Publications

2019

• Properties of the Turbulence Associated with Electron-only Magnetic Reconnection in Earth's Magnetosheath
  
  • Stawarz J. E.
  • Eastwood Jonathan P.
  • Phan T. D.
  • Gingell I. L.
  • Shay M. A.
  • Burch J. L.
  • Ergun R. E.
  • Giles B. L.
  • Gershman D. J.
  • Le Contel Olivier
  • Lindqvist P.-A.
  • Russell C. T.
  • Strangeway R. J.
  • Torbert R. B.
  • Argall M. R.
  • Fischer D.
  • Magnes W.
  • Franci L.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2019, 877 (2), pp.L37. Turbulent plasmas generate intense current structures, which have long been suggested as magnetic reconnection sites. Recent Magnetospheric Multiscale observations in Earth's magnetosheath revealed a novel form of reconnection where the dynamics only couple to electrons, without ion involvement. It was suggested that such dynamics were driven by magnetosheath turbulence. In this study, the fluctuations are examined to determine the properties of the turbulence and if a signature of reconnection is present in the turbulence statistics. The study reveals statistical properties consistent with plasma turbulence with a correlation length of ~10 ion inertial lengths. When reconnection is more prevalent, a steepening of the magnetic spectrum occurs at the length scale of the reconnecting current sheets. The statistics of intense currents suggest the prevalence of electron-scale current sheets favorable for electron reconnection. The results support the hypothesis that electron reconnection is driven by turbulence and highlight diagnostics that may provide insight into reconnection in other turbulent plasmas. (10.3847/2041-8213/ab21c8)
  DOI : 10.3847/2041-8213/ab21c8
• High-Resolution Measurements of the Cross-Shock Potential, Ion Reflection, and Electron Heating at an Interplanetary Shock by MMS
  
  • Cohen Ian J.
  • Schwartz Steven J.
  • Goodrich Katherine A.
  • Ahmadi Narges
  • Ergun Robert E.
  • Fuselier Stephen A.
  • Desai Mihir I.
  • Christian Eric R.
  • Mccomas David J.
  • Zank Gary P.
  • Shuster Jason R.
  • Vines Sarah K.
  • Mauk Barry H.
  • Decker Robert B.
  • Anderson Brian J.
  • Westlake Joseph H.
  • Le Contel Olivier
  • Breuillard Hugo
  • Giles Barbara L.
  • Torbert Roy B.
  • Burch James L.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 124, pp.3961-3978. The Magnetospheric Multiscale (MMS) spacecraft obtained unprecedented high-time resolution multipoint particle and field measurements of an interplanetary shock event on 8 January 2018. The spacecraft encountered the supercritical forward shock of a forward/reverse shock pair in the pristine solar wind upstream of the bow shock near the subsolar point as they neared apogee at 25 R<SUB>E</SUB>. The high-time resolution measurements from the four spacecraft, separated by only 20 km, allowed direct measurement of particle distributions revealing evidence of electron heating and near specularly reflected ions. The cross-shock potential is calculated directly from 3-D electric field measurements. This is the first reported direct high temporal resolution (<1 s) observation at an interplanetary shock of near specularly reflected ions. Calculation of the cross-shock potential yields a potential jump significant enough to reflect at least some of the protons from the incident solar wind beam. The cross-shock potential calculated here is consistent with previous estimations based on particle measurements and numerical/analytical simulations. The ambipolar contribution to the cross-shock potential calculated from the four-spacecraft divergence of the electron pressure tensor is somewhat higher than that inferred form the Liouville-mapped electron energy gain across the shock. Furthermore, the high-time-resolution 3-D electric field measurements reported here reveal small-scale nonlinear structures embedded in the shock layer that contribute to the nonmonotonic shock transition. (10.1029/2018JA026197)
  DOI : 10.1029/2018JA026197
• Signatures of Cold Ions in a Kinetic Simulation of the Reconnecting Magnetopause
  
