding of the generation of x- and gamma rays in atmospheric discharges. In 1991, terrestrial gamma-ray flashes (TGFs) were discovered by the Compton Gamma-Ray Observatory. Those emissions are bremsstrahlung from high energy electrons that run away in electric fields associated with thunderstorms. In this paper, we discuss the runaway threshold definition with a particular interest in the influence of the angular scattering for electron energy close to the threshold. In order to understand the mechanism of runaway, we compare the outcome of different FokkerPlanck and Monte Carlo models with increasing complexity in the description of the scattering. The results show that the inclusion of the stochastic nature of collisions smooths the probability to run away around the threshold. Furthermore, we observe that a significant number of electrons diffuse out of the runaway regime when we take into account the diffusion in angle due to the scattering. Those results suggest using a runaway threshold energy based on the FokkerPlanck model assuming the angular equilibrium that is 1.6 to 1.8 times higher than the one proposed by [1, 2], depending on the magnitude of the ambient electric field. The threshold also is found to be 5 to 26 times higher than the one assuming forward scattering. We give a fitted formula for the threshold field valid over a large range of electric fields. Furthermore, we have shown that the assumption of forward scattering is not valid below 1 MeV where the runaway threshold usually is defined. These results are important for the thermal runaway and the runaway electron avalanche discharge mechanisms suggested to participate in the TGF generation. (10.1088/0741-3335/58/4/044001)
DOI : 10.1088/0741-3335/58/4/044001
Larger sized planar wire arrays of complex configuration on 1.51.8 MA Z-pinch generator
- Safronova Alla S.
- Kantsyrev Viktor L.
- Weller Michael E.
- Shlyaptseva V. V.
- Shrestha I. K.
- Stafford A.
- Schmidt-Petersen M. T.
- Lorance M. Y.
- Schultz K. A.
- Chuvatin Alexandre S.
Physics of Plasmas, American Institute of Physics, 2016, 23, pp.101210. Two new approaches of (i) simultaneous study of implosion and radiative characteristics of different materials in wire array Z-pinch plasmas in one shot and (ii) investigation of larger sized wire arrays (to enhance energy coupling to plasmas and provide better diagnostic access) were developed in experiments with 1.51.8 MA Zebra with a Load Current Multiplier. In particular, the larger sized multi-plane Planar Wire Arrays with two outer planes placed at 9 and 15 mm from each other and then as far as at 19 mm (compared with 6 mm studied before at standard 1 MA current) and with a modified central plane with 8 to 12 empty slots were investigated. Though K-shell Al and L-shell Ni, Cu plasmas have similar electron temperatures and densities, the ablation dynamics and radiation of Al and Ni, Cu planes are somewhat different, which was investigated in detail using the full set of diagnostics and modeling. Advantages of using such wire arrays at higher currents to study plasma flow and radiation from different materials and jets are highlighted. (10.1063/1.4965239)
DOI : 10.1063/1.4965239
Double and Single Planar Wire Arrays on University-Scale Low-Impedance LTD Generator
- Safronova Alla S.
- Kantsyrev Viktor L.
- Weller Michael E.
- Shlyaptseva V. V.
- Shrestha I. K.
- Lorance M. Y.
- Schmidt-Petersen M. T.
- Stafford A.
- Cooper M. C.
- Steiner A. M.
- Yager-Elorriaga D. A.
- Patel S. G.
- Jordan N. M.
- Gilgenbach R. M.
- Chuvatin Alexandre S.
IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers, 2016, 44 (4), pp.432-440. Planar wire array (PWA) experiments were performed on Michigan Accelerator for Inductive Z-pinch Experiments, the University of Michigan's low-impedance linear transformer driver (LTD)-driven generator (0.1 Ω, 0.5-1 MA, and 100-200 ns), for the first time. It was demonstrated that Al wire arrays [both double PWA (DPWA) and single PWA (SPWA)] can be successfully imploded at LTD generator even at the relatively low current of 0.3-0.5 MA. In particular, implosion characteristics and radiative properties of PWAs of different load configurations [for DPWA from Al and stainless steel wires with different wire diameters, interwire gaps, and interplanar gaps (IPGs) and for Al SPWA of different array widths and number of wires] were studied. The major difference from the DPWA experiments on high-impedance Zebra accelerator was in the current rise time that was influenced by the load inductance and was increased up to about 150 ns during the first campaign (and was even longer in the second campaign). The implosion dynamics of DPWAs strongly depends on the critical load parameter, the aspect ratio (the ratio of the array width to IPG) as for Al DPWAs on high-impedance Zebra, but some differences were observed, for low-aspect ratio loads in particular. Analysis of X-ray images and spectroscopy indicates that K-shell Al plasmas from Al PWAs reached the electron temperatures up to more than 450 eV and densities up to 2 x 10²⁰ cm⁻³. Despite the low mass of the loads, opacity effects were observed in the most prominent K-shell Al lines almost in every shot. (10.1109/TPS.2016.2538291)
DOI : 10.1109/TPS.2016.2538291
Orientation of the X-line in asymmetric magnetic reconnection
- Aunai Nicolas
- Hesse Michael
- Lavraud B.
