Publications

2012

• Geomagnetism during solar cycle 23: Characteristics
  
  • Zerbo Jean-Louis
  • Amory-Mazaudier Christine
  • Ouattara F.
  Journal of Advanced Research, Elsevier, 2012, pp.1-25. On the basis of more than 48 years of morphological analysis of yearly and monthly values of the sunspot number, the aa index, the solar wind speed and interplanetary magnetic field, we point out the particularities of geomagnetic activity during the period 1996-2009. We especially investigate the last cycle 23 and the long minimum which followed it. During this period, the lowest values of the yearly averaged IMF (3nT) and yearly averaged solar wind speed (364km/s) are recorded in 1996, and 2009 respectively. The year 2003 shows itself particular by recording the highest value of the averaged solar wind (568 km/s), associated to the highest value of the yearly averaged aa index (37nT). We also find that observations during the year 2003 seem to be related to several coronal holes which are known to generate high-speed wind stream. From the long time (more than one century) study of solar variability, the present period is similar to the beginning of twentieth century. We especially present the morphological features of solar cycle 23 which is followed by a deep solar minimum. (10.1016/j.jare.2012.08.010)
  DOI : 10.1016/j.jare.2012.08.010
• Surface vibrational relaxation of N<SUB>2</SUB> studied by CO<SUB>2</SUB> titration with time-resolved quantum cascade laser absorption spectroscopy
  
  • Marinov Daniil
  • Lopatik D.
  • Guaitella Olivier
  • Hübner M.
  • Ionikh Y.
  • Röpcke J.
  • Rousseau Antoine
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.175201. A new method for determination of the wall de-excitation probability γ _\rm N₂ of vibrationally excited N2 on different surfaces exposed to low-pressure plasmas has been developed. A short dc discharge pulse of only a few milliseconds was applied to a mixture containing 0.051% of CO2 in N2 at a pressure of 133 Pa. Due to a nearly resonant fast vibrational transfer between N2(v) and the asymmetric &#957;3 mode of CO2 the vibrational excitation of these titrating molecules is an image of the degree of vibrational excitation of N2. In the afterglow, the vibrational relaxation of CO2 was monitored in situ using quantum cascade laser absorption spectroscopy. The experimental results were interpreted in terms of a numerical model of non-equilibrium vibrational kinetics in CO2N2 mixtures. Heterogeneous relaxation was the main quenching process of N2(v) under the conditions of this study, which allowed determination of the value of γ _\rm N₂ from the best agreement between the experiment and the model. The new method is suitable for γ _\rm N₂ determination in a single plasma pulse with the discharge tube surface pretreated by a low-pressure plasma. The relaxation probability of the first vibrational level of nitrogen &#947;1 = (1.1 ± 0.15) × 10&#8722;3 found for Pyrex and silica is in reasonable agreement with the literature data. Using the new technique the N2(v = 1) quenching probability was measured on TiO2 surface, &#947;1 = (9 ± 1) × 10&#8722;3. A linear enhancement of the N2(v) wall deactivation probability with an increase in the admixture of CO2 was observed for all studied materials. In order to explain this effect, a vibrational energy transfer mechanism between N2(v) and adsorbed CO2 is proposed. (10.1088/0022-3727/45/17/175201)
  DOI : 10.1088/0022-3727/45/17/175201
• Time dependent behaviors of ion-ion plasmas exposed to various voltage waveforms in the kilohertz to megahertz frequency range
  
  • Oudini N.
  • Garrigues Laurent
  • Meige A.
  • Raimbault Jean-Luc
  • Chabert Pascal
  • Aanesland Ane
  Physics of Plasmas, American Institute of Physics, 2012, 19, pp.103501. An ion-ion plasma, situated between two parallel electrodes, is studied with the use of a time dependent one-dimensional particle-in-cell Monte-Carlo collisions model. This plasma consists of only positively and negatively singly charged ions with the same order of mass and temperature (the electron density is zero). The right electrode is grounded, and the left electrode is biased with a voltage waveform varying from sinusoidal to square with the frequency in the kHz to MHz range. The sheath evolution and the particle flux towards the electrodes, as a function of both space and time, are investigated for the various waveforms and frequencies. The sheath evolution has a strong influence on the time averaged ion energy distribution function (IEDF). The IEDF is broad with a low energy tail for low frequency sinusoidal biases (25kHz) while peaked at low energy for higher frequencies (2 MHz). For square waveforms, the IEDF is mono-energetic with some broadening when the rise time is faster than the typical time to establish the steady state sheath formation(<0.3 us). (10.1063/1.4762855)
  DOI : 10.1063/1.4762855
• Comparative study of Positive and Negative ion flows extracted from downstream plasmas beyond magnetic and electrostatic filters
  