  • Dargent Jérémy
  • Aunai Nicolas
  • Lavraud B.
  • Toledo-Redondo Sergio
  • Califano F.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2019, 124 (4), pp.2497-2514. Abstract At the Earth's magnetopause, a low-energy ion population of ionospheric origin is commonly observed at the magnetospheric side. In this work we use a 2-D fully kinetic simulation to identify several original signatures related to the dynamics of cold ions involved in magnetic reconnection at the asymmetric dayside magnetopause. We identify several original signatures of the cold ions dynamics driven by the development of magnetic reconnection at the asymmetric dayside magnetopause. We find that cold ions tend to rarefy in the diffusion region, while their density is enhanced as a result of compression along magnetospheric separatrices. We also observe the formation of crescent-shaped cold ion distribution functions along the separatrices in the near-exhaust region, and we present an analytical model to explain this signature. Finally, we give evidence of a localized parallel heating of cold ions. These signatures should be detected with the magnetospheric multiscale mission high-resolution observations. (10.1029/2018JA026343)
  DOI : 10.1029/2018JA026343
• Collision risk prediction for constellation design
  
  • Lucken Romain
  • Giolito Damien
  Acta Astronautica, Elsevier, 2019, 161, pp.492-501. INDEMN is an object-oriented program dedicated to the modeling of the evolution of the densities of space objects. Following the work achieved by D. Kessler (1978) and by other authors more recently (G. L. Somma, IAC 2016, A6-IP3; A. Rossi, DPPS 2004, 197), the dynamical model is based on a source and sink approach for various altitudes. The source terms represent the future launches, the explosion of intact spacecrafts, and the collision between objects. Different collision cross sections are used for the various types of objects and the number of debris generated is based on the NASA break-up model. The sink terms are the drag and the end-of-life de-orbitation for the satellites launched after 2009, with a controllable success rate. The code was validated against former simulations performed with statistical and semi-deterministic models. In addition to the classical object types featured in several statistical codes, which are intact objects, explosion debris, and collision debris, a new type representing the satellites of a specific constellation is included. These satellites orbit with altitudes close to 1200&#8239;km and they can perform collision avoidance maneuvers as long as they are fully operational. It is shown that, under realistic assumptions, if only one primary collision occurs at an altitude of 800&#8239;km, the probability of a collision involving a constellation satellite becomes larger than 2% by 2035, which highly jeopardizes the satellite constellation as a whole. (10.1016/j.actaastro.2019.04.003)
  DOI : 10.1016/j.actaastro.2019.04.003
• Correction to: Statistical Analysis of Solar Events Associated with Storm Sudden Commencements over One Year of Solar Maximum During Cycle 23: Propagation from the Sun to the Earth and Effects
  
