Publications

2018

• Numerical study of the influence of surface reaction probabilities on reactive species in an rf atmospheric pressure plasma containing humidity
  
  • Schröter Sandra
  • Gibson Andrew R.
  • Kushner Mark J.
  • Gans Timo
  • O'Connell Deborah
  Plasma Physics and Controlled Fusion, IOP Publishing, 2018, 60. The quantification and control of reactive species (RS) in atmospheric pressure plasmas (APPs) is of great interest for their technological applications, in particular in biomedicine. Of key importance in simulating the densities of these species are fundamental data on their production and destruction. In particular, data concerning particle-surface reaction probabilities in APPs are scarce, with most of these probabilities measured in low-pressure systems. In this work, the role of surface reaction probabilities, gamma, of reactive neutral species (H, O and OH) on neutral particle densities in a He-H<SUB>2</SUB>O radio-frequency micro APP jet (COST-mu APPJ) are investigated using a global model. It is found that the choice of gamma, particularly for low-mass species having large diffusivities, such as H, can change computed species densities significantly. The importance of gamma even at elevated pressures offers potential for tailoring the RS composition of atmospheric pressure microplasmas by choosing different wall materials or plasma geometries. (10.1088/1361-6587/aa8fe9)
  DOI : 10.1088/1361-6587/aa8fe9
• A turbulent cascade model of bounce averaged gyrokinetics
  
  • Xu S.
  • Morel Pierre
  • Gürcan Özgür D.
  Physics of Plasmas, American Institute of Physics, 2018, 25 (2), pp.022304. A shell model is derived for the description of nonlinear bounce averaged gyrokinetics, which is one of the simplest kinetic descriptions in magnetized plasmas. In order to validate the numerical implementation, detailed linear evolution of the system is compared with a linear benchmark based on solving the linear dispersion relation numerically. The resulting wave number spectrum, which extends over 34 decades, is shown to have a robust general structure to model parameters, such as dissipation or the ratio of linear energy injection to nonlinear transfer. In a range of wave numbers where the nonlinear transfer term is dominant, a power law spectrum, roughly of the form k&#8722;4 , is observed for the spectral electrostatic potential energy density. The model, being fully kinetic, can be used to obtain the free energy spectra for ion and electron distribution functions as functions of E. This model constitutes the first numerical implementation of a kinetic shell model. (10.1063/1.5020145)
  DOI : 10.1063/1.5020145
• Reply to Comment on `The case for in situ resource utilisation for oxygen production on Mars by non-equilibrium plasmas
  
  • Guerra Vasco
  • Silva Tiago
  • Ogloblina Polina
  • Grofulovic Marija
  • Terraz Loann
  • Lino da Silva Mário
  • Pintassilgo Carlos D.
  • Alves Luís L.
  • Guaitella Olivier
  Plasma Sources Science and Technology, IOP Publishing, 2018, 27. Not Available (10.1088/1361-6595/aaa570)
  DOI : 10.1088/1361-6595/aaa570
• Study of Ionospheric Variability Using GNSS Observations
  
  • Jouan Taoufiq
  • Bouziani Mourad
  • Azzouzi Rachid
  • Amory-Mazaudier Christine
  Positioning, SCIRP, 2018 (9), pp.79-96. With the increasing number of applications of Global navigation satellite system, the modeling of the ionosphere is a crucial element for precise positioning. Indeed, the ionosphere delays the electromagnetic waves which pass through it and induces a delay of propagation related to the electronic density (TEC) Total Electronic Content and to the frequency of the wave. The impact of this ionospheric error often results in a poor determination of the stations position, particularly in strong solar activity. The first part of this paper focuses on a bibliographic study oriented first of all on the study of the ionosphere in relation to solar activity and secondly on the determination of the total electron content using GNSS measurements from the IGS network reference stations. Measurements were made on two permanent stations RABT, TETN. We selected years of GNSS measurements to evaluate the geomagnetic impact on the ionosphere, 2001, 2009 and 2013. A description of the ionospheric disturbances and geomagnetic storms was analyzed by determination of TEC, especially in high solar activity. The results show a strong dependence of the ionospheric activity with the geomagnetic activity. (10.4236/pos.2018.94006)
  DOI : 10.4236/pos.2018.94006
• Analyzing the Magnetopause Internal Structure: New Possibilities Offered by MMS Tested in a Case Study
  
