Publications

2013

• Commutation rapide déclenchée par filamentation laser femtoseconde
  
  • Larour Jean
  • Arantchouk Léonid
  • Houard Aurélien
  Flash X - La revue scientifique de l'Ecole polytechnique, Ecole polytechnique, 2013, 15, pp.55-57. La revue scientifique de l'Ecole polytechnique (parution annuelle) ISSN : 1775-0385 Le phénomène de filamentation laser dans lair a été décrit dans un précédent numéro (Flash X n° 12) par A. Houard et A. Mysyrowicz, chercheurs du LOA (Laboratoire dOptique appliquée). Rappelons ici quil apparaît spontanément le long dun faisceau laser de durée femtoseconde (fs) quand sa puissance dépasse un seuil critique de quelques GW. un équilibre dynamique entre la diffraction naturelle du faisceau, leffet Kerr, et la défocalisation par lair ionisé, maintient lintensité dans le coeur du faisceau sur une distance qui peut dépasser un mètre, entraînant la formation dun canal de plasma dans le sillage de limpulsion. Le point de démarrage, la longueur des canaux de plasma et leur distribution transverse sont contrôlables en jouant sur les paramètres de limpulsion laser initiale (énergie, durée, diamètre du faisceau).
• Study of reconnection physics using laser-generated B fields
  
  • Smets Roch
  • Belmont Gérard
  • Aunai N.
  • Fuchs J. C.
  • Boniface C.
  • Aulanier G.
  , 2013.
• Structures of dayside whistler-mode waves deduced from conjugate diffuse aurora
  
  • Nishimura Y.
  • Bortnik J.
  • Li W.
  • Thome R.M.
  • Ni B.
  • Lyons L.R.
  • Angelopoulos V.
  • Ebihara Y.
  • Bonnell J. W.
  • Le Contel Olivier
  • Auster U.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (2), pp.664-673. [1] We present simultaneous measurements of dayside diffuse aurora and whistler-mode waves made by the South Pole all-sky imager and two of the THEMIS spacecraft. We found a high correlation between the diffuse aurora intensity at 557.7&#8201;nm near the footprint latitudes of THEMIS and whistler-mode wave intensity measured on board. The power in other wave modes was negligibly small in most cases, indicating that the dayside diffuse aurora is driven by precipitating energetic electrons resonating with whistler-mode waves. The high correlation over a wide L* range (6&#8201;<&#8201;L*&#8201;<&#8201;11) further allowed us to magnetically link the wave and magnetospheric plasma distributions with the auroral patterns. Two distinct regions of whistler-mode waves and ambient plasma density were found outside the plasmasphere near the equator: (1) intense waves in a smooth, low density and (2) moderate waves with enhanced and fluctuating density. The whistler-mode wave intensity in the fluctuating plasma density region is positively correlated with the ambient density variations. The corresponding auroral images show an azimuthally elongated diffuse auroral band on the field lines connected to the low density region, as opposed to a structured diffuse aurora on the fluctuating density field lines. Each structured diffuse auroral patch was stable for a few tens of minutes and slowly drifted azimuthally. The high correlation of waves and auroras indicates that the structured diffuse auroral pattern reflects the spatial distribution of whistler-mode waves and ambient plasma density in space. The enhanced density measured by the spacecraft is quasi-spatial and contributes to enhanced growth of whistler-mode waves. (10.1029/2012JA018242)
  DOI : 10.1029/2012JA018242
• Statistical properties of planetary heavy-ion precipitations toward the Martian ionosphere obtained from Mars Express
  