  • Bocchialini Karine
  • Grison B.
  • Menvielle Michel
  • Chambodut Aude
  • Cornilleau-Wehrlin Nicole
  • Fontaine Dominique
  • Marchaudon Aurélie
  • Pick M.
  • Pitout Frédéric
  • Schmieder Brigitte
  • Régnier S.
  • Zouganelis I.
  Solar Physics, Springer Verlag, 2019, 294, pp.art. 38. Taking the 32 storm sudden commencements (SSCs) listed by the International Service of Geomagnetic Indices (ISGI) of the Observatory de l’Ebre during 2002 (solar activity maximum in Cycle 23) as a starting point, we performed a multi-criterion analysis based on observations (propagation time, velocity comparisons, sense of the magnetic field rotation, radio waves) to associate them with solar sources, identified their effects in the interplanetary medium, and looked at the response of the terrestrial ionized and neutral environment. We find that 28 SSCs can be related to 44 coronal mass ejections (CMEs), 15 with a unique CME and 13 with a series of multiple CMEs, among which 19 (68%) involved halo CMEs. Twelve of the 19 fastest CMEs with speeds greater than 1000 km s−1 are halo CMEs. For the 44 CMEs, including 21 halo CMEs, the corresponding X-ray flare classes are: 4 X-class, 19 M-class, and 21 C-class flares. The probability for an SSC to occur is 75% if the CME is a halo CME. Among the 500, or even more, front-side, non-halo CMEs recorded in 2002, only 23 could be the source of an SSC, i.e. 5%. The complex interactions between two (or more) CMEs and the modification of their trajectories have been examined using joint white-light and multiple-wavelength radio observations. The detection of long-lasting type IV bursts observed at metric–hectometric wavelengths is a very useful criterion for the CME–SSC association. The events associated with the most depressed Dst values are also associated with type IV radio bursts. The four SSCs associated with a single shock at L1 correspond to four radio events exhibiting characteristics different from type IV radio bursts. The solar-wind structures at L1 after the 32 SSCs are 12 magnetic clouds (MCs), 6 interplanetary coronal mass ejections (ICMEs) without an MC structure, 4 miscellaneous structures, which cannot unambiguously be classified as ICMEs, 5 corotating or stream interaction regions (CIRs/SIRs), one CIR caused two SSCs, and 4 shock events; notethat one CIR caused two SSCs. The 11 MCs listed in 3 or more MC catalogs covering the year 2002 are associated with SSCs. For the three most intense geomagnetic storms (based on Dst minima) related to MCs, we note two sudden increases of the Dst, at the arrival of the sheath and the arrival of the MC itself. In terms of geoeffectiveness, the relation between the CME speed and the magnetic-storm intensity, as characterized using the Dst magnetic index, is very complex, but generally CMEs with velocities at the Sun larger than 1000 km s−1 have larger probabilities to trigger moderate or intense storms. The most geoeffective events are MCs, since 92% of them trigger moderate or intense storms, followed by ICMEs (33%). At best, CIRs/SIRs only cause weak storms. We show that these geoeffective events (ICMEs or MCs) trigger an increased and combined auroral kilometric radiation (AKR) and non-thermal continuum (NTC) wave activity in the magnetosphere, an enhanced convection in the ionosphere, and a stronger response in the thermosphere. However, this trend does not appear clearly in the coupling functions, which exhibit relatively weak correlations between the solar-wind energy input and the amplitude of various geomagnetic indices, whereas the role of the southward component of the solar-wind magnetic field is confirmed. Some saturation appears for Dst values <−100nT on the integrated values of the polar and auroral indices. (10.1007/s11207-019-1426-6)
  DOI : 10.1007/s11207-019-1426-6
• [Plasma 2020 Decadal] Multipoint Measurements of the Solar Wind: A Proposed Advance for Studying Magnetized Turbulence of the Solar Wind: A Proposed Advance for Studying Magnetized Turbulence
  
  • G. Klein Kristopher
  • Alexandrova Olga
  • Bookbinder Jay
  • Caprioli D.
  • W. Case A.
  • D. G. Chandran B.
  • J. Chen L.
  • Horbury T.
  • C. Kasper J.
  • Le Contel Olivier
  • A. Maruca B.
  • Matthaeus W. H.
  • Retinò Alessandro
  • Roberts O.
  • Schekochihin A.
  • Skoug Ruth M.
  • W. Smith C.
  • Steinberg John T.
  • Spence H. E.
  • Vasquez B.
  • M. Tenbarge J.
  • Verscharen Daniel
  • Whittlesey P.
  • Jian L.
  , 2019.
• SOTE: A Nonlinear Method for Magnetic Topology Reconstruction in Space Plasmas
  
