Publications

2017

• Induction effects of geomagnetic disturbances in the geo-electric field variations at low latitudes
  
  • Doumbia Vafi
  • Boka Kouadio
  • Kouassi Nguessan
  • Grodji Oswald Didier Franck
  • Amory-Mazaudier Christine
  • Menvielle Michel
  Annales Geophysicae, European Geosciences Union, 2017, 35 (1), pp.39 - 51. In this study we examined the influences of geomagnetic activity on the Earth surface electric field variations at low latitudes. During the International Equatorial Electrojet Year (IEEY) various experiments were performed along 5° W in West Africa from 1992 to 1995. Among other instruments, 10 stations equipped with magnetometers and telluric electric field lines operated along a meridian chain across the geomagnetic dip equator from November 1992 to December 1994. In the present work, the induced effects of space-weather-related geomagnetic disturbances in the equatorial electrojet (EEJ) influence area in West Africa were examined. For that purpose, variations in the north–south (E_x) and east–west (E_y) components of telluric electric field were analyzed, along with that of the three components (H, D and Z) of the geomagnetic field during the geomagnetic storm of 17 February 1993 and the solar flare observed on 4 April 1993. The most important induction effects during these events are associated with brisk impulses like storm sudden commencement (ssc) and solar flare effect (sfe) in the geomagnetic field variations. For the moderate geomagnetic storm that occurred on 17 February 1993, with a minimum Dst index of −110 nT, the geo-electric field responses to the impulse around 11:00 LT at LAM are E_x= 520 mV km⁻¹ and E_y = 400 mV km⁻¹. The geo-electric field responses to the sfe that occurred around 14:30 LT on 4 April 1993 are clearly observed at different stations as well. At LAM the crest-to-crest amplitude of the geo-electric field components associated with the sfe are E_x = 550 mV km⁻¹ and E_y = 340 mV km⁻¹. Note that the sfe impact on the geo-electric field variations decreases with the increasing distance of the stations from the subsolar point, which is located at about 5.13° N on 4 April. This trend does not reflect the sfe increasing amplitude near the dip equator due the high Cowling conductivity in the EEJ belt. (10.5194/angeo-35-39-2017)
  DOI : 10.5194/angeo-35-39-2017
• Hemispheric asymmetries in the ionosphere response observed during the high-speed solar wind streams of the 24-28 August 2010
  
  • Zaourar N.
  • Amory-Mazaudier Christine
  • Fleury Rolland
  Advances in Space Research, Elsevier, 2017. This paper presents the geomagnetic and ionospheric responses to a high speed solar wind stream (HSS) impacting the magnetosphere on 24 August 2010. We focus our study on the interhemispheric conjugated behavior. The solar wind speed remained very high during 5 days from 24 to 28 August 2010. By using magnetometer and ground-based GPS data from various approximately conjugated magnetic observatories and GPS stations, we studied the hemispheric asymmetries in the magnetic signature, Vertical Total Electron Content (VTEC) and scintillation activity during this HSS event. Geomagnetic activity reveals larger disturbances in amplitude in the Northern Hemisphere (NH) than in the southern Hemisphere (SH), and stronger asymmetries at higher latitudes, than at lower latitudes, between the conjugate observatories. VTEC variations reveal large increases in amplitude in the NH; while these effects are less pronounced in the SH. We investigate also the GPS scintillation activities occurring in the conjugated polar regions under HSSs conditions. At auroral latitudes, our results show a good correlation between the rate of VTEC index (ROTI) and auroral Al index, with more intense phase fluctuations in the NH. (10.1016/j.asr.2017.01.048)
  DOI : 10.1016/j.asr.2017.01.048
• Nested polyhedra model of turbulence
  
