Publications

2009

• Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft
  
  • Li W.
  • Thorne R. M.
  • Angelopoulos V.
  • Bortnik J.
  • Cully C. M.
  • Ni B.
  • Le Contel Olivier
  • Roux A.
  • Auster U.
  • Magnes W.
  Geophysical Research Letters, American Geophysical Union, 2009, 36, pp.9104. Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. (10.1029/2009GL037595)
  DOI : 10.1029/2009GL037595
• Time-dependent hydrodynamical simulations of slow solar wind, coronal inflows, and polar plumes
  
  • Pinto Rui
  • Grappin Roland
  • Wang Y-M
  • Léorat Jacques
  Astronomy & Astrophysics - A&A, EDP Sciences, 2009, 497 (2), pp.537--543. Aims. We explore the effects of varying the areal expansion rate and coronal heating function on the solar wind flow. Methods. We use a one-dimensional, time-dependent hydrodynamical code. The computational domain extends from near the photosphere, where nonreflecting boundary conditions are applied, to 30 , and includes a transition region where heat conduction and radiative losses dominate. Results. We confirm that the observed inverse relationship between asymptotic wind speed and expansion factor is obtained if the coronal heating rate is a function of the local magnetic field strength. We show that inflows can be generated by suddenly increasing the rate of flux-tube expansion and suggest that this process may be involved in the closing-down of flux at coronal hole boundaries. We also simulate the formation and decay of a polar plume, by including an additional, time-dependent heating source near the base of the flux tube. (10.1051/0004-6361/200811183)
  DOI : 10.1051/0004-6361/200811183
• Evidence of a Cascade and Dissipation of Solar-Wind Turbulence at the Electron Gyroscale
  
  • Sahraoui Fouad
  • Goldstein M. L.
  • Robert Patrick
  • Khotyaintsev Y. V.
  Physical Review Letters, American Physical Society, 2009, 102 (23), pp.231102. We report the first direct determination of the dissipation range of magnetofluid turbulence in the solar wind at the electron scales. Combining high resolution magnetic and electric field data of the Cluster spacecraft, we computed the spectrum of turbulence and found two distinct breakpoints in the magnetic spectrum at 0.4 and 35 Hz, which correspond, respectively, to the Doppler-shifted proton and electron gyroscales, fρp and fρe. Below fρp, the spectrum follows a Kolmogorov scaling f−1.62, typical of spectra observed at 1 AU. Above fρp, a second inertial range is formed with a scaling f−2.3 down to fρe. Above fρe, the spectrum has a steeper power law ∼f−4.1 down to the noise level of the instrument. We interpret this as the dissipation range and show a remarkable agreement with theoretical predictions of a quasi-two-dimensional cascade into Kinetic Alfvén Waves (KAW). (10.1103/PhysRevLett.102.231102)
  DOI : 10.1103/PhysRevLett.102.231102
• Turbulence in the TORE SUPRA Tokamak: Measurements and Validation of Nonlinear Simulations
  
  • Casati A.
  • Gerbaud T.
  • Hennequin P.
  • Heuraux Stéphane
  Physical Review Letters, American Physical Society, 2009, 102, pp.165005. Turbulence measurements in TORE SUPRA tokamak plasmas have been quantitatively compared to predictions by nonlinear gyrokinetic simulations. For the first time, numerical results simultaneously match within experimental uncertainty (a) the magnitude of effective heat diffusivity, (b) rms values of density fluctuations, and (c) wave-number spectra in both the directions perpendicular to the magnetic field. Moreover, the nonlinear simulations help to revise as an instrumental effect the apparent experimental evidence of strong turbulence anisotropy at spatial scales of the order of ion-sound Larmor radius.
• Unveiling the nature of out-of-equilibrium phase transitions in a system with long-range interactions
  