  • Liu Y. Y.
  • Fu H.S.
  • Olshevsky V.
  • Pontin D. I.
  • Liu C. M.
  • Wang Z.
  • Chen G.
  • Dai L.
  • Retinò Alessandro
  The Astrophysical Journal Supplement, American Astronomical Society / IOP Science, 2019, 244 (2), pp.31. Complex magnetic structures are ubiquitous in turbulent astrophysical plasmas. Such structures can be host to many dynamic processes, such as magnetic reconnection and energy dissipation. Thus, revealing the 3D topologies of these structures is necessary. In this study, we propose a new method to reconstruct complex magnetic topologies in quasi-steady space plasmas, by utilizing eight-point measurements of magnetic fields and particles. Such a method, based on the Second-Order Taylor Expansion (SOTE) of a magnetic field, is nonlinear; it is constrained by \rm∇ · \boldsymbolB=0 and \rm∇ × \boldsymbolB=μ _0\boldsymbolJ, where \boldsymbolJ=ne(\boldsymbolV_\boldsymboli-\boldsymbolV_\boldsymbole) is from particle moments. A benchmark test of this method, using the simulation data, shows that the method can give accurate reconstruction results within an area about three times the size of a spacecraft tetrahedron. By comparing to the previous First-Order Taylor Expansion (FOTE) method, this method (SOTE) gives similar results for reconstructing quasilinear structures but exhibits better accuracy in reconstructing nonlinear structures. Such a method will be useful to the multi-scale missions, such as the future European Space Agency's "cross-scale" mission and China's "self-adaptive" mission. Also, it can be applied to four-point missions, such as Cluster and the Magnetospheric Multiscale Mission. We demonstrated how to apply this method to the four-point missions. In principle, this method will be useful to study shocks, magnetic holes, dipolarization fronts, and other nonlinear structures in space plasmas (10.3847/1538-4365/ab391a)
  DOI : 10.3847/1538-4365/ab391a
• Experimental demonstration of multifrequency impedance matching for tailored voltage waveform plasmas
  
  • Wang Junkang
  • Diné Sébastien
  • Booth Jean-Paul
  • Johnson Erik
  Journal of Vacuum Science & Technology A, American Vacuum Society, 2019, 37 (2), pp.021303. Driving radiofrequency capacitively coupled plasmas by multiharmonic tailored voltage waveforms (TVWs) has been shown to allow considerable control over various plasma properties for surface processing applications. However, industrial adoption of this technology would benefit from more efficient solutions to the challenge of impedance matching the radiofrequency power source to the load simultaneously at multiple harmonic frequencies. The authors report on the design and demonstration of a simple, practical multifrequency matchbox (MFMB) based on a network of LC resonant circuits. The performance of the matchbox was quantified in terms of a range of matchable impedances (when matching a single frequency at a time), as well as for the independence of each match to changes at adjacent harmonics. The effectiveness of the MFMB was demonstrated experimentally on an Ar plasma excited by a three-frequency TVW with a fundamental frequency of 13.56 MHz. Under the plasma conditions studied, the power coupling efficiency (at the generator output) was increased from less than 40% (without impedance matching) to between 80% and 99% for the different exciting frequencies. (10.1116/1.5056205)
  DOI : 10.1116/1.5056205
• Electron Diffusion Regions in Magnetotail Reconnection Under Varying Guide Fields
  
  • Chen L.-J
  • Wang S.
  • Hesse M.
  • Ergun R.
  • Moore T.
  • Giles B.
  • Bessho N.
  • Russell C.
  • Burch J.
  • Torbert R. B
  • Genestreti K. J
  • Paterson W.
  • Pollock C.
  • Lavraud B.
  • Le Contel Olivier
  • Strangeway R.
  • Khotyaintsev Yu V
  • Lindqvist P.-A
  Geophysical Research Letters, American Geophysical Union, 2019, 46 (12), pp.6230-6238. Kinetic structures of electron diffusion regions (EDRs) under finite guide fields in magnetotail reconnection are reported. The EDRs with guide fields 0.14–0.5 (in unit of the reconnecting component) are detected by the Magnetospheric Multiscale spacecraft. The key new features include the following: (1) cold inflowing electrons accelerated along the guide field and demagnetized at the magnetic field minimum while remaining a coherent population with a low perpendicular temperature, (2) wave fluctuations generating strong perpendicular electron flows followed by alternating parallel flows inside the reconnecting current sheet under an intermediate guide field, and (3) gyrophase bunched electrons with high parallel speeds leaving the X‐line region. The normalized reconnection rates for the three EDRs range from 0.05 to 0.3. The measurements reveal that finite guide fields introduce new mechanisms to break the electron frozen‐in condition. (10.1029/2019GL082393)
  DOI : 10.1029/2019GL082393
• [Plasma 2020 Decadal] Disentangling the Spatiotemporal Structure of Turbulence Using Multi-Spacecraft Data
  