  • Gürcan Özgür D.
  Physical Review E, American Physical Society (APS), 2017, 95 (6), pp.063102. A discretization of the wave-number space is proposed, using nested polyhedra, in the form of alternating dodecahedra and icosahedra that are self-similarly scaled. This particular choice allows the possibility of forming triangles using only discretized wave vectors when the scaling between two consecutive dodecahedra is equal to the golden ratio and the icosahedron between the two dodecahedra is the dual of the inner dodecahedron. Alternatively, the same discretization can be described as a logarithmically spaced (with a scaling equal to the golden ratio), nested dodecahedron-icosahedron compounds. A wave vector which points from the origin to a vertex of such a mesh, can always find two other discretized wave vectors that are also on the vertices of the mesh (which is not true for an arbitrary mesh). Thus, the nested polyhedra grid can be thought of as a reduction (or decimation) of the Fourier space using a particular set of self-similar triads arranged approximately in a spherical form. For each vertex (i.e., discretized wave vector) in this space, there are either 9 or 15 pairs of vertices (i.e., wave vectors) with which the initial vertex can interact to form a triangle. This allows the reduction of the convolution integral in the Navier-Stokes equation to a sum over 9 or 15 interaction pairs, transforming the equation in Fourier space to a network of " interacting " nodes that can be constructed as a numerical model, which evolves each component of the velocity vector on each node of the network. This model gives the usual Kolmogorov spectrum of k −5/3. Since the scaling is logarithmic, and the number of nodes for each scale is constant, a very large inertial range (i.e., a very high Reynolds number) with a much lower number of degrees of freedom can be considered. Incidentally, by assuming isotropy and a certain relation between the phases, the model can be used to systematically derive shell models. (10.1103/PhysRevE.95.063102)
  DOI : 10.1103/PhysRevE.95.063102
• Measurements of density fluctuations in magnetic confined plasmas using Doppler backscattering technique
  
  • Vermare Laure
  • Hennequin Pascale
  • Honoré Cyrille
  • Pisarev V.
  • Giacalone J-C.
  , 2017.
• Intermittent energy dissipation by turbulent reconnection
  
  • Fu H.S.
  • Vaivads A.
  • Khotyaintsev Y. V.
  • André M.
  • Cao J.B.
  • Olshevsky V.
  • Eastwood Jonathan P.
  • Retinò Alessandro
  Geophysical Research Letters, American Geophysical Union, 2017, 44 (1), pp.37-43. Magnetic reconnection−-the process responsible for many explosive phenomena in both nature and laboratory−-is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the "diffusion region" at the sub-ion scale. Here we report such a measurement by Cluster−-four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E' s j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines. (10.1002/2016GL071787)
  DOI : 10.1002/2016GL071787
• Near-Earth plasma sheet boundary dynamics during substorm dipolarization
  
  • Nakamura R.
  • Nagai Tsugunobu
  • Birn Joachim
  • Sergeev Victor A.
  • Le Contel Olivier
  • Varsani Ali
  • Baumjohann W.
  • Nakamura T. K. M.
  • Apatenkov Sergey
  • Artemyev A. V.
  • Ergun Robert E.
  • Fuselier Stephen A.
  • Gershman D. J.
  • Giles Barbara J.
  • Khotyaintsev Y. V.
  • Lindqvist Per-Arne
  • Magnes Werner
  • Mauk Barry
  • Russell Christopher T.
  • Singer Howard J.
  • Stawarz J. E.
  • Strangeway Robert J.
  • Anderson Brian
  • Bromund Ken R.
  • Fischer David
  • Kepko Laurence
  • Le Guan
  • Plaschke Ferdinand
  • Slavin J. A.
  • Cohen Ian
  • Jaynes Allison
  • Turner Drew L.
  Earth Planets and Space, Springer / Terra Scientific Publishing Company, 2017, 69, pp.129. We report on the large-scale evolution of dipolarization in the near-Earth plasma sheet during an intense (AL -1000 nT) substorm on August 10, 2016, when multiple spacecraft at radial distances between 4 and 15 R <SUB>E</SUB> were present in the night-side magnetosphere. This global dipolarization consisted of multiple short-timescale (a couple of minutes) B <SUB> z </SUB> disturbances detected by spacecraft distributed over 9 MLT, consistent with the large-scale substorm current wedge observed by ground-based magnetometers. The four spacecraft of the Magnetospheric Multiscale were located in the southern hemisphere plasma sheet and observed fast flow disturbances associated with this dipolarization. The high-time-resolution measurements from MMS enable us to detect the rapid motion of the field structures and flow disturbances separately. A distinct pattern of the flow and field disturbance near the plasma boundaries was found. We suggest that a vortex motion created around the localized flows resulted in another field-aligned current system at the off-equatorial side of the BBF-associated R1/R2 systems, as was predicted by the MHD simulation of a localized reconnection jet. The observations by GOES and Geotail, which were located in the opposite hemisphere and local time, support this view. We demonstrate that the processes of both Earthward flow braking and of accumulated magnetic flux evolving tailward also control the dynamics in the boundary region of the near-Earth plasma sheet.[Figure not available: see fulltext.] (10.1186/s40623-017-0707-2)
  DOI : 10.1186/s40623-017-0707-2
• Magnetospheric Multiscale Observations of Electron Vortex Magnetic Hole in the Turbulent Magnetosheath Plasma
  