  • Rezeau Laurence
  • Belmont Gérard
  • Manuzzo Roberto
  • Aunai Nicolas
  • Dargent Jérémy
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018, 123 (1), pp.227-241. We explore the structure of the magnetopause using a crossing observed by the Magnetospheric Multiscale (MMS) spacecraft on 16 October 2015. Several methods (minimum variance analysis, BV method, and constant velocity analysis) are first applied to compute the normal to the magnetopause considered as a whole. The different results obtained are not identical, and we show that the whole boundary is not stationary and not planar, so that basic assumptions of these methods are not well satisfied. We then analyze more finely the internal structure for investigating the departures from planarity. Using the basic mathematical definition of what is a one‐dimensional physical problem, we introduce a new single spacecraft method, called LNA (local normal analysis) for determining the varying normal, and we compare the results so obtained with those coming from the multispacecraft minimum directional derivative (MDD) tool developed by Shi et al. (2005). This last method gives the dimensionality of the magnetic variations from multipoint measurements and also allows estimating the direction of the local normal when the variations are locally 1‐D. This study shows that the magnetopause does include approximate one‐dimensional substructures but also two‐ and three‐dimensional structures. It also shows that the dimensionality of the magnetic variations can differ from the variations of other fields so that, at some places, the magnetic field can have a 1‐D structure although all the plasma variations do not verify the properties of a global one‐dimensional problem. A generalization of the MDD tool is proposed. (10.1002/2017JA024526)
  DOI : 10.1002/2017JA024526
• Magnetic depression and electron transport in an ion-scale flux rope associated with Kelvin–Helmholtz waves
  
  • Tang Binbin
  • Li Wenya
  • Wang Chi
  • Dai Lei
  • Khotyaintsev Yuri
  • Lindqvist Per-Arne
  • Ergun Robert
  • Le Contel Olivier
  • Pollock Craig
  • Russell Christopher
  • Burch James
  Annales Geophysicae, European Geosciences Union, 2018, 36 (3), pp.879-889. We report an ion-scale magnetic flux rope (the size of the flux rope is ∼ 8.5 ion inertial lengths) at the trailing edge of Kelvin–Helmholtz (KH) waves observed by the Magnetospheric Multiscale (MMS) mission on 27 Septem-ber 2016, which is likely generated by multiple X-line re-connection. The currents of this flux rope are highly filamen-tary: in the central flux rope, the current flows are mainly parallel to the magnetic field, supporting a local magnetic field increase at about 7 nT, while at the edges the current filaments are predominantly along the antiparallel direction, which induce an opposing field that causes a significant magnetic depression along the axis direction (> 20 nT), meaning the overall magnetic field of this flux rope is depressed compared to the ambient magnetic field. Thus, this flux rope, accompanied by the plasma thermal pressure enhancement in the center, is referred to as a crater type. Intense lower hybrid drift waves (LHDWs) are found at the magnetospheric edge of the flux rope, and the wave potential is estimated to be ∼ 17 % of the electron temperature. Though LHDWs may be stabilized by the mechanism of electron resonance broadening , these waves could still effectively enable diffusive electron transports in the cross-field direction, corresponding to a local density dip. This indicates LHDWs could play important roles in the evolution of crater flux ropes. (10.5194/angeo-36-879-2018)
  DOI : 10.5194/angeo-36-879-2018
• The effects of secondary electron emission on plasma sheath characteristics and electron transport in an ExB discharge via kinetic simulations
  
  • Tavant Antoine
  • Croes Vivien
  • Lucken Romain
  • Lafleur Trevor
  • Bourdon Anne
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (12), pp.124001. Hall-effect thrusters, which are electrostatic devices based on an E B ´ plasma discharge, have successfully been used as satellite propulsion systems for the last few decades. However, the presence of anomalous electron cross-field transport is still poorly understood, and involves complex and strongly coupled mechanisms such as azimuthal electron drift instabilities and intense secondary electron emission (SEE) from the thruster walls. The present work focuses on the relative importance of these two phenomena. We use a 2D particle-in-cell/Monte Carlo collision model configured to simulate the radial-azimuthal directions near the thruster exit plane. A constant radial magnetic field and axial electric field are imposed, and electron drift instabilities are observed in the azimuthal (E B ´) direction. A simplified SEE model is implemented and an extensive parametric study is performed to directly determine the effect on electron transport. It is found that, for the operating conditions used in our simulations, SEE enhances the near-wall electron mobility by a factor 2, while reducing the bulk plasma mobility by about 20% (due to electron cooling). However, the dominant contribution to anomalous electron transport is still observed to be caused by electron drift instabilities driven by the E B ´ discharge configuration. SEE modifies the electron mobility profile, but the spatially-averaged value remains relatively constant. Three different operating regimes are identified depending on the SEE rate value: two that are stable, and a third which shows an oscillatory behaviour. In addition to electron transport, the kinetic simulations give new insight into the plasma sheath formation at the radial walls, and comparison with typical analytical sheath models demonstrate important differences. (10.1088/1361-6595/aaeccd)
  DOI : 10.1088/1361-6595/aaeccd
• Cometary plasma response to interplanetary corotating interaction regions during 2016 June–September: a quantitative study by the Rosetta Plasma Consortium
  