  • Firpo Marie-Christine
  EPL - Europhysics Letters, European Physical Society / EDP Sciences / Società Italiana di Fisica / IOP Publishing, 2009, 88, pp.30010. Recently, there has been some vigorous interest in the out-of-equilibrium quasistationary states (QSSs), with lifetimes diverging with the number N of degrees of freedom, emerging from numerical simulations of the ferromagnetic XY Hamiltonian Mean Field (HMF) starting from some special initial conditions. Phase transitions have been reported between low-energy magnetized QSSs and large-energy unexpected, antiferromagnetic-like, QSSs with low magnetization. This issue is addressed here in the Vlasov N→∞ limit. It is argued that the time-asymptotic states emerging in the Vlasov limit can be related to simple generic time-asymptotic forms for the force field. The proposed picture unveils the nature of the out-of-equilibrium phase transitions reported for the ferromagnetic HMF: this is a bifurcation point connecting an effective integrable Vlasov one-particle time-asymptotic dynamics to a partly ergodic one which means a brutal open-up of the Vlasov one-particle phase space. Illustration is given by investigating the time-asymptotic value of the magnetization at the phase transition, under the assumption of a sufficiently rapid time-asymptotic decay of the transient force field. (10.1209/0295-5075/88/30010)
  DOI : 10.1209/0295-5075/88/30010
• Capture of solar wind alpha-particles by the Martian atmosphere
  
  • Chanteur Gérard
  • Dubinin E.
  • Modolo Ronan
  • Fraenz M.
  Geophysical Research Letters, American Geophysical Union, 2009, 36 (23), pp.L23105. Integration along He++ test-particle trajectories in the self-consistent electromagnetic fields generated by three-dimensional hybrid simulations of the solar wind/Mars interaction is used to evaluate the removal of solar wind α-particles due to charge-exchange processes with neutral species of the Martian exosphere. The total removal rate of solar wind He++ ions, transformed into either singly ionised or neutral helium, is equal to 6.7 × 1023 s−1, which corresponds approximately to 30% of the flux of solar α-particles through the planetary cross-section. The deposition rate of helium neutral atoms, created by double electronic capture on exospheric oxygen, impacting the exobase, and penetrating below where it can be trapped, is about 1.5 × 1023 s−1. That means an important contribution of the solar wind source to the helium balance of the Martian atmosphere. The implantation of the solar helium into the Martian atmosphere shows an asymmetry related to the orientation of the motional electric field of the solar wind, −VSW × BIMF. (10.1029/2009GL040235)
  DOI : 10.1029/2009GL040235
• What can we learn from HF signal scattered from a discrete arc?
  
  • Seran Elena
  • Godefroy Michel
  • Kauristie K.
  • Cerisier Jean-Claude
  • Berthelier Jean-Jacques
  • Lester M.
  • Sarri L.-E.
  Annales Geophysicae, European Geosciences Union, 2009, 27 (5), pp.1887-1896. We present observations of a discrete southward propagating arc which appeared in the mid-night sector at latitudes equatorward of main substorm activity. The arc observations were made simultaneously by the ALFA (Auroral Light Fine Analysis) optical camera, the SuperDARN-CUTLASS HF radar and the Demeter satellite during a coordinated multi-instrumental campaign conducted at the KEOPS/ESRANGE site in December 2006. The SuperDARN HF signal which is often lost in the regions of strong electron precipitation yields in our case clear backscatter from an isolated arc of weak intensity. Consequently we are able to study arc dynamics, the formation of meso-scale irregularities of the electron density along the arc, compare the arc motion with the convection of surrounding plasma and discuss the contribution of ionospheric ions in the arc erosion and its propagation. (10.5194/angeo-27-1887-2009)
  DOI : 10.5194/angeo-27-1887-2009
• FLUCTUATION MEASUREMENTS AND THEIR LINK WITH TRANSPORT ON TORE SUPRA
  
  • Sabot R.
  • Hennequin Pascale
  • Colas L.
  Fusion Science and Technology, Taylor & Francis, 2009, 56 (3), pp.1253-1272. Measurement of turbulence properties provides key insight to understand anomalous transport in magnetic fusion devices. On Tore Supra, scattering diagnostics and reflectometers have been used to measure density fluctuations in the plasma core. A cross-polarization scattering diagnostic was also the first diagnostic to measure the turbulence magnetic fluctuations in a fusion plasma core. This paper presents the principle and the experimental setup of these diagnostics, with chosen results illustrating their capabilities to determine the spatial structure of the turbulence and to assess the link between energy transport and fluctuations. These flexible and complementary measurements made it possible to analyze the confinement and fluctuation scaling laws with non-dimensional parameters, which requires a wide variety of plasma conditions.
• Time resolved quantum cascade laser absorption spectroscopy on NO containing pulsed DC discharges
  
  • Welzel S.
  • Guaitella Olivier
  • Pintassilgo C.D.
  • Rousseau Antoine
  • Roepcke J.
  , 2009.
• Dust Detection by the Wave Instrument on STEREO: Nanoparticles Picked up by the Solar Wind?
  