  • Huang S. Y.
  • Sahraoui Fouad
  • Yuan Z. G.
  • He J. S.
  • Zhao J. S.
  • Le Contel Olivier
  • Deng X. H.
  • Zhou M.
  • Fu H.S.
  • Shi Q. Q.
  • Lavraud B.
  • Pang Y.
  • Yang J.
  • Wang D. D.
  • Li H. M.
  • Yu X. D.
  • Pollock C. J.
  • Giles B. L.
  • Torbert R. B.
  • Russell C. T.
  • Goodrich K. A.
  • Gershman D. J.
  • Moore T. E.
  • Ergun R. E.
  • Khotyaintsev Y. V.
  • Lindqvist P.-A.
  • Strangeway R. J.
  • Magnes W.
  • Bromund K.
  • Leinweber H.
  • Plaschke F.
  • Anderson B. J.
  • Burch J. L.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2017, 836 (2), pp.L27. We report on the observations of an electron vortex magnetic hole corresponding to a new type of coherent structure in the turbulent magnetosheath plasma using the Magnetospheric Multiscale mission data. The magnetic hole is characterized by a magnetic depression, a density peak, a total electron temperature increase (with a parallel temperature decrease but a perpendicular temperature increase), and strong currents carried by the electrons. The current has a dip in the core region and a peak in the outer region of the magnetic hole. The estimated size of the magnetic hole is about 0.23 rho <SUB>i</SUB> (~30 rho <SUB>e</SUB>) in the quasi-circular cross-section perpendicular to its axis, where rho <SUB>i</SUB> and rho <SUB>e</SUB> are respectively the proton and electron gyroradius. There are no clear enhancements seen in high-energy electron fluxes. However, there is an enhancement in the perpendicular electron fluxes at 90° pitch angle inside the magnetic hole, implying that the electrons are trapped within it. The variations of the electron velocity components V <SUB>em</SUB> and V <SUB>en</SUB> suggest that an electron vortex is formed by trapping electrons inside the magnetic hole in the cross-section in the M−N plane. These observations demonstrate the existence of a new type of coherent structures behaving as an electron vortex magnetic hole in turbulent space plasmas as predicted by recent kinetic simulations. (10.3847/2041-8213/aa5f50)
  DOI : 10.3847/2041-8213/aa5f50
• MMS Observation of Magnetic Reconnection in the Turbulent Magnetosheath
  
  • Vörös Z.
  • Yordanova E.
  • Varsani A.
  • Genestreti K. J.
  • Khotyaintsev Y. V.
  • Li W.
  • Graham D. B.
  • Norgren C.
  • Nakamura R.
  • Narita Y.
  • Plaschke F.
  • Magnes W.
  • Baumjohann W.
  • Fischer D.
  • Vaivads A.
  • Eriksson E.
  • Lindqvist P.-A.
  • Marklund G.
  • Ergun R. E.
  • Leitner M.
  • Leubner M. P.
  • Strangeway R. J.
  • Le Contel Olivier
  • Pollock C.
  • Giles B. J.
  • Torbert R. B.
  • Burch J. L.
  • Avanov L. A.
  • Dorelli J. C.
  • Gershman D. J.
  • Paterson W. R.
  • Lavraud B.
  • Saito Y.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (11), pp.442-467. In this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi-parallel bow shock. Contrarily to the magnetopause and magnetotail cases, only a few observations of reconnection in the magnetosheath have been reported. The case study in this paper presents, for the first time, both fluid-scale and kinetic-scale signatures of an ongoing reconnection in the turbulent magnetosheath. The spacecraft are crossing the reconnection inflow and outflow regions and the ion diffusion region (IDR). Inside the reconnection outflows D shape ion distributions are observed. Inside the IDR mixing of ion populations, crescent-like velocity distributions and ion accelerations are observed. One of the spacecraft skims the outer region of the electron diffusion region, where parallel electric fields, energy dissipation/conversion, electron pressure tensor agyrotropy, electron temperature anisotropy, and electron accelerations are observed. Some of the difficulties of the observations of magnetic reconnection in turbulent plasma are also outlined. (10.1002/2017JA024535)
  DOI : 10.1002/2017JA024535
• Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en-route to Pluto. Comparison of its Forbush decreases at 1.4, 3.1 and 9.9 AU
  
