Publications

2012

• Shadowgraphic and optical emission spectroscopy investigation of nanosecond discharge in water
  
  • Marinov Ilya
  • Guaitella Olivier
  • Rousseau Antoine
  • Starikovskaia Svetlana
  , 2012.
• Study of a fast gas heating in a capillary nanosecond discharge. Discharge parameters and temperature increase in the afterglow
  
  • Klochko A.V.
  • Popov N.A.
  • Starikovskaia Svetlana
  , 2012.
• Time-resolved electric field measurements in 1-5 atm nanosecond surface dielectric discharge. Ignition of combustible mixtures by surface discharge
  
  • Kosarev I.N.
  • Sagulenko P.N.
  • Khorunzhenko V.I.
  • Popov N.A.
  • Starikovskaia Svetlana
  , 2012.
• Absolute Cl and Cl<SUB>2</SUB> densities in a Cl<SUB>2</SUB> ICP determined by TALIF with a new calibration method
  
  • Booth Jean-Paul
  • Sirse Nishant
  • Azamoum Yasmina
  • Chabert Pascal
  , 2012.
• Enhanced sheath heating in capacitively coupled discharges due to non-sinusoidal voltage waveforms
  
  • Lafleur Trevor
  • Boswell R.W.
  • Booth Jean-Paul
  Applied Physics Letters, American Institute of Physics, 2012, 100, pp.194101. Through the use of particle-in-cell simulations, we demonstrate that the power deposition in capacitively coupled discharges (in argon) can be increased by replacing sinusoidal waveforms with Gaussian-shaped voltage pulses (with a repetition frequency of 13.56 MHz). By changing the Gaussian pulse width, electron heating can be directly controlled, allowing for an increased plasma density and ion flux for the same gas pressure and geometrical operating conditions. Analysis of the power deposition profiles and electron distribution functions shows that enhanced electron-sheath heating is responsible for the increased power absorption. (10.1063/1.4712128)
  DOI : 10.1063/1.4712128
• Absolute atomic chlorine densities in a Cl<SUB>2</SUB> ICP determined by Two-Photon Laser Induced Fluorescence with a new calibration method
  
  • Booth Jean-Paul
  • Azamoum Yasmina
  • Sirse Nishant
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.195201. Absolute densities of chlorine atoms were determined in an inductively coupled plasma in pure chlorine gas as a function of gas pressure and RF power by two-photon laser-induced fluorescence. A new technique is proposed to put the relative two-photon laser-induced fluorescence (TALIF) measurements on an absolute scale, based on photolysis of Cl2 gas (without plasma) with a tripled Nd&#8201;:&#8201;YAG laser at 355 nm. Because the dissociation cross-section and photo-dissociation laser beam energy density are well known, the absolute densities can be determined with high accuracy. We find that the ratio of the Cl atom density normalized to the Cl2 gas density without plasma at the reactor centre increases with RF power and decreases with gas pressure, reaching 20% at 2 mTorr 500 WRF. (10.1088/0022-3727/45/19/195201)
  DOI : 10.1088/0022-3727/45/19/195201
• Analysis of symmetry breaking mechanisms and the role of turbulence self-regulation in intrinsic rotation
  
  • Kwon J.M.
  • Yi Sukyoung
  • Rhee T.
  • Diamond P.H.
  • Miki K.
  • Hahm T.S.
  • Kim J.Y.
  • Gürcan Özgür D.
  • Mcdevitt C.J.
  Nuclear Fusion, IOP Publishing, 2012, 52, pp.013004. We present analyses of mechanisms which convert radial inhomogeneity to broken k||-symmetry and thus produce turbulence driven intrinsic rotation in tokamak plasmas. By performing gyrokinetic simulations of ITG turbulence, we explore the many origins of broken k||-symmetry in the fluctuation spectrum and identify both E × B shear and the radial gradient of turbulence intensitya ubiquitous radial inhomogeneity in tokamak plasmasas important k||-symmetry breaking mechanisms. By studying and comparing the correlations between residual stress, E × B shearing, fluctuation intensity and its radial gradient, we investigate the dynamics of residual stress generation by various symmetry breaking mechanisms and explore the implication of the self-regulating dynamics of fluctuation intensity and E × B shearing for intrinsic rotation generation. Several scalings for intrinsic rotation are reported and are linked to investigations of underlying local dynamics. It is found that stronger intrinsic rotation is generated for higher values of ion temperature gradient, safety factor and weaker magnetic shear. These trends are broadly consistent with the intrinsic rotation scaling found from experimentthe so-called Rice scaling. (10.1088/0029-5515/52/1/013004)
  DOI : 10.1088/0029-5515/52/1/013004
• Coronal heating in coupled photosphere-chromosphere-coronal systems: turbulence and leakage
  