  • Tenbarge J.
  • Alexandrova O.
  • Boldyrev S.
  • Califano F.
  • Cerri S.
  • Chen C.
  • Howes G.
  • Horbury T.
  • Isenberg P.
  • Ji H.
  • Klein K.
  • Krafft C.
  • Kunz M.
  • Loureiro N.
  • Mallet A.
  • Maruca B.
  • Matthaeus W.
  • Meyrand R.
  • Quataert E.
  • Perez J.
  • Roberts O.
  • Sahraoui F.
  • Salem C.
  • Schekochihin A.
  • Spence H.
  • Squire J.
  • Told D.
  • Verscharen D.
  • Wicks R.
  Plasma 2020 Decadal Survey, 2019. This white paper submitted for 2020 Decadal Assessment of Plasma Science concerns the importance of multi-spacecraft missions to address fundamental questions concerning plasma turbulence. Plasma turbulence is ubiquitous in the universe, and it is responsible for the transport of mass, momentum, and energy in such diverse systems as the solar corona and wind, accretion discs, planet formation, and laboratory fusion devices. Turbulence is an inherently multi-scale and multi-process phenomenon, coupling the largest scales of a system to sub-electron scales via a cascade of energy, while simultaneously generating reconnecting current layers, shocks, and a myriad of instabilities and waves. The solar wind is humankind's best resource for studying the naturally occurring turbulent plasmas that permeate the universe. Since launching our first major scientific spacecraft mission, Explorer 1, in 1958, we have made significant progress characterizing solar wind turbulence. Yet, due to the severe limitations imposed by single point measurements, we are unable to characterize sufficiently the spatial and temporal properties of the solar wind, leaving many fundamental questions about plasma turbulence unanswered. Therefore, the time has now come wherein making significant additional progress to determine the dynamical nature of solar wind turbulence requires multi-spacecraft missions spanning a wide range of scales simultaneously. A dedicated multi-spacecraft mission concurrently covering a wide range of scales in the solar wind would not only allow us to directly determine the spatial and temporal structure of plasma turbulence, but it would also mitigate the limitations that current multi-spacecraft missions face, such as non-ideal orbits for observing solar wind turbulence. Some of the fundamentally important questions that can only be addressed by in situ multipoint measurements are discussed. (10.48550/arXiv.1903.05710)
  DOI : 10.48550/arXiv.1903.05710
• MMS Observations of Kinetic-size Magnetic Holes in the Terrestrial Magnetotail Plasma Sheet
  
  • Huang S. Y.
  • He L. H.
  • Yuan Z. G.
  • Sahraoui Fouad
  • Le Contel Olivier
  • Deng X. H.
  • Zhou M.
  • Fu H.S.
  • Jiang K.
  • Yu X. D.
  • Li H. M.
  • Deng D.
  • Pollock C. J.
  • Torbert R. B.
  • Burch J. L.
  The Astrophysical Journal, American Astronomical Society, 2019, 875 (2), pp.113, 8 pp. Kinetic-size magnetic holes (KSMHs) in the terrestrial magnetotail plasma sheet are statistically investigated using the observations from the Magnetospheric Multiscale mission. The scales of KSMHs are found to be smaller than one ion gyroradius or tens of electron gyroradii. The occurrence distributions of KSMHs have dawn−dusk asymmetry (duskside preference) in the magnetotail, which may be caused by the Hall effect. Most events of KSMHs (71.7%) are accompanied by a substorm, implying that substorms may provide favorable conditions for the excitation of KSMHs. However, there is a weak correlation between KSMHs and magnetic reconnection. The statistical results reveal that for most of the events, the electron total temperature and perpendicular temperature increase while the electron parallel temperature decreases inside the KSMHs. The electron temperature anisotropy (T <SUB>e&#8869;</SUB>/T<SUB>e</SUB>| | > 1) is observed in 72% of KSMHs. Whistler-mode waves are frequently observed inside the KSMHs, and most (92%) KSMHs associated with whistler waves have enhancements of electron perpendicular distributions and satisfy the unstable condition of whistler instability. This suggests that the observed electron-scale whistler waves, locally generated by the electron temperature anisotropy, could couple with the electron-scale KSMHs. The observed features of KSMHs and their coupling to electron-scale whistlers are similar to the ones in the turbulent magnetosheath, implying that they are ubiquitous in the space plasmas. The generation of KSMHs in the plasma sheet could be explained by an electron vortex magnetic hole, magnetosonic solitons, and/or ballooning/interchange instabilities. (10.3847/1538-4357/ab0f2f)
  DOI : 10.3847/1538-4357/ab0f2f
• Measurement of the tilt angle of turbulent structures in magnetically confined plasmas using Doppler reflectometry
  