  • Witasse O.
  • Sánchez-Cano B.
  • Mays M.
  • Kajdič P.
  • Opgenoorth H.
  • Elliott H.
  • Richardson G.
  • Zouganelis I.
  • Zender J.
  • Wimmer-Schweingruber R.
  • Turc Lucile
  • Taylor M.
  • Roussos E.
  • Rouillard A.
  • Richter I.
  • Richardson J.
  • Ramstad R.
  • Provan G.
  • Posner A.
  • Plaut J.
  • Odstrcil D.
  • Nilsson H.
  • Niemenen P.
  • Milan S.
  • Mandt K.
  • Lohf H.
  • Lester M.
  • Lebreton Jean-Pierre
  • Kuulkers E.
  • Krupp N.
  • Koenders C.
  • James M.
  • Intzekara D.
  • Holmstrom M.
  • Hassler M.
  • Hall S.
  • Guo J.
  • Goldstein R.
  • Goetz C.
  • Glassmeier H.
  • Génot V.
  • Evans H.
  • Espley J.
  • Edberg N.
  • Dougherty M.
  • Cowley S.
  • Burch J.
  • Behar E.
  • Barabash S.
  • Andrews D. J.
  • Altobelli N.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, Accepted Manuscript (8), pp.64 pages. We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, MAVEN, Mars Odyssey and MSL missions, 44 hours before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a non-ambiguous signature at New Horizons. A potential detection of this ICME by Voyager-2 at 110-111 AU is also discussed. The multi-spacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P and Saturn. (10.1002/2017JA023884)
  DOI : 10.1002/2017JA023884
• Global structure and sodium ion dynamics in Mercury's magnetosphere with the offset dipole
  
  • Yagi Manabu
  • Seki Kanako
  • Matsumoto Y.
  • Delcourt Dominique C.
  • Leblanc François
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2017, 122 (11), pp.10,990–11,002. We conducted global magnetohydrodynamics (MHD) simulation of Mercury's magnetosphere with the dipole offset, which was revealed by MESSENGER observations, in order to investigate its global structure under northward interplanetary magnetic field (IMF) conditions. Sodium ion dynamics originating from the Mercury's exosphere is also investigated based on statistical trajectory tracing in the electric and magnetic fields obtained from the MHD simulations. The results reveal a north-south asymmetry characterized by open field lines around southern polar region, and northward deflection of the plasma sheet in the far tail. The asymmetry of magnetic field structure near the planet drastically affects trajectories of sodium ion, and thus, their pressure distributions and precipitation pattern onto the planet. Weaker magnetic field strength in the southern hemisphere than in the north increases ion loss by precipitation onto the planetary surface in the southern hemisphere. The ‘sodium ring', which is formed by high-energy sodium ions drifting around the planet, is also found in the vicinity of the planet. The 'sodium ring' is almost circular under nominal solar wind conditions. The ring becomes partial under high solar wind density, because dayside magnetosphere is so compressed that there is no space for the sodium ions to drift around. In both cases, the 'sodium ring' is formed by sodium ions that are picked up and accelerated in the magnetosheath just outside the magnetopause and reentered into the magnetosphere due to combined effects of finite Larmor radius and convection electric field in the dawn-side magnetosphere. (10.1002/2017JA024082)
  DOI : 10.1002/2017JA024082
• Ignition of high pressure lean H<SUB>2</SUB>:air mixture along the multiple channels of nanosecond surface discharge
  
  • Shcherbanev S.A.
  • Popov N.A.
  • Starikovskaia Svetlana
  Combustion and Flame, Elsevier, 2017, 176, pp.272284. The initiation of combustion of lean H2 :air mixtures, ER = 0.50.6 , by nanosecond surface dielectric bar- rier discharge (nSDBD) was studied experimentally at high initial pressures, P=36 bar. The discharge was studied in different gas mixtures for the pressure range 112 bar. The ignition was initiated by two different discharge modes: streamer or filamentary nSDBD. The influence of the discharge structure and energy deposition on the ignition was demonstrated. Three regimes of multi-point ignition were observed: ignition with a few kernels, quasi-uniform ignition along the edge of the high voltage electrode and ignition along the plasma channels. The velocities of flame propagation were analyzed. The minimum ignition energy of the discharge and ignition delay time of combustion have been measured and analyzed with the help of kinetic numerical modeling. (10.1016/j.combustflame.2016.07.035)
  DOI : 10.1016/j.combustflame.2016.07.035
• Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause
  