  • Hajra Rajkumar
  • Henri Pierre
  • Myllys Minna
  • Héritier Kevin
  • Galand Marina
  • Simon wedlund Cyril
  • Breuillard Hugo
  • Behar Etienne
  • Edberg Niklas
  • Goetz Charlotte
  • Nilsson Hans
  • Eriksson Anders I.
  • Goldstein Raymond
  • Tsurutani Bruce T
  • Moré Jérome
  • Vallières Xavier
  • Wattieaux Gaëtan
  Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2018, 480 (4), pp.4544-4556. Four interplanetary corotating interaction regions (CIRs) were identified during 2016 June–September by the Rosetta Plasma Consortium (RPC) monitoring in situ the plasma environment of the comet 67P/Churyumov–Gerasimenko (67P) at heliocentric distances of ∼3–3.8 au. The CIRs, formed in the interface region between low- and high-speed solar wind streams with speeds of ∼320–400 km s−1 and ∼580–640 km s−1, respectively, are characterized by relative increases in solar wind proton density by factors of ∼13–29, in proton temperature by ∼7–29, and in magnetic field by ∼1–4 with respect to the pre-CIR values. The CIR boundaries are well defined with interplanetary discontinuities. Out of 10 discontinuities, four are determined to be forward waves and five are reverse waves, propagating at ∼5–92 per cent of the magnetosonic speed at angles of ∼20°–87° relative to ambient magnetic field. Only one is identified to be a quasi-parallel forward shock with magnetosonic Mach number of ∼1.48 and shock normal angle of ∼41°. The cometary ionosphere response was monitored by Rosetta from cometocentric distances of ∼4–30 km. A quiet time plasma density map was developed by considering dependences on cometary latitude, longitude, and cometocentric distance of Rosetta observations before and after each of the CIR intervals. The CIRs lead to plasma density enhancements of ∼500–1000 per cent with respect to the quiet time reference level. Ionospheric modelling shows that increased ionization rate due to enhanced ionizing (>12–200 eV) electron impact is the prime cause of the large cometary plasma density enhancements during the CIRs. Plausible origin mechanisms of the cometary ionizing electron enhancements are discussed. (10.1093/mnras/sty2166)
  DOI : 10.1093/mnras/sty2166
• Kinetic study of CO<SUB>2</SUB> plasmas under non-equilibrium conditions. II. Input of vibrational energy
  
  • Grofulovic Marija
  • Silva Tiago
  • Klarenaar Bart
  • Morillo-Candas Ana-Sofia
  • Guaitella Olivier
  • Engeln Richard
  • Pintassilgo C.D.
  • Guerra V.
  Plasma Sources Science and Technology, IOP Publishing, 2018. This is the second of two papers presenting the study of vibrational energy exchanges in non-equilibrium CO₂ plasmas in low-excitation conditions. The companion paper addresses a theoretical and experimental investigation of the time relaxation of ~70 individual vibrational levels of ground-state CO₂(X¹&#931;) molecules during the afterglow of a pulsed DC glow discharge, operating at pressures of a few Torr and discharge currents around 50 mA, where the rate coefficients for vibration-translation (V-T) and vibration-vibration (V-V) energy transfers among these levels are validated. Herein the investigation focus the active discharge, by extending the model with the inclusion of electron impact processes for vibrational excitation and de-excitation (e-V). The time-dependent calculated densities of the different vibrational levels are compared with experimental data obtained from time-resolved in situ Fourier Transform Infrared spectroscopy. It is shown that the vibrational temperature of the asymmetric stretching mode is always larger than the vibrational temperatures of the bending and symmetric stretching modes along the discharge pulse, the latter two remaining very nearly the same and close to the gas temperature. The general good agreement between the model predictions and the experimental results validates the e-V rate coefficients used and gives confidence that the proposed kinetic scheme provides a solid basis to understand the vibrational energy exchanges occurring in CO₂ plasmas. (10.1088/1361-6595/aadb60)
  DOI : 10.1088/1361-6595/aadb60
• 3D Anisotropy of Solar Wind Turbulence, Tubes, or Ribbons?
  