  • Verdini Andrea
  • Grappin Roland
  • Velli Marco
  Astronomy & Astrophysics - A&A, EDP Sciences, 2012, 538, pp.70. Context. Coronal loops act as resonant cavities for low-frequency fluctuations that are transmitted from the deeper layers of the solar atmosphere. These fluctuations are amplified in the corona and lead to the development of turbulence that in turn is able to dissipate the accumulated energy, thus heating the corona. However, trapping is not perfect, because some energy leaks down to the chromosphere on a long timescale, limiting the turbulent heating. Aims. We consider the combined effects of turbulence and energy leakage from the corona to the photosphere in determining the turbulent energy level and associated heating rate in models of coronal loops, which include the chromosphere and transition region. Methods. We use a piece-wise constant model for the Alfvén speed in loops and a reduced MHD-shell model to describe the interplay between turbulent dynamics in the direction perpendicular to the mean field and propagation along the field. Turbulence is sustained by incoming fluctuations that are equivalent, in the line-tied case, to forcing by the photospheric shear flows. While varying the turbulence strength, we systematically compare the average coronal energy level and dissipation in three models with increasing complexity: the classical closed model, the open corona, and the open corona including chromosphere (or three-layer model), with the last two models allowing energy leakage. Results. We find that (i) leakage always plays a role. Even for strong turbulence, the dissipation time never becomes much lower than the leakage time, at least in the three-layer model; therefore, both the energy and the dissipation levels are systematically lower than in the line-tied model; (ii) in all models, the energy level is close to the resonant prediction, i.e., assuming an effective turbulent correlation time longer than the Alfvén coronal crossing time; (iii) the heating rate is close to the value given by the ratio of photospheric energy divided by the Alfvén crossing time; (iv) the coronal spectral range is divided in two: an inertial range with 5/3 spectral slope, and a large-scale peak where nonlinear couplings are inhibited by trapped resonant modes; (v) in the realistic three-layer model, the two-component spectrum leads to a global decrease in damping equal to Kolmogorov damping reduced by a factor urms/Vac where Vac is the coronal Alfvén speed. (10.1051/0004-6361/201118046)
  DOI : 10.1051/0004-6361/201118046
• Source location of falling tone chorus
  
  • Kurita S.
  • Misawa H.
  • Cully C. M.
  • Le Contel Olivier
  • Angelopoulos V.
  Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.22102. Chorus is characterized by its fine structures consisting of rising or falling tones believed to result from nonlinear wave-particle interactions. However, previous studies have showed that the intensity and propagation characteristics of rising and falling tone chorus are quite different, suggesting that their generation processes might be different. In this paper, the propagation direction of falling tone chorus is statistically investigated to identify its source region based on the Poynting vector measurement with THEMIS. The result shows that the falling tone chorus propagates from the magnetic equator to higher latitude both in the northern and southern hemispheres, in the same way as rising tone chorus. Our result shows that the magnetic equator is the common source location for both rising and falling tone chorus. The result emphasizes that the different properties between rising and falling tone chorus originate from their generation mechanism rather than source region. (10.1029/2012GL053929)
  DOI : 10.1029/2012GL053929
• Detection of Small-Scale Structures in the Dissipation Regime of Solar-Wind Turbulence
  