  • Hara T.
  • Seki K.
  • Futaana Y.
  • Yamauchi M.
  • Barabash S.
  • Fedorov A. O.
  • Yagi M.
  • Delcourt Dominique C.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (8), pp.5348-5357. The interplanetary magnetic field (IMF) embedded in the solar wind interacts with the Martian crustal magnetic field and atmosphere. The IMF orientation is one of the important parameters to control the acceleration and precipitation of planetary heavy ions (PHIs). We statistically investigate the effects of the IMF orientation on PHI precipitations toward the ionosphere based on observations by Mars Express (MEX). We identified 59 PHI precipitation events between July 2007 and September 2009. To estimate the IMF orientation without magnetometer that MEX does not carry, we used the velocity distribution of exospheric-origin pickup protons. We estimated the IMF orientation without its polarity for 10 events. The results show that the precipitations of PHIs tend to be observed around pole regions in the MSE (Mars-centered, solar electrical) coordinates determined from the solar wind electric field (Esw), in which the pole axis directs to the parallel or antiparallel to Esw due to the ambiguity in the IMF polarity determination. The observed precipitating PHIs are accelerated only up to a few keV. This feature may reflect the short distance from the picked-up region. For one of these 10 events, we estimated the IMF polarity by comparing the velocity distribution of exospheric-origin pickup protons observed by MEX with those obtained from statistical trajectory tracing simulations under two cases of possible IMF polarity conditions. The estimated polarity indicates that the PHI precipitation in this event is observed in the downward electric field hemisphere in MSE, where Esw points to Mars in the pole region. (10.1002/jgra.50494)
  DOI : 10.1002/jgra.50494
• Capacitively coupled radio-frequency plasmas excited by tailored voltage waveforms
  
  • Lafleur Trevor
  • Delattre Pierre-Alexandre
  • Johnson E.V.
  • Booth Jean-Paul
  Plasma Physics and Controlled Fusion, IOP Publishing, 2013, 55 (12), pp.124002. By applying certain types of ?tailored? voltage waveforms (TVWs) to capacitively coupled plasmas, a dc self-bias and an asymmetric plasma response can be produced, even in geometrically symmetric reactors. Furthermore, these arbitrary applied waveforms can produce a number of interesting phenomena that are not present in typical single-frequency sinusoidal discharges. This electrical asymmetry effect presents emerging possibilities for the improved control of the ion energy and ion flux in these systems; parameters of vital importance to both etching and deposition applications for materials processing. With a combined research approach utilizing both experimental measurements, and particle-in-cell simulations, we review and extend recent investigations that study a particular class of TVW. The waveforms used have a pulse-type shape and are composed of a varying number of harmonic frequencies. This allows a strong self-bias to be produced, and causes most of the applied voltage to be dropped across a single sheath. Additionally, decreasing the pulse width (by increasing the number of harmonics), allows the plasma density and ion flux to be increased. Simulation and experimental results both demonstrate that this type of waveform can be used to separately control the ion flux and ion energy, while still producing a uniform plasma over large area (50 cm diameter) rf electrodes. (10.1088/0741-3335/55/12/124002)
  DOI : 10.1088/0741-3335/55/12/124002
• Theory for the self-bias formation in capacitively coupled plasmas excited by arbitrary waveforms
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner M.M.
  • Booth Jean-Paul
  Plasma Sources Science and Technology, IOP Publishing, 2013, 22 (6), pp.065013. We develop a semi-analytical theory for the self-bias formation in capacitively coupled plasmas excited by arbitrary radio-frequency (rf) waveforms. The requirement of rf current continuity and voltage balance across the discharge results in the need for a self-bias voltage to develop with non-sinusoidal excitations, even in geometrically symmetric systems. The theory is compared extensively with a wide range of experimental and particle-in-cell (PIC) simulation data within the literature, and is found to be in excellent agreement. Furthermore, it is shown that the present theory is formally equivalent to the original model proposed by Heil et al (2008 J. Phys. D: Appl. Phys. 41 165202), but goes further by explicitly allowing the time-varying sheath voltages and symmetry parameter to be evaluated without input from PIC simulations. (10.1088/0963-0252/22/6/065013)
  DOI : 10.1088/0963-0252/22/6/065013
• Simulation benchmarks for low-pressure plasmas: Capacitive discharges
  