  • Verdini Andrea
  • Grappin Roland
  • Alexandrova Olga
  • Lion Sonny
  The Astrophysical Journal, American Astronomical Society, 2018, 853 (1), pp.85. We study the anisotropy with respect to the local magnetic field of turbulent magnetic fluctuations at magnetofluid scales in the solar wind. Previous measurements in the fast solar wind obtained axisymmetric anisotropy, despite that the analysis method allows nonaxisymmetric structures. These results are probably contaminated by the wind expansion that introduces another symmetry axis, namely, the radial direction, as indicated by recent numerical simulations. These simulations also show that while the expansion is strong, the principal fluctuations are in the plane perpendicular to the radial direction. Using this property, we separate 11 yr of Wind spacecraft data into two subsets characterized by strong and weak expansion and determine the corresponding turbulence anisotropy. Under strong expansion, the small-scale anisotropy is consistent with the Goldreich & Sridhar critical balance. As in previous works, when the radial symmetry axis is not eliminated, the turbulent structures are field-aligned tubes. Under weak expansion, we find 3D anisotropy predicted by the Boldyrev model, that is, turbulent structures are ribbons and not tubes. However, the very basis of the Boldyrev phenomenology, namely, a cross-helicity increasing at small scales, is not observed in the solar wind: the origin of the ribbon formation is unknown. (10.3847/1538-4357/aaa433)
  DOI : 10.3847/1538-4357/aaa433
• Solar Wind Turbulence Studies Using MMS Fast Plasma Investigation Data
  
  • Bandyopadhyay Riddhi
  • Chasapis A.
  • Chhiber R.
  • Parashar T. N.
  • Maruca B. A.
  • Matthaeus W. H.
  • Schwartz S. J.
  • Eriksson S.
  • Le Contel Olivier
  • Breuillard Hugo
  • Burch J. L.
  • Moore T. E.
  • Pollock C. J.
  • Giles B. L.
  • Paterson W. R.
  • Dorelli J. C.
  • Gershman D. J.
  • Torbert R. B.
  • Russell C. T.
  • Strangeway R. J.
  The Astrophysical Journal, American Astronomical Society, 2018, 866 (2), pp.81. Studies of solar wind turbulence traditionally employ high-resolution magnetic field data, but high-resolution measurements of ion and electron moments have been possible only recently. We report the first turbulence studies of ion and electron velocity moments accumulated in pristine solar wind by the Fast Plasma Investigation (FPI) instrument on board the Magnetospheric Multiscale Mission. Use of these data is made possible by a novel implementation of a frequency domain Hampel filter, described herein. After presenting procedures for processing of the data, we discuss statistical properties of solar wind turbulence extending into the kinetic range. Magnetic field fluctuations dominate electron and ion-velocity fluctuation spectra throughout the energy-containing and inertial ranges. However, a multispacecraft analysis indicates that at scales shorter than the ion inertial length, electron velocity fluctuations become larger than ion-velocity and magnetic field fluctuations. The kurtosis of ion-velocity peaks around a few ion inertial lengths and returns to a near Gaussian value at sub-ion scales. (10.3847/1538-4357/aade93)
  DOI : 10.3847/1538-4357/aade93
• Wave Phenomena and Beam-Plasma Interactions at the Magnetopause Reconnection Region
  
  • Burch J. L.
  • Webster J. M.
  • Genestreti K. J.
  • Torbert R. B.
  • Giles B. L.
  • Fuselier S. A.
  • Dorelli J. C.
  • Rager A. C.
  • Phan T. D.
  • Allen R. C.
  • Chen L.-J.
  • Wang S.
  • Le Contel Olivier
  • Russell C. T.
  • Strangeway R. J.
  • Ergun R. E.
  • Jaynes A. N.
  • Lindqvist P.-A.
  • Graham D. B.
  • Wilder F. D.
  • Hwang K.-J.
  • Goldstein J.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018, 123 (2), pp.1118-1133. This paper reports on Magnetospheric Multiscale observations of whistler mode chorus and higher-frequency electrostatic waves near and within a reconnection diffusion region on 23 November 2016. The diffusion region is bounded by crescent-shaped electron distributions and associated dissipation just upstream of the X-line and by magnetic field-aligned currents and electric fields leading to dissipation near the electron stagnation point. Measurements were made southward of the X-line as determined by southward directed ion and electron jets. We show that electrostatic wave generation is due to magnetosheath electron beams formed by the electron jets as they interact with a cold background plasma and more energetic population of magnetospheric electrons. On the magnetosphere side of the X-line the electron beams are accompanied by a strong perpendicular electron temperature anisotropy, which is shown to be the source of an observed rising-tone whistler mode chorus event. We show that the apex of the chorus event and the onset of electrostatic waves coincide with the opening of magnetic field lines at the electron stagnation point. (10.1002/2017JA024789)
  DOI : 10.1002/2017JA024789
• Electron bulk acceleration and thermalization at Earth's quasi-perpendicular bow shock
  