  • Dudin S.
  • Rafalskyi D.V.
  • Popelier Lara
  • Aanesland Ane
  Physical Surface Engineering, 2012, 10, pp.22-28. In the present paper we compare the positive and negative ion flows created using a recently developed electrostatic grid-type filter with the flows formed using a magnetic filter. Langmuir probe measurements show electron cooling with both filters, allowing effective formation of negative ions via electron dissociative attachment in the region of low electron temperature. The energy distribution functions of positive and negative ions extracted from the filtered plasmas are measured in both systems showing an almost monoenergetic nature of the ions with the energy corresponding to the imposed extraction potential. It is shown that in both cases strongly electronegative plasmas where the negative ion density is much larger than the electron density can be formed downstream of the filter. Biasing an internal electrode or the electrostatic filter grid allows control of the plasma potential. In the case of the electrostatic filter the plasma could be biased negatively compared to ground and effective extraction of negative ion was achieved.
• The 2012 Plasma Roadmap (Invited Review Article)
  
  • Samukawa S.
  • Hori M.
  • Rauf S.
  • Tachibana K.
  • Bruggeman P.
  • Kroesen G.
  • Whitehead J.C.
  • Murphy A.B.
  • Gutsol A.F.
  • Starikovskaia Svetlana
  • Kortshagen U.
  • Buf J.-P.
  • Sommerer T.J.
  • Kushner M.J.
  • Czarnetzki U.
  • Mason N.
  Journal of Physics D: Applied Physics, IOP Publishing, 2012, 45, pp.253001. Low-temperature plasma physics and technology are diverse and interdisciplinary fields. The plasma parameters can span many orders of magnitude and applications are found in quite different areas of daily life and industrial production. As a consequence, the trends in research, science and technology are difficult to follow and it is not easy to identify the major challenges of the field and their many sub-fields. Even for experts the road to the future is sometimes lost in the mist. Journal of Physics D: Applied Physics is addressing this need for clarity and thus providing guidance to the field by this special Review article, The 2012 Plasma Roadmap. Although roadmaps are common in the microelectronic industry and other fields of research and development, constructing a roadmap for the field of low-temperature plasmas is perhaps a unique undertaking. Realizing the difficulty of this task for any individual, the plasma section of the Journal of Physics D Board decided to meet the challenge of developing a roadmap through an unusual and novel concept. The roadmap was divided into 16 formalized short subsections each addressing a particular key topic. For each topic a renowned expert in the sub-field was invited to express his/her individual visions on the status, current and future challenges, and to identify advances in science and technology required to meet these challenges. Together these contributions form a detailed snapshot of the current state of the art which clearly shows the lifelines of the field and the challenges ahead. Novel technologies, fresh ideas and concepts, and new applications discussed by our authors demonstrate that the road to the future is wide and far reaching. We hope that this special plasma science and technology roadmap will provide guidance for colleagues, funding agencies and government institutions. If successful in doing so, the roadmap will be periodically updated to continue to help in guiding the field. (10.1088/0022-3727/45/25/253001)
  DOI : 10.1088/0022-3727/45/25/253001
• Analysis of Ni-60 alloy precursor wire array experiments on the 1.7 MA Zebra generator at UNR
  