  • Turner M.M.
  • Derzsi A.
  • Donkó Z.
  • Eremin D.
  • Kelly S.J.
  • Lafleur Trevor
  • Mussenbrock T.
  Physics of Plasmas, American Institute of Physics, 2013, 20, pp.013507. Benchmarking is generally accepted as an important element in demonstrating the correctness of computer simulations. In the modern sense, a benchmark is a computer simulation result that has evidence of correctness, is accompanied by estimates of relevant errors, and which can thus be used as a basis for judging the accuracy and efficiency of other codes. In this paper, we present four benchmark cases related to capacitively coupled discharges. These benchmarks prescribe all relevant physical and numerical parameters. We have simulated the benchmark conditions using five independently developed particle-in-cell codes. We show that the results of these simulations are statistically indistinguishable, within bounds of uncertainty that we define. We, therefore, claim that the results of these simulations represent strong benchmarks, which can be used as a basis for evaluating the accuracy of other codes. These other codes could include other approaches than particle-in-cell simulations, where benchmarking could examine not just implementation accuracy and efficiency, but also the fidelity of different physical models, such as moment or hybrid models. We discuss an example of this kind in the Appendix. Of course, the methodology that we have developed can also be readily extended to a suite of benchmarks with coverage of a wider range of physical and chemical phenomena. (10.1063/1.4775084)
  DOI : 10.1063/1.4775084
• Anomalous collisionality in low-pressure plasmas
  
  • Lafleur Trevor
  • Chabert Pascal
  • Turner M.M.
  • Booth Jean-Paul
  Physics of Plasmas, American Institute of Physics, 2013, 20 (12), pp.124503. Based on a theoretical argument from fundamental kinetic theory, by way of simple worked examples, and through the use of particle-in-cell simulations of capacitively coupled plasmas, we demonstrate that conventional methods for calculating the momentum transfer collision frequency in low-pressure plasmas can be seriously erroneous. This potentially plays an important and previously unconsidered role in many low-pressure discharges, and at least in part provides a possible explanation for anomalous behaviour often encountered in these plasmas. (10.1063/1.4859155)
  DOI : 10.1063/1.4859155
• Action diffusion and lifetimes of quasistationary states in the Hamiltonian Mean Field model
  
  • Ettoumi Wahb
  • Firpo Marie-Christine
  Physical Review E, American Physical Society (APS), 2013, 87, pp.030102(R). Out-of-equilibrium quasistationary states (QSSs) are one of the signatures of a broken ergodicity in long-range interacting systems. For the widely studied Hamiltonian Mean-Field model, the lifetime of some QSSs has been shown to diverge with the number N of degrees of freedom with a puzzling N¹.7 scaling law, contradicting the otherwise widespread N scaling law. It is shown here that this peculiar scaling arises from the locality properties of the dynamics captured through the computation of the diffusion coefficient in terms of the action variable. The use of a mean first passage time approach proves to be successful in explaining the non-trivial scaling at stake here, and sheds some light on another case, where lifetimes diverging as e^N above some critical energy have been reported. (10.1103/PhysRevE.87.030102)
  DOI : 10.1103/PhysRevE.87.030102
• Microsecond ramp compression of a metallic liner driven by a 5 MA current on the SPHINX machine using a dynamic load current multiplier pulse shaping
  
  • d'Almeida Thierry
  • Lassalle Francis
  • Morell Alain
  • Grunenwald Julien
  • Zucchini Frédéric
  • Loyen Arnaud
  • Maysonnave Thomas
  • Chuvatin Alexandre S.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (09), pp.092512. SPHINX is a 6 MA, 1-&#956;s Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20&#8201;MA, 1-&#956;s LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23&#8201;GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liner's interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics. (10.1063/1.4823720)
  DOI : 10.1063/1.4823720
• Chlorine atom densities in the (3p<SUP>5</SUP>)<SUP>2</SUP> P<SUP>0</SUP><SUB>1/2</SUB> excited spin-orbit state measured by two-photon absorption laser-induced fluorescence in a chlorine inductively coupled plasma
  