  • Chen L.-J.
  • Wang S.
  • Wilson Iii L. B.
  • Schwartz S. J.
  • Bessho N.
  • Moore T. E.
  • Gershman D. J.
  • Giles B. L.
  • Malaspina D. M.
  • Wilder F. D.
  • Ergun R. E.
  • Hesse Michael
  • Lai H.
  • Russell C. T.
  • Strangeway R. J.
  • Torbert R. B.
  • Viñas A.-F.
  • Burch J. L.
  • Lee S.
  • Pollock C.
  • Dorelli J. C.
  • Paterson W. R.
  • Ahmadi N.
  • Goodrich K. A.
  • Lavraud B.
  • Le Contel Olivier
  • Khotyaintsev Y. V.
  • Lindqvist P.-A.
  • Boardsen S.
  • Wei H.
  • Le A.
  • Avanov L. A.
  Physical Review Letters, American Physical Society, 2018, 120, pp.225101. Electron heating at Earth's quasiperpendicular bow shock has been surmised to be due to the combined effects of a quasistatic electric potential and scattering through wave-particle interaction. Here we report the observation of electron distribution functions indicating a new electron heating process occurring at the leading edge of the shock front. Incident solar wind electrons are accelerated parallel to the magnetic field toward downstream, reaching an electron-ion relative drift speed exceeding the electron thermal speed. The bulk acceleration is associated with an electric field pulse embedded in a whistler-mode wave. The high electron-ion relative drift is relaxed primarily through a nonlinear current-driven instability. The relaxed distributions contain a beam traveling toward the shock as a remnant of the accelerated electrons. Similar distribution functions prevail throughout the shock transition layer, suggesting that the observed acceleration and thermalization is essential to the cross-shock electron heating. (10.1103/PhysRevLett.120.225101)
  DOI : 10.1103/PhysRevLett.120.225101
• Multi-variable comprehensive analysis of two great geomagnetic storms of 2015
  
  • Kashcheyey Anton
  • Migoya-Orué Yenca
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Nava B.
  • Alazo-Cuartas K.
  • Radicella S.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018. During the year 2015 two great geomagnetic storms (Dst<-200nT) occurred on 17 March and 22 June. These two geomagnetic storms have similarities. They occurred during the same decreasing phase of the sunspot cycle 24. The interplanetary and magnetospheric environments were calm before the beginning of the storms. Both events were due to Coronal Mass Ejections (CME)and High Speed Solar Wind (HSSW). Variations of the solar wind velocity and the Bz component of the interplanetary magnetic field (IMF) were also similar. Two key features that are different for these storms are UT time of the beginning(04:45UT for 17March and 18:33UT for 22June) and season (equinox and solstice). The comparison of the impact of the storms on the Earth ionosphere and magnetosphere have been performed using diverse parameters including global ionospheric maps (GIMs) of vertical total electron content (VTEC), data from individual GNSS receivers, ionosondes, magnetometers and instruments from different space missions. Visualizing GIM data as the difference of VTEC between consecutive days allowed understanding better the effect of the storms as a function of time of the beginning of the storm and of the season. It is shown that the presence or absence of scintillations in GNSS signals during these 2 storms in African longitude sector is clearly related to the local time at a given station at the beginning of the storm. (10.1029/2017JA024900)
  DOI : 10.1029/2017JA024900
• Poloidal asymmetries of flows in the Tore Supra tokamak
  
  • Vermare Laure
  • Hennequin Pascale
  • Gürcan Özgür D.
  • Garbet X.
  • Honoré Cyrille
  • Clairet F.
  • Giacalone J-C.
  • Morel Pierre
  • Storelli A.
  • Team Tore Supra
  Physics of Plasmas, American Institute of Physics, 2018, 25 (2), pp.020704. Simultaneous measurements of binormal velocity of density fluctuations using two separate Doppler backscattering systems at the low field side and at the top of the plasma show significant poloidal asymmetry. The measurements are performed in the core region between the radii 0.7&#8201;<&#8201;&#961;&#8201; < &#8201;0.95, over a limited number of L-mode discharges covering a wide range of plasma conditions in the Tore Supra tokamak. A possible generation mechanism by the ballooned structure of the underlying turbulence, in the form of convective cells, is proposed for explaining the observation of these poloidally asymmetric mean flows. (10.1063/1.5022122)
  DOI : 10.1063/1.5022122
• Fast gas heating and radial distribution of active species in nanosecond capillary discharge in pure nitrogen and N<SUB>2</SUB>:O<SUB>2</SUB> mixtures
  
  • Lepikhin N D
  • Popov N.A.
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2018, 27, pp.055005. Fast gas heating is studied experimentally and numerically using pulsed nanosecond capillary discharge in pure nitrogen and N2:O2 mixtures under the conditions of high specific deposited energy (up to 1 eV/molecule) and high reduced electric fields (100300 Td). Deposited energy, electric field and gas temperature are measured as functions of time. The radial distribution of active species is analyzed experimentally. The roles of processes involving excited N2 molecules and ions and N(2D) excited nitrogen species leading to heat release are analyzed using numerical modeling in the framework of 1D axial approximation. (10.1088/1361-6595/aab74e)
  DOI : 10.1088/1361-6595/aab74e
• SMILEI : A collaborative, open-source, multi-purpose particle-in-cell code for plasma simulation
  