  • Perri S.
  • Goldstein M. L.
  • Dorelli J. C.
  • Sahraoui Fouad
  Physical Review Letters, American Physical Society, 2012, 109 (19), pp.191101. Recent observations of the solar wind have pointed out the existence of a cascade of magnetic energy from the scale of the proton Larmor radius &#961;p down to the electron Larmor radius &#961;e scale. In this Letter we study the spatial properties of magnetic field fluctuations in the solar wind and find that at small scales the magnetic field does not resemble a sea of homogeneous fluctuations, but rather a two-dimensional plane containing thin current sheets and discontinuities with spatial sizes ranging from l&#8819;&#961;p down to &#961;e and below. These isolated structures may be manifestations of intermittency that localize sites of turbulent dissipation. Studying the relationship between turbulent dissipation, reconnection, and intermittency is crucial for understanding the dynamics of laboratory and astrophysical plasmas. (10.1103/PhysRevLett.109.191101)
  DOI : 10.1103/PhysRevLett.109.191101
• Effect of sheared flow on the growth rate and turbulence decorrelation
  
  • Gürcan Özgür D.
  Physical Review Letters, American Physical Society, 2012, 109, pp.155006. The effect of a large scale flow shear on a linearly unstable turbulent system is considered. A cubic equation describing the effective growth rate is obtained, which is shown to reduce to well-known forms in weak and strong shear limits. A shear suppression rule is derived which corresponds to the point where the effective growth rate becomes negative. The effect of flow shear on nonlinear mode coupling of drift or Rossby waves is also considered, and it is shown that the resonance manifold shrinks and weakens as the vortices are sheared. This leads to a reduction of the efficiency of three-wave interactions. Tilted eddies can then only couple to the large scale sheared flows, because the resonance condition for that interaction is trivially satisfied. It is argued that this leads to absorbtion of the sheared vortices by large scale flow structures. Studying the form of the effective growth rate for weak shear, it was shown that in addition to reducing the overall growth rate, a weak flow shear also reduces the wave number where the fluctuations are most unstable. (10.1103/PhysRevLett.109.155006)
  DOI : 10.1103/PhysRevLett.109.155006
• Separate control of the ion flux and ion energy in capacitively coupled rf discharges using voltage waveform tailoring
  
  • Lafleur Trevor
  • Delattre Pierre-Alexandre
  • Johnson E.V.
  • Booth Jean-Paul
  Applied Physics Letters, American Institute of Physics, 2012, 101, pp.124104. We experimentally characterize an argon plasma in a geometrically symmetric, capacitively coupled rf discharge, excited by pulse-type tailored waveforms (generated using multiple voltage harmonics). The results confirm a number of predictions made by recent particle-in-cell simulations of a similar system and demonstrate a unique form of control over the ion flux and ion energy in capacitively coupled plasmas; by increasing the number of applied harmonics (equivalent to decreasing the pulse width), it is possible to increase the plasma density and ion flux (together with the power deposition) while keeping the average ion energy on one of the electrodes low and constant. (10.1063/1.4754692)
  DOI : 10.1063/1.4754692
• A global hybrid model for Mercury's interaction with the solar wind: Case study of the dipole representation
  
  • Richer Emilie
  • Modolo Ronan
  • Chanteur Gérard
  • Hess Sebastien
  • Leblanc François
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117 (A10), pp.A10228. The interaction of the solar wind (SW) with the magnetic field of Mercury is investigated by means of a three dimensional parallelized multispecies hybrid model. A comparison between two mathematical representations of Mercury's intrinsic magnetic field is studied. The first model is an Offset Dipole (OD) having the offset and dipolar moment reported by Anderson et al. (2011). The second model is a combination of a Dipole and a Quadrupole (DQ), the total field is fitted to the offset dipolar field, for northern latitudes greater than 50°. Simulations reproduce the features which characterize Mercury's interaction with the SW, encompassing the Bow Shock (BS), the magnetosheath, the magnetotail, the "cusps" region and the neutral current sheet. Global hybrid simulations of the Hermean magnetosphere run for the OD and DQ models demonstrate that the southern parts of the magnetospheres produced by the OD and DQ models differ greatly in topology and volume meanwhile their northern parts-are quite similar. In particular the DQ model exhibits a dome of closed field lines around the south pole in contrast to the OD. Without further information on the intrinsic magnetic field of the planet in the southern region which should be provided by BepiColombo after year 2020, we can only speculate on the influence of the different magnetic topologies on the magnetospheric dynamics. (10.1029/2012JA017898)
  DOI : 10.1029/2012JA017898
• A dc-pulsed capacitively coupled planar Langmuir probe for plasma process diagnostics and monitoring
  