  • Pinzon Javier
  • Estrada Teresa
  • Happel T.
  • Hennequin Pascale
  • Blanco E.
  • Stroth Ulrich
  Plasma Physics and Controlled Fusion, IOP Publishing, 2019. The mean tilt angle of turbulent structures is a key element for describing the turbulence and its interplay with plasma flows in magnetically confined plasmas. It is a quantity predicted by theories and gyrokinetic simulations, which can provide information on the type of the dominant micro-instability, and also on the turbulence anisotropy induced by sheared flows. A new method for measuring the tilt angle of turbulent structures using Doppler reflectometry has been recently introduced [J R Pinzón <i>et al</i> 2019 Nucl. Fusion 59 074002]. It is based on the time delay of the cross-correlation between microwaves backscattered at radially displaced positions. In this paper, the method is presented in detail and is successfully applied on the ASDEX Upgrade tokamak and the TJ-II stellarator. Measurements of the tilt angle in the core of both machines are reported, in the TJ-II case, for the first time. (10.1088/1361-6587/ab394d)
  DOI : 10.1088/1361-6587/ab394d
• Turbulence and Microprocesses in Inhomogeneous Solar Wind Plasmas
  
  • Krafft C.
  • Volokitin A. S.
  • Gauthier Gaétan
  Fluids, MDPI, 2019, 4 (2), pp.69. The random density fluctuations observed in the solar wind plasma crucially influence on the Langmuir wave turbulence generated by energetic electron beams ejected during solar bursts. Those are powerful phenomena consisting of a chain of successive processes leading ultimately to strong electromagnetic emissions. The small-scale processes governing the interactions between the waves, the beams and the inhomogeneous plasmas need to be studied to explain such macroscopic phenomena. Moreover, the complexity induced by the plasma irregularities requires to find new approaches and modelling. Therefore theoretical and numerical tools were built to describe the Langmuir wave turbulence and the beams dynamics in inhomogeneous plasmas, in the form of a self-consistent Hamiltonian model including a fluid description for the plasma and a kinetic approach for the beam. On this basis, numerical simulations were performed in order to shed light on the impact of the density fluctuations on the beam dynamics, the electromagnetic wave radiation, the generation of Langmuir wave turbulence, the waves coupling and decay phenomena involving Langmuir and low frequency waves, the acceleration of beam electrons, their diffusion mechanisms, the modulation of the Langmuir waveforms and the statistical properties of the radiated fields distributions. The paper presents the main results obtained in the form of a review. (10.3390/fluids4020069)
  DOI : 10.3390/fluids4020069
• Linear discriminant analysis based predator-prey analysis of hot electron effects on the X-pinch plasma produced K-shell Aluminum spectra
  