  • Ergun R. E.
  • Chen L.-J.
  • Wilder F. D.
  • Ahmadi N.
  • Eriksson S.
  • Usanova M. E.
  • Goodrich K. A.
  • Holmes J. C.
  • Sturner A. P.
  • Malaspina D. M.
  • Newman D. L.
  • Torbert R. B.
  • Argall M. R.
  • Lindqvist P.-A.
  • Burch J. L.
  • Webster J. M.
  • Drake J. F.
  • Price L.
  • Cassak P. A.
  • Swisdak M.
  • Shay M. A.
  • Graham D. B.
  • Strangeway R. J.
  • Russell C. T.
  • Giles B. L.
  • Dorelli J. C.
  • Gershman D. J.
  • Avanov L.
  • Hesse Michael
  • Lavraud B.
  • Le Contel Olivier
  • Retinò Alessandro
  • Phan T. D.
  • Goldman M. V.
  • Stawarz J. E.
  • Schwartz S. J.
  • Eastwood Jonathan P.
  • Hwang K.-J.
  • Nakamura R.
  • Wang S.
  Geophysical Research Letters, American Geophysical Union, 2017, 44 (7), pp.2978-2986. Observations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large-amplitude parallel electric fields (E<SUB>||</SUB>). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large-amplitude ( 100 mV/m) E<SUB>||</SUB> in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process. (10.1002/2016GL072493)
  DOI : 10.1002/2016GL072493
• Saturation of energetic-particle-driven geodesic acoustic modes due to wave–particle nonlinearity
  
  • Biancalani A.
  • Chavdarovski I.
  • Qiu Z.
  • Bottino A.
  • Sarto D. Del
  • Ghizzo A.
  • Gürcan Özgür D.
  • Morel Pierre
  • Novikau I.
  Journal of Plasma Physics, Cambridge University Press (CUP), 2017, 83 (6), pp.725830602. The nonlinear dynamics of energetic-particle (EP) driven geodesic acoustic modes (EGAM) is investigated here. A numerical analysis with the global gyrokinetic particle-in-cell code ORB5 is performed, and the results are interpreted with the analytical theory, in close comparison with the theory of the beam-plasma instability. Only axisymmetric modes are considered, with a nonlinear dynamics determined by wave-particle interaction. Quadratic scalings of the saturated electric field with respect to the linear growth rate are found for the case of interest. As a main result, the formula for the saturation level is provided. Near the saturation, we observe a transition from adiabatic to non-adiabatic dynamics, i.e., the frequency chirping rate becomes comparable to the resonant EP bounce frequency. The numerical analysis is performed here with electrostatic simulations with circular flux surfaces, and kinetic effects of the electrons are neglected. (10.1017/S0022377817000976)
  DOI : 10.1017/S0022377817000976
• E x B staircases and barrier permeability in magnetised plasmas
  
  • Hornung G.
  • Dif-Pradalier Guilhem
  • Clairet F.
  • Sarazin Y.
  • Sabot R.
  • Hennequin Pascale
  • Verdoolaege G.
  Nuclear Fusion, IOP Publishing, 2017, 57 (1), pp.014006. In-depth experimental characterisation of spontaneous shear flow patterning into a so-called ##IMG## [http://ej.iop.org/images/0029-5515/57/1/014006/nfaa42aaieqn003.gif] \mathbfE× \mathbfB staircase?named after its planetary analogue?is shown in magnetised plasma turbulence, using ultrafast-sweeping reflectometry in the Tore Supra tokamak. Staircase signatures are found in a large variety of L-mode plasma conditions. Sensitivity to the dominant source of free energy is highlighted for the first time. A connection between staircase shear layer permeability and deviation from gyro-Bohm confinement scaling is strongly suggested, opening new routes to understanding confinement in drift-wave turbulence. (10.1088/0029-5515/57/1/014006)
  DOI : 10.1088/0029-5515/57/1/014006
• Evidence and relevance of spatially chaotic magnetic field lines in MCF devices
  