  • Samara V.
  • Booth Jean-Paul
  • de Marneffe J.-F.
  • Milenin A.P.
  • Brouri M.
  • Boullart W.
  Plasma Sources Science and Technology, IOP Publishing, 2012, 21, pp.065004. An improvement to the RF-biased planar Langmuir probe technique proposed by Braithwaite et al (1996 Plasma Sources Sci. Technol. 5 67) is demonstrated, and applied to the case of an industrial CCP reactor. Compared with the RF-biased probe, the new technique uses dc pulses instead of RF bursts, which provides similar results but with simpler electronics. The ion fluxes determined by both techniques are compared under the same O2/Ar plasma conditions using available literature data for the RF-biased case. The data show not only the same trends but very close absolute values of ion fluxes for all studied plasma conditions after correcting for the chamber-area difference. Furthermore, the new technique has the additional benefit of providing information on the 'electron transition region' of the IV curve, as well as allowing the resistance and capacitance of films deposited on the probe to be determined. Finally, both experimental data and numerical simulations of the IV characteristics and the film parameters are presented for different oxidizing plasmas. (10.1088/0963-0252/21/6/065004)
  DOI : 10.1088/0963-0252/21/6/065004
• Solar wind charge exchange X-ray emission from Mars.
  
  • Koutroumpa Dimitra
  • Modolo Ronan
  • Chanteur Gérard
  • Chaufray Jean-Yves
  • Kharchenko V.
  • Lallement Rosine
  Astronomy & Astrophysics - A&A, EDP Sciences, 2012, 545, pp.A153. We study the soft X-ray emission induced by charge exchange (CX) collisions between solar-wind, highly charged ions and neutral atoms of the Martian exosphere. A 3D multi species hybrid simulation model with improved spatial resolution (130 km) is used to describe the interaction between the solar wind and the Martian neutrals. We calculated velocity and density distributions of the solar wind plasma in the Martian environment with realistic planetary ions description, using spherically symmetric exospheric H and O profiles. Following that, a 3D test-particle model was developed to compute the X-ray emission produced by CX collisions between neutrals and solar wind minor ions. The model results are compared to XMM-Newton observations of Mars. We calculate projected X-ray emission maps for the XMM-Newton observing conditions and demonstrate how the X-ray emission reflects the Martian electromagnetic structure in accordance with the observed X-ray images. Our maps confirm that X-ray images are a powerful tool for the study of solar wind - planetary interfaces. However, the simulation results reveal several quantitative discrepancies compared to the observations. Typical solar wind and neutral coronae conditions corresponding to the 2003 observation period of Mars cannot reproduce the high luminosity or the corresponding very extended halo observed with XMM-Newton. Potential explanations of these discrepancies are discussed. (10.1051/0004-6361/201219720)
  DOI : 10.1051/0004-6361/201219720
• ON THE NATURE OF THE SOLAR WIND FROM CORONAL PSEUDOSTREAMERS
  
  • Wang Y-M
  • Grappin Roland
  • Robbrecht E.
  • Sheeley N R
  The Astrophysical Journal, American Astronomical Society, 2012, 749 (2), pp.182. Coronal pseudostreamers, which separate like-polarity coronal holes, do not have current sheet extensions, unlike the familiar helmet streamers that separate opposite-polarity holes. Both types of streamers taper into narrow plasma sheets that are maintained by continual interchange reconnection with the adjacent open magnetic field lines. White-light observations show that pseudostreamers do not emit plasma blobs; this important difference from helmet streamers is due to the convergence of like-polarity field lines above the X-point, which prevents the underlying loops from expanding outward and pinching off. The main component of the pseudostreamer wind has the form of steady outflow along the open field lines rooted just inside the boundaries of the adjacent coronal holes. These flux tubes are characterized by very rapid expansion below the X-point, followed by reconvergence at greater heights. Analysis of an idealized pseudostreamer configuration shows that, as the separation between the underlying holes increases, the X-point rises and the expansion factor f ss at the source surface increases. In situ observations of pseudostreamer crossings indicate wind speeds v ranging from ~350 to ~550 km s1, with O7 /O6 ratios that are enhanced compared with those in high-speed streams but substantially lower than in the slow solar wind. Hydrodynamic energy-balance models show that the empirical v-f ss relation overestimates the wind speeds from nonmonotonically expanding flux tubes, particularly when the X-point is located at low heights and f ss is small. We conclude that pseudostreamers produce a "hybrid" type of outflow that is intermediate between classical slow and fast solar wind. (10.1088/0004-637X/749/2/182)
  DOI : 10.1088/0004-637X/749/2/182
• A hemispherical retarding field energy analyzer to characterize spatially and angularly extended electron beams
  