  • Derouillat J.
  • Beck A.
  • Pérez F.
  • Vinci T.
  • Chiaramello M.
  • Grassi A.
  • Flé M.
  • Bouchard G.
  • Plotnikov I.
  • Aunai Nicolas
  • Dargent Jérémy
  • Riconda C.
  • Grech M.
  Computer Physics Communications, Elsevier, 2018, 222, pp.351-373. SMILEI is a collaborative, open-source, object-oriented (C ) particle-in-cell code. To benefit from the latest advances in high-performance computing (HPC), SMILEI is co-developed by both physicists and HPC experts. The code's structures, capabilities, parallelization strategy and performances are discussed. Additional modules (e.g. to treat ionization or collisions), benchmarks and physics highlights are also presented. Multi-purpose and evolutive, SMILEI is applied today to a wide range of physics studies, from relativistic laser-plasma interaction to astrophysical plasmas. (10.1016/j.cpc.2017.09.024)
  DOI : 10.1016/j.cpc.2017.09.024
• Electron Power-Law Spectra in Solar and Space Plasmas
  
  • Oka Mitsuo
  • Birn J.
  • Battaglia Marina
  • Chaston C. C.
  • Hatch S. M.
  • Livadiotis G.
  • Imada S.
  • Miyoshi Y.
  • Kuhar M.
  • Effenberger F.
  • Eriksson E.
  • Khotyaintsev Yu. V.
  • Retinò Alessandro
  Space Science Reviews, Springer Verlag, 2018, 214. Particles are accelerated to very high, non-thermal energies in solar and space plasma environments. While energy spectra of accelerated electrons often exhibit a power law, it remains unclear how electrons are accelerated to high energies and what processes determine the power-law index delta . Here, we review previous observations of the power-law index delta in a variety of different plasma environments with a particular focus on sub-relativistic electrons. It appears that in regions more closely related to magnetic reconnection (such as the `above-the-looptop' solar hard X-ray source and the plasma sheet in Earth's magnetotail), the spectra are typically soft (delta &gsim;4). This is in contrast to the typically hard spectra (delta &lsim;4) that are observed in coincidence with shocks. The difference implies that shocks are more efficient in producing a larger non-thermal fraction of electron energies when compared to magnetic reconnection. A caveat is that during active times in Earth's magnetotail, delta values seem spatially uniform in the plasma sheet, while power-law distributions still exist even in quiet times. The role of magnetotail reconnection in the electron power-law formation could therefore be confounded with these background conditions. Because different regions have been studied with different instrumentations and methodologies, we point out a need for more systematic and coordinated studies of power-law distributions for a better understanding of possible scaling laws in particle acceleration as well as their universality. (10.1007/s11214-018-0515-4)
  DOI : 10.1007/s11214-018-0515-4
• Experimental benchmark of kinetic simulations of capacitively coupled plasmas in molecular gases
  
  • Donkó Z.
  • Derzsi A.
  • Korolov Ihor
  • Hartmann P.
  • Brandt S.
  • Schulze J.
  • Berger B.
  • Koepke M.
  • Bruneau Bastien
  • Johnson Erik
  • Lafleur Trevor
  • Booth Jean-Paul
  • Gibson Andrew
  • O'Connell D.
  • Gans T.
  Plasma Physics and Controlled Fusion, IOP Publishing, 2018, 60 (1), pp.014010. We discuss the origin of uncertainties in the results of numerical simulations of low-temperature plasma sources, focusing on capacitively coupled plasmas. These sources can be operated in various gases/gas mixtures, over a wide domain of excitation frequency, voltage, and gas pressure. At low pressures, the non-equilibrium character of the charged particle transport prevails and particle-based simulations become the primary tools for their numerical description. The particle-in-cell method, complemented with Monte Carlo type description of collision processes, is a well-established approach for this purpose. Codes based on this technique have been developed by several authors/groups, and have been benchmarked with each other in some cases. Such benchmarking demonstrates the correctness of the codes, but the underlying physical model remains unvalidated. This is a key point, as this model should ideally account for all important plasma chemical reactions as well as for the plasma-surface interaction via including specific surface reaction coefficients (electron yields, sticking coefficients, etc). In order to test the models rigorously, comparison with experimental ?benchmark data? is necessary. Examples will be given regarding the studies of electron power absorption modes in O 2 , and CF 4 ?Ar discharges, as well as on the effect of modifications of the parameters of certain elementary processes on the computed discharge characteristics in O 2 capacitively coupled plasmas. (10.1088/1361-6587/aa8378)
  DOI : 10.1088/1361-6587/aa8378
• Local energy transfer rate and kinetic signatures in solar wind turbulence
  