  • Meyer-Vernet N.
  • Maksimovic M.
  • Czechowski A.
  • Mann I. R.
  • Zouganelis I.
  • Goetz K.
  • Kaiser M. L.
  • St. Cyr O. C.
  • Bougeret J.-L.
  • Bale S. D.
  Solar Physics, Springer Verlag, 2009, 256 (1-2), pp.463-474. The STEREO wave instrument (S/WAVES) has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit. (10.1007/s11207-009-9349-2)
  DOI : 10.1007/s11207-009-9349-2
• Kelvin-Helmholtz waves at the Earth's magnetopause: Multiscale development and associated reconnection
  
  • Hasegawa H.
  • Retinò Alessandro
  • Vaivads A.
  • Khotyaintsev Y. V.
  • André M.
  • Nakamura T. K. M.
  • Teh W.-L.
  • Sonnerup B. U. Ö.
  • Schwartz S. J.
  • Seki Y.
  • Fujimoto M.
  • Saito Y.
  • Rème H.
  • Canu Patrick
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2009, 114, pp.12207. We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at 1500 magnetic local time (MLT) by the Geotail spacecraft and at 1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 × 10<SUP>4</SUP> km) is considerably longer than predicted by the linear theory from the thickness (1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length 100 km and of magnetic shear 60° existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvénic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster. (10.1029/2009JA014042)
  DOI : 10.1029/2009JA014042
• A review of new wire arrays with open and closed magnetic configurations at the 1.6 MA Zebra generator for radiative properties and opacity effects
  
  • Kantsyrev Viktor L.
  • Safronova Alla S.
  • Esaulov A. A.
  • Williamson Kenneth M.
  • Shrestha Ishor
  • Yilmaz M. F.
  • Osborne Glenn C.
  • Weller Michael E.
  • Ouart N. D.
  • Shlyaptseva V. V.
  • Rudakov Leonid I.
  • Chuvatin Alexandre S.
  • Velikovich A. L.
  High Energy Density Physics, Elsevier, 2009, 5 (3), pp.115-123. Review The studies emphasize investigation of plasma formation, implosion, and radiation features as a function of two load configurations: compact multi-planar and cylindrical wire arrays. Experiments with different Z-pinch loads were performed on 1.6 MA, 100 ns, Zebra generator at University of Nevada, Reno. The multi-planar wire arrays (PWAs) were studied in open and closed configurations with Al, Cu, brass, Mo and W wires. In the open magnetic configurations (single, double, triple PWAs) magnetic fields are present inside the arrays from the beginning of discharge, while in closed configurations (prism-like PWA) the global magnetic field is excluded inside before plasma flow occurs. The new prism-like PWA allows high flexibility in control of implosion dynamics and precursor formation. The spectral modeling, magneto-hydrodynamic (MHD) and wire ablation dynamic model (WADM) codes were used to describe the plasma evolution and plasma parameters. Experimentally observed electron temperature and density in multiple bright spots reached 1.4 keV and 5 × 10E21 cm&#8722;3, respectively. Two types of bright spots were observed. With peak currents up to 1.3 MA opacity effects became more pronounced and led to a limiting of the X-ray yields from compact cylindrical arrays. Despite different magnetic energy to plasma coupling mechanisms early in the implosion a comparison of compact double PWA and cylindrical WA results indicates that during the stagnation stage the same plasma heating mechanism may occur. The double PWA was found to be the best radiator tested at University scale 1 MA generator. It is characterized by a combination of larger yield and power, mm-scale size, and provides the possibility of radiation pulse shaping. Further, the newer configuration, the double PWA with skewed wires, was tested and showed the possibility of a more effective X-ray generation. (10.1016/j.hedp.2009.04.001)
  DOI : 10.1016/j.hedp.2009.04.001
• International Heliophysical Year: GPS Network in Africa
  