  • Fatih Yilmaz Mehmet
  • Danisman Yusuf
  • Larour Jean
  • Arantchouk Léonid
  Scientific Reports, Nature Publishing Group, 2019, 9, pp.11867. In this study, Linear Discriminant Analysis (LDA) is applied to investigate the electron beam effects on the X-pinch produced K-shell Aluminum plasma. The radiating plasma is produced by the explosion of two 25-μm Al wires on a compact L-C (40 kV, 200 kA and 200 ns) generator, and the time integrated spectra are recorded using de Broglie spectrographs. The ion and electron oscillations of K-shell Al plasma are extracted using LDA of spectral database of non-LTE K-shell Al model. A three dimensional representation of LDA shows that the presence of electron beam exhibits outward spirals of Langmuir turbulence and the center region of the spirals recieves lower electron temperatures of 50–100 eV. These spirals then are modeled by logistic growth of predator-prey model. This modeling suggests that the ions (LD1: most dominant eigenvector of LDA) and electrons (LD2: second most dominant eigenvector of LDA) represent the predators and preys, respectively. Besides, addition of electron beams transforms evanescent oscillations to the standing ones. (10.1038/s41598-019-47997-6)
  DOI : 10.1038/s41598-019-47997-6
• Fluidization of collisionless plasma turbulence
  
  • Meyrand Romain
  • Kanekar Anjor
  • Dorland William
  • Schekochihin Alexander A.
  Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 2019, 116, pp.1185. In a collisionless, magnetized plasma, particles may stream freely alongmagnetic field lines, leading to ?phase mixing? of their distributionfunction and consequently, to smoothing out of any ?compressive?fluctuations (of density, pressure, etc.). This rapid mixing underliesLandau damping of these fluctuations in a quiescent plasma?one of themost fundamental physical phenomena that makes plasma different from aconventional fluid. Nevertheless, broad power law spectra of compressivefluctuations are observed in turbulent astrophysical plasmas (mostvividly, in the solar wind) under conditions conducive to strong Landaudamping. Elsewhere in nature, such spectra are normally associated withfluid turbulence, where energy cannot be dissipated in the inertial-scale range and is, therefore, cascaded from large scales to small. Bydirect numerical simulations and theoretical arguments, it is shown herethat turbulence of compressive fluctuations in collisionless plasmasstrongly resembles one in a collisional fluid and does have broad powerlaw spectra. This ?fluidization? of collisionless plasmas occurs,because phase mixing is strongly suppressed on average by ?stochasticechoes,? arising due to nonlinear advection of the particle distributionby turbulent motions. Other than resolving the long-standing puzzle ofobserved compressive fluctuations in the solar wind, our results suggesta conceptual shift for understanding kinetic plasma turbulencegenerally: rather than being a system where Landau damping plays therole of dissipation, a collisionless plasma is effectivelydissipationless, except at very small scales. The universality of?fluid? turbulence physics is thus reaffirmed even for a kinetic,collisionless system. (10.1073/pnas.1813913116)
  DOI : 10.1073/pnas.1813913116
• Energy Cascade Rate Measured in a Collisionless Space Plasma with MMS Data and Compressible Hall Magnetohydrodynamic Turbulence Theory
  
  • Andrés Nahuel
  • Sahraoui Fouad
  • Galtier Sébastien
  • Hadid Lina
  • Ferrand R.
  • Huang S.Y.
  Physical Review Letters, American Physical Society, 2019, 123 (24), pp.055102. The first complete estimation of the compressible energy cascade rate lεCl at magnetohydrodynamic (MHD) and subion scales is obtained in Earth’s magnetosheath using Magnetospheric MultiScale spacecraft data and an exact law derived recently for compressible Hall MHD turbulence. A multispacecraft technique is used to compute the velocity and magnetic gradients, and then all the correlation functions involved in the exact relation. It is shown that when the density fluctuations are relatively small, lεCl identifies well with its incompressible analog jεIj at MHD scales but becomes much larger than jεIj at subion scales. For larger density fluctuations, lεCl is larger than jεIj at every scale with a value significantly higher than for smaller density fluctuations. Our study reveals also that for both small and large density fluctuations, the nonflux terms remain always negligible with respect to the flux terms and that the major contribution to lεCl at subion scales comes from the compressible Hall flux. (10.1103/PhysRevLett.123.245101)
  DOI : 10.1103/PhysRevLett.123.245101
• The Role of Upper Hybrid Waves in the Magnetotail Reconnection Electron Diffusion Region
  