  • Stafford A.
  • Safronova Alla S.
  • Kantsyrev Viktor L.
  • Esaulov A. A.
  • Weller Michael E.
  • Osborne Glenn C.
  • Shrestha Ishor
  • Keim S. F.
  • Shlyaptseva V. V.
  • Zunino H. A.
  • Coverdale C. A.
  • Chuvatin Alexandre S.
  , 2012, pp.2P88. Previous experiments on the 1 MA Zebra generator at UNR were dedicated to studies of precursor plasmas with Ni-60 alloy (96% Cu) cylindrical wire arrays (CWA). Those precursor plasmas were shown to consistently have electron temperatures >; 400 eV 1. Continuing research at 1 MA on Zebra found precursors for Cu, Brass, and Alumel CWAs to be consistent with Ni-60 results2. Recent Ni-60 CWA experiments have been performed on Zebra using a Load Current Multiplier (LCM) that raises the current up to 1.7 MA. A full set of diagnostics included 10 beam lines. These CWAs consist of 6 wires evenly spaced in a 12 mm diameter as before, but with a linear mass density of 100 &#956;g/cm, more than double the mass density in 1 MA experiments. The radiation yield per unit length in the LCM shots, 16 kJ/cm, (with decreased 1 cm anode-cathode gap) has approximately doubled the previous Ni-60 yield per unit length. The total integrated PCD energy (filtered with 8 &#956;m Be) increases up to 750 J (compared to the previous ~500 J). The percentage of the PCD due to the precursor has remained relatively constant. Non-LTE kinetic models of Cu and Ni have been applied to account for the L-shell radiation from the precursor and main x-ray burst plasmas. The resulting plasma parameters from modeling of TGSI and TISR spectra together with analysis of corresponding images allow for the study of precursor plasma formation in time and in space, respectively. (10.1109/PLASMA.2012.6383644)
  DOI : 10.1109/PLASMA.2012.6383644
• SOLAR WIND TURBULENT SPECTRUM AT PLASMA KINETIC SCALES
  
  • Alexandrova Olga
  • Lacombe C.
  • Mangeney Anne
  • Grappin Roland
  • Maksimovic M.
  The Astrophysical Journal, American Astronomical Society, 2012, 760 (2), pp.121. The description of the turbulent spectrum of magnetic fluctuations in the solar wind in the kinetic range of scales is not yet completely established. Here, we perform a statistical study of 100 spectra measured by the STAFF instrument on the Cluster mission, which allows us to resolve turbulent fluctuations from ion scales down to a fraction of electron scales, i.e., from ~102 km to ~300 m. We show that for k &#8869;&#961; e in [0.03, 3] (which corresponds approximately to the frequency in the spacecraft frame f in [3, 300] Hz), all the observed spectra can be described by a general law E(k &#8869;)vpropk 8/3 &#8869;exp ( k &#8869;&#961; e ), where k &#8869; is the wavevector component normal to the background magnetic field and &#961; e the electron Larmor radius. This exponential tail found in the solar wind seems compatible with the Landau damping of magnetic fluctuations onto electrons. (10.1088/0004-637X/760/2/121)
  DOI : 10.1088/0004-637X/760/2/121
• IMPALAS: Investigation of MagnetoPause Activity using Longitudinally-Aligned Satellites―-a mission concept proposed for the ESA M3 2020/2022 launch
  
  • Owen C. J.
  • Amm Olaf
  • Bruno Roberto
  • de Keyser J.
  • Dunlop M. W.
  • Eastwood Jonathan P.
  • Fazakerley A.
  • Fontaine Dominique
  • Forsyth C.
  • Hasegawa H.
  • Hellinger P.
  • Hercik David
  • Jacquey C.
  • Milan Steven
  • Raeder Joachim
  • Sibeck David G.
  • Stverak Stepan
  • Travnicek Pavel
  • Walsh Andrew P.
  • Wild James A.
  Experimental Astronomy, Springer Link, 2012, 33 (2-3), pp.365-401. The dayside magnetopause is the primary site of energy transfer from the solar wind into the magnetosphere, and modulates the activity observed within the magnetosphere itself. Specific plasma processes operating on the magnetopause include magnetic reconnection, generation of boundary waves, propagation of pressure-pulse induced deformations of the boundary, formation of boundary layers and generation of Alfvén waves and field-aligned current systems connecting the boundary to the inner magnetosphere and ionosphere. However, many of the details of these processes are not fully understood. For example, magnetic reconnection occurs sporadically, producing flux transfer events, but how and where these arise, and their importance to the global dynamics of the magnetospheric system remain unresolved. Many of these phenomena involve propagation across the magnetopause surface. Measurements at widely-spaced (Delta 5 R<SUB>E</SUB>) intervals along the direction of dayside terrestrial field lines at the magnetopause would be decisive in resolving these issues. We describe a mission carrying a fields and plasmas payload (including magnetometer, ion and electron spectrometer and energetic particle telescopes) on three identical spacecraft in synchronized orbits. These provide the needed separations, with each spacecraft skimming the dayside magnetopause and continuously sampling this boundary for many hours. The orbits are phased such that (i) all three spacecraft maintain common longitude and thus sample along the same magnetopause field line; (ii) the three spacecraft reach local midday when northern European ground-based facilities also lie near local midday, enabling simultaneous sampling of magnetopause field lines and their footprints. (10.1007/s10686-011-9245-2)
  DOI : 10.1007/s10686-011-9245-2
• EIDOSCOPE: particle acceleration at plasma boundaries
  