  • Cipriani Fabrice
  • Leblanc Frédéric
  • Illiano Jean-Marie
  • Berthelier Jean-Jacques
  European Physical Journal: Applied Physics, EDP Sciences, 2012, 60 (2), pp.21002 (7 p.). We have designed and built a hemispherical retarding field energy analyzer in order to facilitate characterization of large area electron emitters (typically field emitter arrays with active areas up to 1 cm2) with large angular aperture. A complete numerical model of the analyzer has been built, including perturbations due to secondary particles, in order to determine the analyzer performances. The analyzer energy resolution is better than 100 meV for an energy range up to 120 eV. The analyzer has a global field of view of 112° and allows measurements of the energy distribution of the beam as a function of the emission angle, as well as measurements of the beam intensity profile along any section of the beam. We have successfully used the analyzer to characterize the electron beam emitted by 1 cm2 Mo microtips-based field emitter arrays. (10.1051/epjap/2012120011)
  DOI : 10.1051/epjap/2012120011
• Multi-scale Cluster observations of reconnection jet fronts/braking regions and associated particle energization in near-Earth magnetotail
  
  • Retinò Alessandro
  • Vaivads A.
  • Zieger B.
  • Fujimoto M.
  • Kasahara S.
  • Nakamura R.
  , 2012. Reconnection jet fronts, the boundaries separating jetting from ambient plasma, and jet braking regions, where jets eventually stop/dissipate, play a key role for the near-Earth magnetotail e.g. in terms of particle energization. Recent Cluster orbits, where two spacecraft are separated by ~ 100 km (sub-proton scales) while being separated from the others by ~ 10000 km (MHD scales), allow the unique possibility to study jet fronts/braking regions and associated particle energization at different scales. Here we present Cluster observations from such orbits, focusing in particular on the datasets from the upcoming Cluster Guest Investigator campaign.
• New Insight into Short-wavelength Solar Wind Fluctuations from Vlasov Theory
  
  • Sahraoui Fouad
  • Belmont Gérard
  • Goldstein M. L.
  The Astrophysical Journal, American Astronomical Society, 2012, 748, pp.100. The nature of solar wind (SW) turbulence below the proton gyroscale is a topic that is being investigated extensively nowadays, both theoretically and observationally. Although recent observations gave evidence of the dominance of kinetic Alfvén waves (KAWs) at sub-ion scales with omega < omega<SUB>ci</SUB>, other studies suggest that the KAW mode cannot carry the turbulence cascade down to electron scales and that the whistler mode (i.e., omega > omega<SUB>ci</SUB>) is more relevant. Here, we study key properties of the short-wavelength plasma modes under limited, but realistic, SW conditions, typically beta<SUB> i </SUB> >~ beta<SUB> e </SUB> ~ 1 and for high oblique angles of propagation 80° <= Theta<SUB> kB </SUB> < 90° as observed from the Cluster spacecraft data. The linear properties of the plasma modes under these conditions are poorly known, which contrasts with the well-documented cold plasma limit and/or moderate oblique angles of propagation (Theta<SUB> kB </SUB> < 80°). Based on linear solutions of the Vlasov kinetic theory, we discuss the relevance of each plasma mode (fast, Bernstein, KAW, whistler) in carrying the energy cascade down to electron scales. We show, in particular, that the shear Alfvén mode (known in the magnetohydrodynamic limit) extends at scales krho<SUB> i </SUB> >~ 1 to frequencies either larger or smaller than omega<SUB>ci</SUB>, depending on the anisotropy k <SUB>par</SUB>/k <SUB></SUB>. This extension into small scales is more readily called whistler (omega > omega<SUB>ci</SUB>) or KAW (omega < omega<SUB>ci</SUB>), although the mode is essentially the same. This contrasts with the well-accepted idea that the whistler branch always develops as a continuation at high frequencies of the fast magnetosonic mode. We show, furthermore, that the whistler branch is more damped than the KAW one, which makes the latter the more relevant candidate to carry the energy cascade down to electron scales. We discuss how these new findings may facilitate resolution of the controversy concerning the nature of the small-scale turbulence, and we discuss the implications for present and future spacecraft wave measurements in the SW. (10.1088/0004-637X/748/2/100)
  DOI : 10.1088/0004-637X/748/2/100
• Observations of turbulence within reconnection jet in the presence of guide field
  