  • Amory-Mazaudier Christine
  • Basu S.
  • Bock Olivier
  • Combrink A.
  • Groves K.
  • Fuller Rowell T.
  • Lassudrie-Duchesne Patrick
  • Petitdidier Monique
  • Yizengaw E.
  Earth, Moon, and Planets, Springer Verlag, 2009, 104 (1-4), pp.263-270. The main scientific objectives of the International Heliophysical Year are to discover and study all the physical processes coupling the Earth to the Sun. During the IHY a number of scientific instruments are being deployed all over the world. This brief report presents the scientific objectives, the GPS receiver network over Africa and the long lasting research planned for the next decades in Africa (10.1007/s11038-008-9273-8)
  DOI : 10.1007/s11038-008-9273-8
• Slow Solar Wind From Open Regions with Strong Low-Coronal Heating
  
  • Wang Y-M
  • Ko Y-K
  • Grappin Roland
  The Astrophysical Journal, American Astronomical Society, 2009, 691 (1), pp.760--769. By comparing solar wind data taken by the Advanced Composition Explorer during 1998-2007 with extrapolations of the observed photospheric magnetic field, we verify that high O7 /O6 and Fe/O ratios are associated with low wind speeds, large expansion factors, strong footpoint fields, and high mass and energy flux densities at the coronal base. As demonstrated by model calculations, these correlations are consistent with the idea that the bulk of the slow wind originates from regions of rapidly diverging open flux, where the coronal heating is concentrated at low heights. We identify two main components of the slow wind, one emanating from small coronal holes near active regions and characterized by particularly strong low-coronal heating, the other coming from just inside the polar-hole boundaries and characterized by weaker low-coronal heating and intermediate O7 /O6 and Fe/O ratios. (10.1088/0004-637X/691/1/760)
  DOI : 10.1088/0004-637X/691/1/760
• A comment on the paper 'Solar activity and its influence on climate' Author C. de Jager Published in Netherlands Journal of Geosciences-Geologie en Mijnbouw, 87-3, pp 207213, 3 2008
  
  • Amory-Mazaudier Christine
  • Legrand J.P.
  NETHERLANDS JOURNAL OF GEOSCIENCES-GEOLOGIE EN MIJNBOUW, 2009, [88-3] 177, pp.[88-3] 177. The purpose of this comment is not to criticize the results obtained by Dr C. de Jager, and we agree for example with his prediction of the next sunspot cycle amplitude - 68 with σ= 17.
• Quasi-parallel whistler mode waves observed by THEMIS during near-earth dipolarizations
  
  • Le Contel Olivier
  • Roux A.
  • Jacquey C.
  • Robert Patrick
  • Berthomier Matthieu
  • Chust Thomas
  • Grison B.
  • Angelopoulos V.
  • Sibeck David G.
  • Chaston C. C.
  • Cully C. M.
  • Ergun B.
  • Glassmeier K.-H.
  • Auster U.
  • Mcfadden J. P.
  • Carlson C. W.
  • Larson D. E.
  • Bonnell J. W.
  • Mende S. B.
  • Russell C. T.
  • Donovan E.
  • Mann I. R.
  • Singer H.
  Annales Geophysicae, European Geosciences Union, 2009, 27, pp.2259-2275. We report on quasi-parallel whistler emissions detected by the near-earth satellites of the THEMIS mission before, during, and after local dipolarization. These emissions are associated with an electron temperature anisotropy alpha=T<SUB>&#8869;e</SUB>/T<SUB>||e</SUB>>1 consistent with the linear theory of whistler mode anisotropy instability. When the whistler mode emissions are observed the measured electron anisotropy varies inversely with beta<SUB>||e</SUB> (the ratio of the electron parallel pressure to the magnetic pressure) as predicted by Gary and Wang (1996). Narrow band whistler emissions correspond to the small alpha existing before dipolarization whereas the broad band emissions correspond to large alpha observed during and after dipolarization. The energy in the whistler mode is leaving the current sheet and is propagating along the background magnetic field, towards the Earth. A simple time-independent description based on the Liouville's theorem indicates that the electron temperature anisotropy decreases with the distance along the magnetic field from the equator. Once this variation of alpha is taken into account, the linear theory predicts an equatorial origin for the whistler mode. The linear theory is also consistent with the observed bandwidth of wave emissions. Yet, the anisotropy required to be fully consistent with the observations is somewhat larger than the measured one. Although the discrepancy remains within the instrumental error bars, this could be due to time-dependent effects which have been neglected. The possible role of the whistler waves in the substorm process is discussed. (10.5194/angeo-27-2259-2009)
  DOI : 10.5194/angeo-27-2259-2009
• Toroidal Rotation Driven by the Polarization Drift
  