  • Firpo Marie-Christine
  • Lifschitz Agustin
  • Ettoumi Wahb
  • Farengo Ricardo
  • Ferrari Hugo
  • Garcia-Martinez Pablo Luis
  Plasma Physics and Controlled Fusion, IOP Publishing, 2017, 59 (3). Numerical evidence for the existence of spatially chaotic magnetic field lines about the collapse phase of tokamak sawteeth with incomplete reconnection is presented. This uses the results of extensive test particle simulations in different sets of electromagnetic perturbations tested against experimental JET measurements. In tokamak sawteeth, that form a laboratory prototype of magnetic reconnection, the relative magnetic perturbation δB/B may reach a few percents. This does not apply to tokamak operating regimes dominated by turbulence where δB/B is usually not larger than 10 −4. However, this small magnetic perturbation being sustained by a large spectrum of modes is shown to be sufficient to ensure the existence of stochastic magnetic field lines. This has important consequences for magnetic confinement fusion (MCF) where electrons are dominantly governed by the magnetic force. Indeed some overlap between magnetic resonances can locally induce chaotic magnetic field lines enabling the spatial redistribution of the electron population and of its thermal content. As they are the swiftest plasma particles, electrons feed back the most rapid perturbations of the magnetic field (10.1088/1361-6587/aa570d)
  DOI : 10.1088/1361-6587/aa570d
• Counterpropagating radiative shock experiments on the Orion laser
  
  • Suzuki-Vidal Francisco
  • Clayson Thomas
  • Stehlé Chantal
  • Swadling G. F.
  • Foster J.
  • Skidmore J.
  • Graham P.
  • Burdiak G.
  • Lebedev S. V.
  • Chaulagain Uddhab
  • Singh Raj Laxmi
  • Gumbrell E.
  • Patankar S.
  • Spindloe C.
  • Larour Jean
  • Kozlová Michaela
  • Rodriguez Perez R.
  • Gil J. M.
  • Espinosa G.
  • Velarde P.
  • Danson C.
  Physical Review Letters, American Physical Society, 2017, 119 (05), pp.055001. We present new experiments to study the formation of radiative shocks and the interaction between two counterpropagating radiative shocks. The experiments are performed at the Orion laser facility, which is used to drive shocks in xenon inside large aspect ratio gas cells. The collision between the two shocks and their respective radiative precursors, combined with the formation of inherently three-dimensional shocks, provides a novel platform particularly suited for the benchmarking of numerical codes. The dynamics of the shocks before and after the collision are investigated using point-projection x-ray backlighting while, simultaneously, the electron density in the radiative precursor was measured via optical laser interferometry. Modeling of the experiments using the 2D radiation hydrodynamic codes NYM and PETRA shows very good agreement with the experimental results. (10.1103/PhysRevLett.119.055001)
  DOI : 10.1103/PhysRevLett.119.055001
• Long-lived laser-induced arc discharges for energy channeling applications
  
  • Point Guillaume
  • Arantchouk Léonid
  • Thouin Emmanuelle
  • Carbonnel Jérôme
  • Mysyrowicz André
  • Houard Aurélien
  Scientific Reports, Nature Publishing Group, 2017, 7 (1), pp.13801. Laser filamentation offers a promising way for the remote handling of large electrical power in the form of guided arc discharges. We here report that it is possible to increase by several orders of magnitude the lifetime of straight plasma channels from filamentation-guided sparks in atmospheric air. A 30 ms lifetime can be reached using a low-intensity, 100 mA current pulse. Stability of the plasma shape is maintained over such a timescale through a continuous Joule heating from the current. This paves the way for applications based on the generation of straight, long duration plasma channels, like virtual plasma antennas or contactless transfer of electric energy. (10.1038/s41598-017-14054-z)
  DOI : 10.1038/s41598-017-14054-z
• Erratum: "On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence" (2017, ApJL, 836, L10)
  
  • Huang S. Y.
  • Hadid Lina
  • Sahraoui Fouad
  • Yuan Z. G.
  • Deng X. H.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2017, 837 (2), pp.L31. Not Available (10.3847/2041-8213/aa633c)
  DOI : 10.3847/2041-8213/aa633c
• Controlling plasma properties under differing degrees of electronegativity using odd harmonic dual frequency excitation
  
  • Gibson Andrew R.
  • Gans Timo
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26. The charged particle dynamics in low-pressure oxygen plasmas excited by odd harmonic dual frequency waveforms (low frequency of 13.56 MHz and high frequency of 40.68 MHz) are investigated using a one-dimensional numerical simulation in regimes of both low and high electronegativity. In the low electronegativity regime, the time and space averaged electron and negative ion densities are approximately equal and plasma sustainment is dominated by ionisation at the sheath expansion for all combinations of low and high frequency and the phase shift between them. In the high electronegativity regime, the negative ion density is a factor of 15--20 greater than the low electronegativity cases. In these cases, plasma sustainment is dominated by ionisation inside the bulk plasma and at the collapsing sheath edge when the contribution of the high frequency to the overall voltage waveform is low. As the high frequency component contribution to the waveform increases, sheath expansion ionisation begins to dominate. It is found that the control of the average voltage drop across the plasma sheath and the average ion flux to the powered electrode are similar in both regimes of electronegativity, despite the differing electron dynamics using the considered dual frequency approach. This offers potential for similar control of ion dynamics under a range of process conditions, independent of the electronegativity. This is in contrast to ion control offered by electrically asymmetric waveforms where the relationship between the ion flux and ion bombardment energy is dependent upon the electronegativity. (10.1088/1361-6595/aa8dcd)
  DOI : 10.1088/1361-6595/aa8dcd
• Influence of neutral pressure on instability enhanced friction and ion velocities at the sheath edge of two-ion-species plasmas
  