  • Vaivads A.
  • Andersson G.
  • Bale S. D.
  • Cully C. M.
  • de Keyser J.
  • Fujimoto M.
  • Grahn S.
  • Haaland S.
  • Ji H.
  • Khotyaintsev Y. V.
  • Lazarian A.
  • Lavraud B.
  • Mann I. R.
  • Nakamura R.
  • Nakamura T. K. M.
  • Narita Y.
  • Retinò Alessandro
  • Sahraoui Fouad
  • Schekochihin A.
  • Schwartz S. J.
  • Shinohara I.
  • Sorriso-Valvo L.
  Experimental Astronomy, Springer Link, 2012, 33 (2-3), pp.491-527. We describe the mission concept of how ESA can make a major contribution to the Japanese Canadian multi-spacecraft mission SCOPE by adding one cost-effective spacecraft EIDO (Electron and Ion Dynamics Observatory), which has a comprehensive and optimized plasma payload to address the physics of particle acceleration. The combined mission EIDOSCOPE will distinguish amongst and quantify the governing processes of particle acceleration at several important plasma boundaries and their associated boundary layers: collisionless shocks, plasma jet fronts, thin current sheets and turbulent boundary layers. Particle acceleration and associated cross-scale coupling is one of the key outstanding topics to be addressed in the Plasma Universe. The very important science questions that only the combined EIDOSCOPE mission will be able to tackle are: 1) Quantitatively, what are the processes and efficiencies with which both electrons and ions are selectively injected and subsequently accelerated by collisionless shocks? 2) How does small-scale electron and ion acceleration at jet fronts due to kinetic processes couple simultaneously to large scale acceleration due to fluid (MHD) mechanisms? 3) How does multi-scale coupling govern acceleration mechanisms at electron, ion and fluid scales in thin current sheets? 4) How do particle acceleration processes inside turbulent boundary layers depend on turbulence properties at ion/electron scales? EIDO particle instruments are capable of resolving full 3D particle distribution functions in both thermal and suprathermal regimes and at high enough temporal resolution to resolve the relevant scales even in very dynamic plasma processes. The EIDO spin axis is designed to be sun-pointing, allowing EIDO to carry out the most sensitive electric field measurements ever accomplished in the outer magnetosphere. Combined with a nearby SCOPE Far Daughter satellite, EIDO will form a second pair (in addition to SCOPE Mother-Near Daughter) of closely separated satellites that provides the unique capability to measure the 3D electric field with high accuracy and sensitivity. All EIDO instrumentation are state-of-the-art technology with heritage from many recent missions. The EIDOSCOPE orbit will be close to equatorial with apogee 25-30 RE and perigee 8-10 RE. In the course of one year the orbit will cross all the major plasma boundaries in the outer magnetosphere; bow shock, magnetopause and magnetotail current sheets, jet fronts and turbulent boundary layers. EIDO offers excellent cost/benefits for ESA, as for only a fraction of an M-class mission cost ESA can become an integral part of a major multi-agency L-class level mission that addresses outstanding science questions for the benefit of the European science community. (10.1007/s10686-011-9233-6)
  DOI : 10.1007/s10686-011-9233-6
• In Situ Observations of Reconnection and Associated Particle Energization in Turbulent Plasmas
  
  • Retinò Alessandro
  • Vaivads A.
  • Chasapis A.
  • Sundkvist D.
  • Sahraoui Fouad
  , 2012.
• 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 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
• 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
• 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
• 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
• 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
• 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
• 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
• 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