  • Jiang K.
  • Huang S. Y.
  • Yuan Z. G.
  • Sahraoui Fouad
  • Deng X. H.
  • Yu X. D.
  • He L. H.
  • Deng D.
  • Wei Y. Y.
  • Xu S. B.
  The Astrophysical Journal Letters, Bristol : IOP Publishing, 2019, 881 (2), pp.L28. Plasma waves are believed to play an important role during magnetic reconnection. However, the specific role of upper hybrid (UH) waves in the electron diffusion region (EDR) remains elusive, owing to the absence of high-resolution measurements. We analyze one EDR event in the magnetotail on 2017 July 11 observed by the Magnetospheric Multiscale (MMS) mission. To the best of our knowledge, this is the first time that intense UH waves have been observed in the EDR by MMS. The agyrotropic crescent-shaped electron distributions could result in the observed UH waves. Concomitant with the observations of UH waves, the agyrotropy parameter of the electrons decreases, implying that the UH waves could effectively scatter the electrons in the EDR. The good accordance of positive energy conversion (, likely dissipation) and the observed UH waves indicates that UH waves may contribute to the energy conversion from the magnetic fields to the plasma particles during magnetic reconnection. (10.3847/2041-8213/ab36b9)
  DOI : 10.3847/2041-8213/ab36b9
• Oxygen (<SUP>3</SUP>P) atom recombination on a Pyrex surface in an O<SUB>2</SUB> plasma
  
  • Booth Jean-Paul
  • Guaitella Olivier
  • Chatterjee Abhyuday
  • Drag Cyril
  • Guerra V.
  • Lopaev Dmitry
  • Zyryanov Sergey
  • Rakhimova Tatyana
  • Voloshin Dmitry
  • Mankelevich Y.
  Plasma Sources Science and Technology, IOP Publishing, 2019, 28 (5), pp.055005. The recombination of O (3P) atoms on the surface of a Pyrex tube containing a DC glow discharge in pure O2 was studied over a wide range of pressure (0.210 Torr) and discharge current (1040 mA) for two fixed surface temperatures ( 50 °C and 5 °C). The recombination probability, &#947;, was deduced from the observed atom loss rate (dominated by surface recombination) determined by time-resolved optical emission actinometry in partially-modulated (amplitude ~15%17%) discharges. The value of &#947; increased with discharge current at all pressures studied. As a function of pressure it passes through a minimum at ~0.75 Torr. At pressures above this minimum &#947; is well-correlated with the gas temperature, T g, (determined from the rotational structure of the O2 (b1&#931;g , v = 0) &#8594; O2(X3&#931;g &#8722;, v = 0) emission spectrum) which increases with pressure and current. The temperature of the atoms incident at the surface was deduced from a model, calibrated by measurements of the spatially-averaged gas temperature and validated by radial temperature profile measurements. The value of &#947; follows an Arrhenius law depending on the incident atom temperature, with an activation energy in the range 0.130.16 eV. At the higher surface temperature the activation energy is the same, but the pre-exponential factor is smaller. Under conditions where the O flux to the surface is low &#947; falls below this Arrhenius law. These results are well explained by an EleyRideal (ER) mechanism with incident O atoms recombining with both chemisorbed and more weakly bonded physisorbed atoms on the surface, with the kinetic energy of the incident atoms providing the energy to overcome the activation energy barrier. A phenomenological ER model is proposed that explains both the decrease in recombination probability with surface temperature as well as the deviations from the Arrhenius law when the O flux is low. At pressures below 0.75 Torr &#947; increases significantly, and also increases strongly with the discharge current. We attribute this effect to incident ions and fast neutrals arriving with sufficient energy to clean or chemically modify the surface, generating new adsorption sites. Discharge modeling confirms that at pressures below ~0.3 Torr a noticeable fraction of the ions arriving at the surface have adequate kinetic energy to break surface chemical bonds (>35 eV). (10.1088/1361-6595/ab13e8)
  DOI : 10.1088/1361-6595/ab13e8