  • Huang S. Y.
  • Zhou M.
  • Sahraoui Fouad
  • Vaivads A.
  • Deng X. H.
  • André M.
  • He J. S.
  • Fu H.S.
  • Li H. M.
  • Yuan Z. G.
  • Wang D. D.
  Geophysical Research Letters, American Geophysical Union, 2012, 39, pp.L11104. We present the first comprehensive observations of turbulence properties within high speed reconnection jet in the plasma sheet with moderate guide field. The power spectral density index is about &#8722;1.73 in the inertial range, and follows the value of &#8722;2.86 in the ion dissipation range. The turbulence is strongly anisotropic in the wave-vector space with the major power having its wave-vector highly oblique to the ambient magnetic field, suggesting that the turbulence is quasi-2D. The measured dispersion relations obtained using the k-filtering technique are compared with theory and are found to be consistent with the Alfvén-Whistler mode. In addition, both Probability Distribution Functions and flatness results show that the turbulence in the reconnection jet is intermittent (multifractal) at scales less than the proton gyroradius/inertial lengths. The estimated electric field provided by anomalous resistivity caused by turbulence is about 3 mV/m, which is close to the typical reconnection electric field in the magnetotail. (10.1029/2012GL052210)
  DOI : 10.1029/2012GL052210
• Inductance and near fields of a loop antenna in a cold magnetoplasma in the whistler frequency band
  
  • Korobkov S. V.
  • Kostrov A. V.
  • Gushchin M. E.
  • Zaboronkova T. M.
  • Krafft C.
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.093301. The influence of a magnetoplasma on the inductance of a circular loop antenna oriented perpendicular to the ambient static magnetic field and operated in the whistler frequency band is studied. Based on a strict electrodynamic approach, the analytical treatment of the antenna reactance is performed for a uniform rf current distribution along the antenna wire. Calculations are made for plasma parameters and operating frequencies typical for active ionospheric experiments and laboratory rf (helicon) sources of dense magnetized plasmas. It is shown that the plasma influence on the inductance of the loop antenna remains relatively weak, even for antennas with dimensions close to half of the longitudinal whistler wavelength, when the rf field distribution in the antenna near zone is strongly different from that in vacuum. The theoretical predictions are confirmed by measurements performed on the large KROT plasma device. The results obtained are of crucial importance for the preparation of active ionospheric experiments and for the matching of loop antennas used in laboratory rf sources of dense magnetized plasmas. (10.1063/1.4745611)
  DOI : 10.1063/1.4745611
• Thin current sheets in the presence of a guiding magnetic field in Earth's magnetosphere
  