- Dargent Jérémy
- Smets Roch
Journal of Plasma Physics, Cambridge University Press (CUP), 2016, 82 (4), pp.535820401. Magnetic reconnection can occur in current sheets separating magnetic fields sheared by any angle and of arbitrarily different amplitudes. In such asymmetric and non-coplanar systems, it is not yet understood what the orientation of the X-line will be. Studying how this orientation is determined locally by the reconnection process is important to understand systems such as the Earth magnetopause, where reconnection occurs in regions with large differences in upstream plasma and field properties. This study aims at determining what the local X-line orientation is for different upstream magnetic shear angles in an asymmetric set-up relevant to the Earth's magnetopause. We use two-dimensional hybrid simulations and vary the simulation plane orientation with regard to the fixed magnetic field profile and search for the plane maximizing the reconnection rate. We find that the plane defined by the bisector of upstream fields maximizes the reconnection rate and this appears not to depend on the magnetic shear angle, domain size or upstream plasma and asymmetries. (10.1017/S0022377816000647)
DOI : 10.1017/S0022377816000647
Electron scale structures and magnetic reconnection signatures in the turbulent magnetosheath
- Yordanova E.
- Vörös Z.
- Varsani A.
- Graham D. B.
- Norgren C.
- Khotyaintsev Y. V.
- Vaivads A.
- Eriksson E.
- Nakamura R.
- Lindqvist P.-A.
- Marklund G.
- Ergun R. E.
- Magnes W.
- Baumjohann W.
- Fischer D.
- Plaschke F.
- Narita Y.
- Russell C. T.
- Strangeway R. J.
- Le Contel Olivier
- Pollock C.
- Torbert R. B.
- Giles B. J.
- Burch J. L.
- Avanov L. A.
- Dorelli J. C.
- Gershman D. J.
- Paterson W. R.
- Lavraud B.
- Saito Y.
Geophysical Research Letters, American Geophysical Union, 2016, 43 (12), pp.5969-5978. Collisionless space plasma turbulence can generate reconnecting thin current sheets as suggested by recent results of numerical magnetohydrodynamic simulations. The Magnetospheric Multiscale (MMS) mission provides the first serious opportunity to verify whether small ion-electron-scale reconnection, generated by turbulence, resembles the reconnection events frequently observed in the magnetotail or at the magnetopause. Here we investigate field and particle observations obtained by the MMS fleet in the turbulent terrestrial magnetosheath behind quasi-parallel bow shock geometry. We observe multiple small-scale current sheets during the event and present a detailed look of one of the detected structures. The emergence of thin current sheets can lead to electron scale structures. Within these structures, we see signatures of ion demagnetization, electron jets, electron heating, and agyrotropy suggesting that MMS spacecraft observe reconnection at these scales. (10.1002/2016GL069191)
DOI : 10.1002/2016GL069191
Ion injection at Quasi-parallel Shocks Seen by the Cluster Spacecraft
- Johlander A.
- Vaivads A.
- Khotyaintsev Y. V.
- Retinò Alessandro
- Dandouras I.
The Astrophysical Journal Letters, Bristol : IOP Publishing, 2016, 817 (1), pp.L4. Collisionless shocks in space plasma are known to be capable of accelerating ions to very high energies through diffusive shock acceleration (DSA). This process requires an injection of suprathermal ions, but the mechanisms producing such a suprathermal ion seed population are still not fully understood. We study acceleration of solar wind ions resulting from reflection off short large-amplitude magnetic structures (SLAMSs) in the quasi-parallel bow shock of Earth using in situ data from the four Cluster spacecraft. Nearly specularly reflected solar wind ions are observed just upstream of a SLAMS. The reflected ions are undergoing shock drift acceleration (SDA) and obtain energies higher than the solar wind energy upstream of the SLAMS. Our test particle simulations show that solar wind ions with lower energy are more likely to be reflected off the SLAMS, while high-energy ions pass through the SLAMS, which is consistent with the observations. The process of SDA at SLAMSs can provide an effective way of accelerating solar wind ions to suprathermal energies. Therefore, this could be a mechanism of ion injection into DSA in astrophysical plasmas. (10.3847/2041-8205/817/1/L4)
DOI : 10.3847/2041-8205/817/1/L4

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