  • Sirse Nishant
  • Booth Jean-Paul
  • Chabert Pascal
  • Surzhykov A.
  • Indelicato P.
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46 (29), pp.295203. Chlorine atom densities in the spinorbit excited state were measured by two-photon absorption laser-induced fluorescence (TALIF) in an inductively coupled plasma discharge in pure Cl2. The atoms were excited by two photons at 235.702 nm to the state and detected by fluorescence to the (4s) 4P5/2 state at 726 nm. The population of this state relative to that in the ground state, was determined from the relative TALIF signal intensity from the two states, combined with new calculations of the two-photon absorption cross-sections. was found to increase continuously with radio-frequency power (50500 W), whereas with Cl2 pressure (590 mTorr) it passes through a maximum at 10 mTorr, reaching ~30% at 500 W. This maximum corresponds to the maximum of electron density in the discharge. Combining this density ratio measurement with previous measurements of the absolute ground state chlorine atom density [1] allows the absolute spin-orbit excited state density to be estimated. A significant fraction of the total chlorine atom density is in this excited state which should be included in plasma chemistry models. (10.1088/0022-3727/46/29/295203)
  DOI : 10.1088/0022-3727/46/29/295203
• Spatio-temporal evolution of the H -> L back transition
  
  • Miki K.
  • Diamond P.H.
  • Schmitz L.
  • Mcdonald D. C.
  • Estrada T.
  • Gürcan Özgür D.
  • Tynan G.R.
  Physics of Plasmas, American Institute of Physics, 2013, 20 (6), pp.062304. Since ITER will operate close to threshold and with limited control, the H&#8201;&#8594;&#8201;L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model [Miki et al., Phys. Plasmas 19, 092306 (2012)], we investigate ELM-free H&#8201;&#8594;&#8201;L back transition dynamics in order to isolate transport physics effects. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-downs reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in the profile gradient scale length is characterized by the Nusselt number, where Nu = &#967;i,turb/&#967;i,neo. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number, where Prped = Dped/&#967;i,neo. We expect the H-mode to be somewhat more resilient in density than in temperature. (10.1063/1.4812555)
  DOI : 10.1063/1.4812555
• Radial correlation of density fluctuations by coupling IPP and LPP W-band Doppler reflectometers on ASDEX Upgrade
  
  • Hennequin Pascale
  • Happel T.
  • Conway G. D.
  • Honoré Cyrille
  • Vermare Laure
  • Pisarev V.
  • Giacalone J-C.
  • Gürcan Özgür D.
  • Asdex Upgrade Team
  , 2013 (oral).
• Electron nongyrotropy in the context of collisionless magnetic reconnection
  
  • Aunai Nicolas
  • Hesse Michael
  • Kuznetsova M. M.
  Physics of Plasmas, American Institute of Physics, 2013, 20, pp.2903. Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions. (10.1063/1.4820953)
  DOI : 10.1063/1.4820953
• Inner radiation belt particle acceleration and energy structuring by drift resonance with ULF waves during geomagnetic storms
  
  • Delcourt Dominique C.
  • Benoist C.
  • Penou E.
  • Chen Y.
  • Russell C. T.
  • Sauvaud J.-A.
  • Walt M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (4), pp.1723-1736. Geomagnetic storms are frequently associated with the formation of multiple bands of energetic electrons inside the inner radiation belt at L = 1.1-1.9 and with prominent energy structures of protons inside the slot region at L = 2.2-3.5. These structures typically from 100 keV up to the MeV range result from coherent interactions of energetic particles with quasi-monochromatic ultra-low frequency (ULF) waves. These waves are induced by magnetospheric changes due to the arrival of dense solar material and related nightside injections of particles from the outer magnetosphere that destabilize field lines in the inner magnetosphere down to L = 1.1. Using low-altitude data from the polar orbiting Demeter spacecraft, we perform case and statistical studies of these structures. We show that with such a spacecraft, these structures are best seen near the South Atlantic Anomaly because of lowering of the belt particle mirror point. As evidenced from ground measurements, energy bands are associated with quasi-sinusoidal ULF Pc5 and Pc4 waves with periods in the 1000 s range for L = 1.1-1.9 and in the 60 s range for L = 2.2-3.5. Numerical simulations of the coherent drift resonance of energetic particles with ultra-low frequency waves show how the particles are accelerated and how the observed structures build up. (10.1002/jgra.50125)
  DOI : 10.1002/jgra.50125
• Energetic electron acceleration by unsteady magnetic reconnection
  