  • Adrian P. J.
  • Baalrud S. D.
  • Lafleur T.
  Physics of Plasmas, American Institute of Physics, 2017, 24. The Instability Enhanced Friction theory [Baalrud et al., Phys. Rev. Lett. 103, 205002 (2009)] is extended to account for the influence of neutral pressure in predicting the flow speed of each ion species at the sheath edge of plasmas containing two ion species. Particle-in-cell simulations show that the theory accurately predicts both the neutral pressure cutoff of ion-ion two-stream instabilities and the ion flow speeds at the sheath edge as pressure is varied over several orders of magnitude. The simulations are used to directly calculate the instability-enhanced ion-ion friction force. At sufficiently high neutral pressure, the simulations also provide evidence for collisional modifications to the Bohm criterion. (10.1063/1.4986239)
  DOI : 10.1063/1.4986239
• Enhanced control of the ionization rate in radio-frequency plasmas with structured electrodes via tailored voltage waveforms
  
  • Doyle Scott J.
  • Lafleur Trevor
  • Gibson Andrew R.
  • Tian Peng
  • Kushner Mark J.
  • Dedrick James
  Plasma Sources Science and Technology, IOP Publishing, 2017, 26. Radio-frequency capacitively coupled plasmas that incorporate structured electrodes enable increases in the electron density within spatially localized regions through the hollow cathode effect (HCE). This enables enhanced control over the spatial profile of the plasma density, which is useful for several applications including materials processing, lighting and spacecraft propulsion. However, asymmetries in the powered and grounded electrode areas inherent to the hollow cathode geometry lead to the formation of a time averaged dc self-bias voltage at the powered electrode. This bias alters the energy and flux of secondary electrons leaving the surface of the cathode and consequentially can moderate the increased localized ionization afforded by the hollow cathode discharge. In this work, two-dimensional fluid-kinetic simulations are used to demonstrate control of the dc self-bias voltage in a dual-frequency driven (13.56, 27.12 MHz), hollow cathode enhanced, capacitively coupled argon plasma over the 66.6--200 Pa (0.5--1.5 Torr) pressure range. By varying the phase offset of the 27.12 MHz voltage waveform, the dc self-bias voltage varies by 10%--15% over an applied peak-to-peak voltage range of 600--1000 V, with lower voltages showing higher modulation. Resulting ionization rates due to secondary electrons within the hollow cathode cavity vary by a factor of 3 at constant voltage amplitude, demonstrating the ability to control plasma properties relevant for maintaining and enhancing the HCE. (10.1088/1361-6595/aa96e5)
  DOI : 10.1088/1361-6595/aa96e5
• MMS observations of whistler waves in electron diffusion region
  
  • Cao D.
  • Fu H.S.
  • Cao J.B.
  • Wang T. Y.
  • Graham D. B.
  • Chen Z. Z.
  • Peng F. Z.
  • Huang S. Y.
  • Khotyaintsev Y. V.
  • André M.
  • Russell C. T.
  • Giles B. L.
  • Lindqvist P.-A.
  • Torbert R. B.
  • Ergun R. E.
  • Le Contel Olivier
  • Burch J. L.
  Geophysical Research Letters, American Geophysical Union, 2017, 44 (9), pp.3954-3962. Whistler waves that can produce anomalous resistivity by affecting electrons' motion have been suggested as one of the mechanisms responsible for magnetic reconnection in the electron diffusion region (EDR). Such type of waves, however, has rarely been observed inside the EDR so far. In this study, we report such an observation by Magnetospheric Multiscale (MMS) mission. We find large-amplitude whistler waves propagating away from the X line with a very small wave-normal angle. These waves are probably generated by the perpendicular temperature anisotropy of the 300 eV electrons inside the EDR, according to our analysis of dispersion relation and cyclotron resonance condition; they significantly affect the electron-scale dynamics of magnetic reconnection and thus support previous simulations. (10.1002/2017GL072703)
  DOI : 10.1002/2017GL072703
• Differential kinetic physics of solar-wind minor ions
  