  • Malova H. V.
  • Popov V. Y.
  • Mingalev O. V.
  • Mingalev I. V.
  • Melnik M. N.
  • Artemyev A. V.
  • Petrukovich A. A.
  • Delcourt Dominique C.
  • Shen C.
  • Zelenyi L. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2012, 117, pp.4212. A self-consistent theory of relatively thin anisotropic current sheets (TCS) in collisionless plasma is developed, taking into account the presence of a guiding field B<SUB>y</SUB> (all notations are used in the GSM coordinate system). TCS configurations with a finite value of guiding field B<SUB>y</SUB> are often observed in Earth's magnetotail and are typical for Earth's magnetopause. A characteristic signature of such configurations is the existence of a magnetic field component along the direction of TCS current. A general case is considered in this paper with global sheared magnetic field B<SUB>y</SUB> = const. Analytical and numerical (particle-in-cell) models for such plasma equilibria are analyzed and compared with each other as well as with Cluster observations. It is shown that, in contrast to the case with B<SUB>y</SUB> = 0, the character of ``particle-current sheet'' interaction is drastically changed in the case of a global magnetic shear. Specifically, serpentine-like parts of ion trajectories in the neutral plane become more tortuous, leading to a thicker current sheet. The reflection coefficient of particles coming from northern and southern sources also becomes asymmetric and depends upon the value of the B<SUB>y</SUB> component. As a result, the degree of asymmetry of magnetic field, plasma, and current density profiles appears characteristic of current sheets with a constant B<SUB>y</SUB>. In addition, in the presence of nonzero guiding field, the curvature current of electrons in the center of the current sheet decreases, yielding an effective thickening of the sheet. Implications of these results for current sheets in Earth's magnetosphere are discussed. (10.1029/2011JA017359)
  DOI : 10.1029/2011JA017359
• Coupling between whistler waves and slow-mode solitary waves
  
  • Tenerani Anna
  • Califano F.
  • Pegoraro F.
  • Le Contel Olivier
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.052103. The interplay between electron- and ion-scale phenomena is of general interest for both laboratory and space plasma physics. In this paper, we investigate the linear coupling between whistler waves and slow magnetosonic solitons through two-fluid numerical simulations. Whistler waves can be trapped in the presence of inhomogeneous external fields such as a density hump or hole where they can propagate for times much longer than their characteristic time scale, as shown by laboratory experiments and space measurements. Space measurements have detected whistler waves also in correspondence to magnetic holes, i.e., to density humps with magnetic field minima extending on ion-scales. This raises the interesting question of how ion-scale structures can couple to whistler waves. Slow magnetosonic solitons share some of the main features of a magnetic hole. Using the ducting properties of an inhomogeneous plasma as a guide, we present a numerical study of whistler waves that are trapped and transported inside propagating slow magnetosonic solitons. (10.1063/1.4717764)
  DOI : 10.1063/1.4717764
• Kinetic equilibrium for an asymmetric tangential layer, Physics of Plasmas
  
  • Belmont Gérard
  • Aunai Nicolas
  • Smets Roch
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.022108. Finding kinetic (Vlasov) equilibria for tangential current layers is a long standing problem, especially in the context of reconnection studies, when the magnetic field reverses. Its solution is of pivotal interest for both theoretical and technical reasons when such layers must be used for initializing kinetic simulations. The famous Harris equilibrium is known to be limited to symmetric layers surrounded by vacuum, with constant ion and electron flow velocities, and with current variation purely dependent on density variation. It is clearly not suited for the magnetopause-like layers, which separate two plasmas of different densities and temperatures, and for which the localization of the current density j=n&#948;v is due to the localization of the electron-to-ion velocity difference &#948;v and not of the density n. We present here a practical method for constructing a Vlasov stationary solution in the asymmetric case, extending the standard theoretical methods based on the particle motion invariants. We show that, in the case investigated of a coplanar reversal of the magnetic field without electrostatic field, the distribution function must necessarily be a multi-valued function of the invariants to get asymmetric profiles for the plasma parameters together with a symmetric current profile. We show also how the concept of accessibility makes these multi-valued functions possible, due to the particle excursion inside the layer being limited by the Larmor radius. In the presented method, the current profile across the layer is chosen as an input, while the ion density and temperature profiles in between the two asymptotic imposed values are a result of the calculation. It is shown that, assuming the distribution is continuous along the layer normal, these profiles have always a more complex profile than the profile of the current density and extends on a larger thickness. The different components of the pressure tensor are also outputs of the calculation and some conclusions concerning the symmetries of this tensor are pointed out. (10.1063/1.3685707)
  DOI : 10.1063/1.3685707
• Variation of F2 layer critical frequency with solar cycle at Dakar station
  
  • Thiam N. M.
  • Ouattara Frédéric Martial
  • Gnabahou Doua Allain
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Lassurdie-Duchesne P.
  Journal des sciences, Université Cheikh Anta Diop, 2012, 11 (2), pp.16-20. ...