  • Fu H.S.
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Retinò Alessandro
  • André M.
  Nature Physics, Nature Publishing Group [2005-....], 2013, 9, pp.426-430. The mechanism that produces energetic electrons during magnetic reconnection is poorly understood. This is a fundamental process responsible for stellar flares, substorms, and disruptions in fusion experiments. Observations in the solar chromosphere and the Earth's magnetosphere indicate significant electron acceleration during reconnection, whereas in the solar wind, energetic electrons are absent. Here we show that energetic electron acceleration is caused by unsteady reconnection. In the Earth's magnetosphere and the solar chromosphere, reconnection is unsteady, so energetic electrons are produced; in the solar wind, reconnection is steady, so energetic electrons are absent. The acceleration mechanism is quasi-adiabatic: betatron and Fermi acceleration in outflow jets are two processes contributing to electron energization during unsteady reconnection. The localized betatron acceleration in the outflow is responsible for at least half of the energy gain for the peak observed fluxes. (10.1038/nphys2664)
  DOI : 10.1038/nphys2664
• Antisunward structure of thin current sheets in the Earth's magnetotail : Implications of quasi-adiabatic theory
  
  • Malova H. V.
  • Popov V. Y.
  • Delcourt Dominique C.
  • Petrukovich A. A.
  • Zelenyi L. M.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118. We developed a self-consistent kinetic model of thin current sheets (TCS), taking into account the inhomogeneity of TCS parameters in the antisunward direction. We show that the charged particle dynamics depending on the magnetic field distribution in the downtail direction completely determines the magnetotail equilibrium structure. We demonstrate that transient ions as well as electrons are the main current carriers in this system, but the first ones support mostly the background (1-D) structure of the current sheet. The influence of electrons and quasi-trapped ions is found to vary depending upon downtail distance along the sheet. Assuming the conservation of the so-called quasi-adiabatic invariant, we show that quasi-trapped particles are distributed along the current sheet in such a way that they concentrate in the region with large values of normal magnetic field component. As a result quasi-trapped ions can dominate near the earthward edge of TCS. In contrast, the electron current becomes stronger in the TCS tailward region where the normal magnetic field component becomes weaker, and field line curvature drifts are enhanced. Our quasi-adiabatic model predicts that thin current sheets in the Earth's magnetotail should have weakly 2-D configuration which, similar to its 1-D analog considered earlier, conserves the multiscale matreshka structure with multiple embedded layers. (10.1002/jgra.50390)
  DOI : 10.1002/jgra.50390
• Autocalibration Method for Anisotropic Magnetoresistive Sensors Using Offset Coils
  
  • Mohamadabadi K.
  • Jeandet Alexis
  • Hillion M.
  • Coillot Christophe
  IEEE Sensors Journal, Institute of Electrical and Electronics Engineers, 2013, 13 (2), pp.772-776. In this paper, we present a zero-cost indoor calibration method for anisotropic magnetoresistive (AMR) sensors. The implemented circuit is designed to calibrate AMR sensors using integrated coils. A microcontroller is used to generate an artificial three-dimensional magnetic field by injecting three separate currents into three offset coils. We show the similarity of the results for residual calibration norm by using this method compared with the calibration of the sensor in free Earth's magnetic field. Furthermore, this method does not need any other instruments such as Helmholtz coils or a platform for rotating the sensor. Here the sensor is placed inside a mu-metal box during calibration, and the calibration process is completely autonomous. (10.1109/JSEN.2012.2227595)
  DOI : 10.1109/JSEN.2012.2227595
• Effects of the surface conductivity and the IMF strength on the dynamics of planetary ions in Mercury's magnetosphere
  