  • Mcdevitt C.J.
  • Diamond P.H.
  • Gürcan Özgür D.
  • Hahm T.S.
  Physical Review Letters, American Physical Society, 2009, 103, pp.205003. Starting from a phase space conserving gyrokinetic formulation, a systematic derivation of parallel momentum conservation uncovers a novel mechanism by which microturbulence may drive intrinsic rotation. This mechanism, which appears in the gyrokinetic formulation through the parallel nonlinearity, emerges due to charge separation induced by the polarization drift. The derivation and physical discussion of this mechanism will be pursued throughout this Letter. (10.1103/PhysRevLett.103.205003)
  DOI : 10.1103/PhysRevLett.103.205003
• Dispersion relations of electron density fluctuations in a Hall thruster plasma, observed by collective light scattering
  
  • Tsikata Sedina
  • Lemoine N.
  • Pisarev V.
  • Grésillon D.
  Physics of Plasmas, American Institute of Physics, 2009, 16, pp.033506. Kinetic models and numerical simulations of E×B plasma discharges predict microfluctuations at the scales of the electron cyclotron drift radius and the ion plasma frequency. With the help of a specially designed collective scattering device, the first experimental observations of small-scale electron density fluctuations inside the plasma volume are obtained, and observed in the expected ranges of spatial and time scales. The anisotropy, dispersion relations, form factor, amplitude, and spatial distribution of these electron density fluctuations are described and compared to theoretical expectations. (10.1063/1.3093261)
  DOI : 10.1063/1.3093261
• Quantum Cascade Laser Absorption Spectroscopy Study on the Influence of Plasma Stimulated Surface Adsorptions to the NO Destruction Kinetics
  
  • Hübner M.
  • Guaitella Olivier
  • Rousseau Antoine
  • Welzel S.
  • Roepcke J.
  , 2009.
• Dielectric Barrier Discharge (DBD) and Zeolite Coupling: Butane case
  
  • Youssef Joseph
  • Bouamra K.
  • Makarov M.
  • Guaitella Olivier
  • Rousseau Antoine
  , 2009.
• Time resolved studies on pulsed DC discharges using QCL
  
  • Welzel S.
  • Guaitella Olivier
  • Lazzaroni Claudia
  • Gatilova Lina
  • Rousseau Antoine
  • Roepcke J.
  , 2009.
• Time resolved shadow imaging of a pulsed corona in water
  
  • Ceccato P H
  • Guaitella Olivier
  • Rousseau Antoine
  , 2009.
• On the long term change in the geomagnetic activity during the 20th century
  
  • Ouattara F.
  • Amory-Mazaudier Christine
  • Menvielle Michel
  • Simon P.
  • Legrand J.-P.
  Annales Geophysicae, European Geosciences Union, 2009, 27 (5), pp.2045-2051. The analysis of the aa index series presented in this paper clearly shows that during the last century (1900 to 2000) the number of quiet days (Aa<20 nT) drastically di- minished from a mean annual value greater than 270 days per year at the end of the nineteenth century to a mean value of 160 quiet days per year one hundred years later. This de- crease is mainly due to the decrease of the number of very quiet days (Aa<13 nT). We show that the so-evidenced de- crease in the number of quiet days cannot be accounted for by drift in the aa baseline resulting in a systematic underes- timation of aa during the first quarter of the century: a 2– 3 nT overestimation in the aa increase during the 20th cen- tury would lead to a 20–40% overestimation in the decrease of the number of quiet days during the same period. The quiet days and very quiet days correspond to periods during which the Earth encounters slow solar wind streams flowing in the heliosheet during the period where the solar magnetic field has a dipolar geometry. Therefore, the ob- served change in the number of quiet days is the signature of a long term evolution of the solar coronal field topology. It may be interpreted in terms of an increase in the magnitude of the solar dipole, the associated decrease of the heliosheet thickness accounting for the observed decrease in the number of quiet days. (10.5194/angeo-27-2045-2009)
  DOI : 10.5194/angeo-27-2045-2009
• West African equatorial ionospheric parameters climatology based on Ouagadougou ionosonde station data from June 1966 to February 1998
  