  • Sorriso-Valvo L.
  • Catapano F.
  • Retinò Alessandro
  • Greco A.
  • Perri S.
  • Marino R.
  • Pezzi O.
  • Perrone D.
  • Bruno Roberto
  • Valentini F.
  • Servidio S.
  • Panebianco V.
  , 2018, 20, pp.EGU2018-6552. The transfer of energy from large to small scales in space turbulence is an important ingredient of the longstanding question about the mechanism of the interplanetary plasma heating. Statistical analysis in the context of magnetohydrodynamic (MHD) turbulence provided evidence that the total transported energy is compatible with the observed heating of the solar wind as it expands in the heliosphere. However, in order to understand which processes contribute to the plasma heating, it is necessary to have a local description of the energy flux across scales. To this aim, we propose a proxy of the scale-dependent, local energy transfer that includes magnetic, velocity and cross-helicity terms, and is based on the third-order moment scaling law for MHD turbulence. Data from Helios2 are used to determine the statistical properties of such a proxy in comparison with the magnetic and velocity fields PVI, and the correlation with local solar wind heating is pointed out. MMS data are used to study the correlation with kinetic-scale features, as for example the temperature anisotropy, the heat flux, the agyrothropy of the pressure tensor, and the deviation form Maxwellian. A comparison with hybrid direct numerical simulations of the Vlasov-Maxwell system, including alpha particles, is also performed. The good correlation between the turbulent local energy flux and the indicators of kinetic processes found in the data and in the simulations suggests an important role played by this proxy in the study of plasma energy dissipation.
• Compressible Magnetohydrodynamic Turbulence in the Earth’s Magnetosheath: Estimation of the Energy Cascade Rate Using in situ Spacecraft Data
  
  • Hadid Lina
  • Sahraoui Fouad
  • Galtier Sébastien
  • Huang S.Y.
  Physical Review Letters, American Physical Society, 2018, 120 (5), pp.055102. The first estimation of the energy cascade rate |εC| of magnetosheath turbulence is obtained using the CLUSTER and THEMIS spacecraft data and an exact law of compressible isothermal magnetohydrodynamics turbulence. |εC | is found to be of the order of 10−13J.m−3.s−1, at least two orders of magnitude larger than its value in the solar wind (order of 10−16 J.m−3.s−1 in the fast wind). Two types of turbulence are evidenced and shown to be dominated either by incompressible Alfénic or compressible magnetosonic-like fluctuations. Density fluctuations are shown to amplify the cascade rate and its spatial anisotropy in comparison with incompressible Alfv´enic turbulence. Furthermore, for compressible magnetosonic fluctuations, large cascade rates are found to lie mostly near the linear kinetic instability of the mirror mode. New empirical power-laws relating |C | to the turbulent Mach number and to the internal energy are evidenced. These new finding have potential applications in distant astrophysical plasmas that are not accessible to in situ measurements. (10.1103/PhysRevLett.120.055102)
  DOI : 10.1103/PhysRevLett.120.055102
• Magnetospheric Multiscale Observations of an Ion Diffusion Region With Large Guide Field at the Magnetopause: Current System, Electron Heating, and Plasma Waves
  
  • Zhou M.
  • Berchem J.
  • Walker R. J.
  • El-Alaoui M.
  • Goldstein M. L.
  • Lapenta G.
  • Deng X.
  • Li J.
  • Le Contel Olivier
  • Graham D. B.
  • Lavraud B.
  • Paterson W. R.
  • Giles B. L.
  • Burch J. L.
  • Torbert R. B.
  • Russell C. T.
  • Strangeway R. J.
  • Zhao C.
  • Ergun R. E.
  • Lindqvist P.-A.
  • Marklund G.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2018, 123 (3), pp.1834-1852. We report Magnetospheric Multiscale (MMS) observations of a reconnecting current sheet in the presence of a weak density asymmetry with large guide field at the dayside magnetopause. An ion diffusion region (IDR) was detected associated with this current sheet. Parallel current dominated over the perpendicular current in the IDR, as found in previous studies of component reconnection. Electrons were preferentially heated parallel to the magnetic field within the IDR. The heating was manifested as a flattop distribution below 400eV. Two types of electromagnetic electron whistler waves were observed within the regions where electrons were heated. One type of whistler wave was associated with nonlinear structures in E-|| with amplitudes up to 20mV/m. The other type was not associated with any structures in E-||. Poynting fluxes of these two types of whistler waves were directed away from the X-line. We suggest that the nonlinear evolution of the oblique whistler waves gave rise to the solitary structures in E-||. There was a perpendicular super-Alfvenic outflow jet that was carried by magnetized electrons. Intense electrostatic lower hybrid drift waves were localized in the current sheet center and were probably driven by the super-Alfvenic electron jet, the velocity of which was approximately equal to the diamagnetic drift of demagnetized ions. Our observations suggest that the guide field significantly modified the structures (Hall electromagnetic fields and current system) and wave properties in the IDR. (10.1002/2017JA024517)
  DOI : 10.1002/2017JA024517
• The role of instability-enhanced friction on anomalous electron and ion transport in Hall-effect thrusters
  