  • Seki Kanako
  • Terada Naoki
  • Yagi Manabu
  • Delcourt Dominique C.
  • Leblanc François
  • Ogino Tatsuki
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2013, 118 (6), pp.3233-3242. To examine the effects of planetary surface conductivity and the southward IMF strength on ion dynamics, systematic trajectory tracings of Na⁺ ions were performed in the electric and magnetic field configurations obtained from magnetohydrodynamics (MHD) simulations of the solar wind-Mercury interaction. Comparison with a previous study, which used an analytical model that rescaled the Earth's magnetosphere and assumed the existence of the distant neutral line (DNL) in Mercury's magnetotail, shows a drastic change in the Na⁺ precipitation pattern onto due to the formation of the near-Mercury neutral line (NMNL) in MHD simulations. The Na⁺ precipitation band at approximately 30 degrees of latitude (LAT), which was obtained in the previous study, disappeared in the equivalent low-conductivity MHD case due to the NMNL formation, while the NMNL formation causes high-energy Na⁺ precipitation in the equatorial region. The change in strength of the southward IMF (sBz) alters the location of the NMNL and the Na⁺ precipitation pattern. In the low-conductivity sBz = 5 case, both the equatorial precipitation and the Na⁺ band at approximately LAT = 30 are formed. In the high-conductivity sBz = 5 case, magnetospheric convection through the polar regions is suppressed, which results in a region of dense Na⁺ near the planet. These results suggest that the precipitation pattern of planetary ions onto Mercury's surface changes significantly with the activity level of Mercury's magnetosphere. It is also suggested that observations of the magnetospheric convection, the distribution of Na⁺ ions around the planet, or the precipitation pattern of Na⁺ ions onto the planetary surface can provide us information about the surface conductivity. (10.1002/jgra.50181)
  DOI : 10.1002/jgra.50181
• Role of the terrestrial bow shock on magnetic clouds structure: 2. 3D analytical MHD model
  
  • Turc Lucile
  • Fontaine Dominique
  • Kilpua E. K. J.
  • Savoini Philippe
  , 2013. Magnetic clouds (MC) figure among the most important drivers of magnetic storms. In the solar wind, they present a very distinctive structure. However, before reaching the magnetosphere, MCs encounter the bow shock which modifies their structure, and therefore may influence their geoeffectivity. In order to understand how the magnetic structure of MCs is altered by the shock, a simple 3D MHD model is used to calculate the magnetic field strength and direction inside the magnetosheath. We present several outputs of the model, corresponding to different MC axis orientations and to different impact parameters. The variation of the magnetic field direction from the solar wind to the magnetosheath appears to be strongly driven by the shock obliquity. Asymmetries due to different shock configurations may arise inside the magnetosheath. Moreover, the Bz component can even reverse in some parts of the magnetosheath. The model outputs are compared with spacecraft observations. Finally, we discuss the impact of our conclusions on MCs geoeffectivity.
• Direct observation of ozone formation on SiO<SUB>2</SUB> surfaces in O<SUB>2</SUB> discharges
  
  • Marinov Daniil
  • Guaitella Olivier
  • Booth Jean-Paul
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2013, 46, pp.032001. Ozone production is studied in a pulsed O2 discharge at pressures in the range 1.36.7 mbar. Time-resolved absolute concentrations of O3 and O are measured in the post-discharge using UV absorption spectroscopy and two-photon absorption laser-induced fluorescence. In a bare silica discharge tube ozone is formed mainly by three-body gas-phase recombination. When the tube surface is covered by a high specific surface silica catalyst heterogeneous formation becomes the main source of ozone. The efficiency of this surface process increases with O2 pressure and is favoured by the presence of OH groups and adsorbed H2O on the surface. At p = 6.7 mbar ozone production accounts for up to 25% of the atomic oxygen losses on the surface. (10.1088/0022-3727/46/3/032001)
  DOI : 10.1088/0022-3727/46/3/032001
• Cavitation in the vicinity of the high-voltage electrode as a key step of nanosecond breakdown in liquids
  
  • Marinov Ilya
  • Guaitella Olivier
  • Rousseau Antoine
  • Starikovskaia Svetlana
  Plasma Sources Science and Technology, IOP Publishing, 2013, 22, pp.042001. Fast shadowgraphy of nanosecond discharge in liquids with different dielectric permittivity, namely in water, ethanol and n-pentane, has been performed. Formation of a gas cavity at a nanosecond time scale was observed as a pre-breakdown phenomenon at amplitudes of the high-voltage pulse close to the breakdown threshold. This phenomenon is considered as a possible key step of high-voltage breakdown in polar liquids. (10.1088/0963-0252/22/4/042001)
  DOI : 10.1088/0963-0252/22/4/042001
• On the supply of heavy planetary material to the magnetotail of Mercury
  