  • Ouattara F.
  • Amory-Mazaudier Christine
  • Fleury Rolland
  • Lassudrie Duchesne Patrick
  • Vila P.
  • Petitdidier Monique
  Annales Geophysicae, European Geosciences Union, 2009, 27 (6), pp.2503-2514. This study is the first which gives the climatology of West African equatorial ionosphere by using Ouagadougou station through three solar cycles. It has permitted to show the complete morphology of ionosphere parameters by analyzing yearly variation, solar cycle and geomagnetic activity, seasonal evolution and diurnal development. This work shows that almost all ionospheric parameters have 11-year solar cycle evolution. Seasonal variation shows that only foF2 exhibits annual, winter and semiannual anomaly. foF2 seasonal variation has permitted us to identify and characterize solar events effects on F2 layer in this area. In fact (1) during quiet geomagnetic condition foF2 presents winter and semiannual anomalies asymmetric peaks in March/April and October. (2) The absence of winter anomaly and the presence of equinoctial peaks are the most visible effects of fluctuating activity in foF2 seasonal time profiles. (3) Solar wind shock activity does not modify the profile of foF2 but increases ionization. (4) The absence of asymmetry peaks, the location of the peaks in March and October and the increase of ionization characterize recurrent storm activity. F1 layers shows increasing trend from cycle 20 to cycle 21. Moreover, E layer parameters seasonal variations exhibit complex structure. It seems impossible to detect fluctuating activity effect in E layer parameters seasonal variations but shock activity and wind stream activity act to decrease E layer ionization. It can be seen from Es layer parameters seasonal variations that wind stream activity effect is fairly independent of solar cycle. E and Es layers critical frequencies and virtual heights diurnal variations let us see the effects of the greenhouse gases in these layers. (10.5194/angeo-27-2503-2009)
  DOI : 10.5194/angeo-27-2503-2009
• Disk formation during collapse of magnetized protostellar cores
  
  • Hennebelle Patrick
  • Ciardi Andrea
  Astronomy & Astrophysics - A&A, EDP Sciences, 2009, 506, pp.L29-L32. Context: In the context of star and planet formation, understanding the formation of disks is of fundamental importance. Aims: Previous studies found that the magnetic field has a very strong impact on the collapse of a prestellar cloud, by possibly suppressing the formation of a disk even for relatively modest values of the magnetic intensity. Since observations infer that cores have a substantial level of magnetization, this raises the question of how disks form. However, most studies have been restricted to the case in which the initial angle, alpha, between the magnetic field and the rotation axis equals 0°. Here we explore and analyse the influence of non aligned configurations on disk formation. Methods: We perform 3D ideal MHD, AMR numerical simulations for various values of mu, the ratio of the mass-to-flux to the critical mass-to-flux, and various values of alpha. Results: We find that disks form more easily as alpha increases from 0 to 90°. We propose that as the magnetized pseudo-disks become thicker with increasing alpha, the magnetic braking efficiency is lowered. We also find that even small values of alpha (~=10-20°) show significant differences with the aligned case. Conclusions: Within the framework of ideal MHD, and for our choice of initial conditions, centrifugally supported disks cannot form for values of mu smaller than ~=3 when the magnetic field and the rotation axis are perpendicular, and smaller than about ~=5-10 when they are perfectly aligned. (10.1051/0004-6361/200913008)
  DOI : 10.1051/0004-6361/200913008