  • Lafleur Trevor
  • Chabert Pascal
  Plasma Sources Science and Technology, IOP Publishing, 2018, 27 (1), pp.015003. Using a self-consistent 2D particle-in-cell (PIC) simulation, we investigate the electron transport in Hall-effect thrusters. The PIC simulation is explicit in time and models the axial and azimuthal directions of a thruster without using any artificial parametric or geometric scaling factors. The applied discharge voltage and external magnetic field causes electrons to drift in the azimuthal direction, and this drives an instability in the plasma that produces large amplitude oscillations in both the plasma density and azimuthal electric field. A Fourier transform in time and space shows that the oscillations follow a dispersion relation similiar to that for an ion acoustic instability (in agreement with a recent kinetic theory). Correlated with the presence of this instability is an enhanced electron cross-field transport; even in the absence of electron-wall collisions and secondary electron emission. The amplitude of plasma density oscillations (but not electric field oscillations) is found to decrease significantly in a region just downstream of the thruster exit (before then increasing again), and reaches levels similar to those measured experimentally with collective light scattering techniques. By taking relevant velocity moments of the electron distribution function in the PIC simulations, we reconstruct each term in the electron momentum conservation equation and demonstrate that anomalous electron transport can be explained entirely due to an instability-enhanced friction force between electrons and ions. This friction force acts as an additional momentum loss allowing electrons to cross the magnetic field, and as an accelerating force causing ions to rotate azimuthally in the same direction as the electrons. Clear evidence of ion-wave trapping in the instability electric field is observed. (10.1088/1361-6595/aa9efe)
  DOI : 10.1088/1361-6595/aa9efe
• Nested polyhedra model of isotropic magnetohydrodynamic turbulence
  
  • Gürcan Özgür D.
  Physical Review E, American Physical Society (APS), 2018, 97, pp.063111. A nested polyhedra model has been developed for magnetohydrodynamic turbulence. Driving only the velocity field at large scales with random, divergence-free forcing results in a clear, stationary k^-5/3 spectrum for both kinetic and magnetic energies. Since the model naturally effaces disparate scale interactions, does not have a guide field, and avoids injecting any sign of helicity by random forcing, the resulting three-dimensional k spectrum is statistically isotropic. The strengths and weaknesses of the model are demonstrated by considering large or small magnetic Prandtl numbers. It was also observed that the timescale for the equipartition offset with those of the smallest scales shows a k^-1/2 scaling. (10.1103/PhysRevE.97.063111)
  DOI : 10.1103/PhysRevE.97.063111
• The effect of liquid target on a nonthermal plasma jet−imaging, electric fields, visualization of gas flow and optical emission spectroscopy
  
  • Kovačević Vv
  • Sretenović Gb
  • Slikboer Elmar
  • Guaitella Olivier
  • Sobota Ana
  • Kuraica Mm
  Journal of Physics D: Applied Physics, IOP Publishing, 2018, 51 (6), pp.065202. The article describes the complex study of the interaction of a helium plasma jet with distilled water and saline. The discharge development, spatial distribution of the excited species, electric field measurement results and the results of the Schlieren imaging are presented. The results of the experiments showed that the plasmaliquid interaction could be prolonged with the proper choice of the gas composition between the jet nozzle and the target. This depends on the gas flow and the target distance. Increased conductivity of the liquid does not affect the discharge properties significantly. An increase of the gas flow enables an extension of the plasma duration on the liquid surface up to 10 µs, but with a moderate electric field strength in the ionization wave. In contrast, there is a significant enhancement of the electric field on the liquid surface, up to 30&#8201;kV cm&#8722;1 for low flows, but with a shorter time of the overall plasma liquid interaction. Ignition of the plasma jet induces a gas flow modification and may cause turbulences in the gas flow. A significant influence of the plasma jet causing a mixing in the liquid is also recorded and it is found that the plasma jet ignition changes the direction of the liquid circulation. (10.1088/1361-6463/aaa288)
  DOI : 10.1088/1361-6463/aaa288