  • Delcourt Dominique C.
  Annales Geophysicae, European Geosciences Union, 2013, 31 (10), pp.1673-1679. We examine the transport of low-energy heavy ions of planetary origin (O + , Na + , Ca +) in the magneto-sphere of Mercury. We show that, in contrast to Earth, these ions are abruptly energized after ejection into the magneto-sphere due to enhanced curvature-related parallel acceleration. Regardless of their mass-to-charge ratio, the parallel speed of these ions is rapidly raised up to ∼ 2 V E×B (denoting by V E×B the magnitude of the local E × B drift speed), in a like manner to Fermi-type acceleration by a moving magnetic mirror. This parallel energization is such that ions with very low initial energies (a few tenths of eVs) can overcome gravity and, regardless of species or convection rate, are transported over comparable distances into the night-side magnetosphere. The region of space where these ions reach the magnetotail is found to extend over altitudes similar to those where enhanced densities are noticeable in the MESSENGER data, viz., from ∼ 1000 km up to ∼ 6000 km in the pre-midnight sector. The observed density enhancements may thus follow from E × B related focusing of planetary material of dayside origin into the magnetotail. Due to the planetary magnetic field offset, an asymmetry is found between drift paths anchored in the Northern and Southern hemispheres, which puts forward a predominant role of heavy material originating in the Northern Hemisphere in populating the innermost region of Mercury's magnetotail. (10.5194/angeo-31-1673-2013)
  DOI : 10.5194/angeo-31-1673-2013
• Remote sensing of a NTC radio source from a Cluster tilted spacecraft pair
  
  • Décréau Pierrette
  • Kougblénou S.
  • Lointier G.
  • Rauch Jean-Louis
  • Trotignon Jean-Gabriel
  • Vallières X.
  • Canu Patrick
  • Rochel Grimald S.
  • El-Lemdani Mazouz Farida
  • Darrouzet F.
  Annales Geophysicae, European Geosciences Union, 2013, 31 (11), pp.2097-2121. The Cluster mission operated a "tilt campaign" during the month of May 2008. Two of the four identical Cluster spacecraft were placed at a close distance (~50 km) from each other and the spin axis of one of the spacecraft pair was tilted by an angle of ~46°. This gave the opportunity, for the first time in space, to measure global characteristics of AC electric field, at the sensitivity available with long boom (88 m) antennas, simultaneously from the specific configuration of the tilted pair of satellites and from the available base of three satellites placed at a large characteristic separation (~1 RE). This paper describes how global characteristics of radio waves, in this case the configuration of the electric field polarization ellipse in 3-D-space, are identified from in situ measurements of spin modulation features by the tilted pair, validating a novel experimental concept. In the event selected for analysis, non-thermal continuum (NTC) waves in the 15-25 kHz frequency range are observed from the Cluster constellation placed above the polar cap. The observed intensity variations with spin angle are those of plane waves, with an electric field polarization close to circular, at an ellipticity ratio e = 0.87. We derive the source position in 3-D by two different methods. The first one uses ray path orientation (measured by the tilted pair) combined with spectral signature of magnetic field magnitude at source. The second one is obtained via triangulation from the three spacecraft baseline, using estimation of directivity angles under assumption of circular polarization. The two results are not compatible, placing sources widely apart. We present a general study of the level of systematic errors due to the assumption of circular polarization, linked to the second approach, and show how this approach can lead to poor triangulation and wrong source positioning. The estimation derived from the first method places the NTC source region in the dawn sector, at a large L value (L ~ 10) and a medium geomagnetic latitude (35° S). We discuss these untypical results within the frame of the geophysical conditions prevailing that day, i.e. a particularly quiet long time interval, followed by a short increase of magnetic activity. (10.5194/angeo-31-2097-2013)
  DOI : 10.5194/angeo-31-2097-2013
• Plasma/surface interaction: example of air plasmas and plasma bullets
  
  • Guaitella Olivier
  , 2013.