  • Perrone Denise
  • Valentini F.
  • Servidio S.
  • Stabile S.
  • Pezzi O.
  • Sorriso-Valvo L.
  • de Marco R.
  • Marcucci M. F.
  • Brienza D.
  • Bruno Roberto
  • Lavraud Benoit
  • Retinò Alessandro
  • Vaivads A.
  • Consolini G.
  • de Keyser J.
  • Salatti M.
  • Veltri P.
  , 2017, 19, pp.13382. The solar wind, although predominantly constituted of protons, is also made up of a finite amount of alpha particles, together with a few percent of heavier ions. The kinetic properties of heavy ions in the solar wind are known to behave in a well organized way under most solar-wind flow conditions: their speeds are faster than that of hydrogen by about the local Alfvén speed, and their kinetic temperatures are more than proportional to their mass. Preferential heating and acceleration of heavy ions in the solar wind and corona represent a long-standing theoretical problem in space physics, and are distinct experimental signatures of kinetic processes occurring in collisionless plasmas. However, due to very scarce measurements of heavy ions at time resolutions comparable with their kinetic scales, energy partition between species in turbulent plasma dissipation is basically unexplored. For the moment, most of the information comes from numerical simulations and a crucial support is given by self-consistent, fully nonlinear Vlasov models. Here, hybrid Vlasov-Maxwell simulations are used to investigate the role of kinetic effects in a two-dimensional turbulent multi-ion plasma, composed of kinetic protons and alpha particles, and fluid electrons. The response of different ion species to the fluctuating electromagnetic fields appears to be different. In particular, a significant differential heating of alpha particles with respect to protons is observed, localized nearby the peaks of ion vorticity and where strong deviations from thermodynamic equilibrium are recovered. Then, the understanding of the complex process of particle heating results strongly related to the study of the non-Maxwellian features on the three-dimensional ion velocity distributions. These numerical results highlight the importance for the future space missions to provide detailed ion measurements to make a significant step forward in the problem of heating in turbulent space plasmas.
• Transfer of microwave energy along a filament plasma column in air
  
  • Prade Bernard
  • Houard Aurélien
  • Larour Jean
  • Pellet Michel
  • Mysyrowicz André
  Applied Physics B - Laser and Optics, Springer Verlag, 2017, 123, pp.40. We demonstrate the coupling of microwave radiation into a plasma channel formed by laser filamentation in air, leading to the amplification by two orders of magnitude of longitudinal oscillations of the plasma. Transfer of this longitudinal excitation towards unexcited region of the plasma column occurs over more than 10 cm, in good agreement with a theoretical model describing the propagation of a TM wave guided along the surface between air and plasma. We foresee that high power low frequency electromagnetic waves injected into a multi-filament plasma could initiate and sustain a long-lived plasma over several meters distance. (10.1007/s00340-016-6616-4)
  DOI : 10.1007/s00340-016-6616-4
• Acceleration of energetic electrons by waves in inhomogeneous solar wind plasmas
  
  • Krafft C.
  • Volokitin A.
  Journal of Plasma Physics, Cambridge University Press (CUP), 2017, 83 (2), pp.705830201. The paper studies the influence of the background plasma density fluctuations on the dynamics of the Langmuir turbulence generated by electron beams, for parameters typical for solar type III beams and plasmas near 1 AU. A self-consistent Hamiltonian model based on the Zakharov and the Newton equations is used, which presents several advantages compared to the Vlasov approach. Beams generating Langmuir turbulence can be accelerated as a result of wave transformation effects or/and decay cascade processes; in both cases, the beam-driven Langmuir waves transfer part of their energy to waves of smaller wavenumbers, which can be reabsorbed later on by beam particles of higher velocities. As a consequence, beams can conserve a large part of their initial kinetic energy while propagating and radiating wave turbulence over long distances in inhomogeneous plasmas. Beam particles can also be accelerated in quasi-homogeneous plasmas due to the second cascade of wave decay, the wave transformation processes being very weak in this case. The net gains and losses of energy of a beam and the wave turbulence it radiates are calculated as a function of the average level of plasma density fluctuations and the beam parameters. The results obtained provide relevant information on the mechanism of energy reabsorption by beams radiating Langmuir turbulence in solar wind plasmas. (10.1017/S0022377817000174)
  DOI : 10.1017/S0022377817000174