Publications

2011

• A global model of micro-hollow cathode discharges in the stationary regime
  
  • Lazzaroni Claudia
  • Chabert Pascal
  Journal of Physics D: Applied Physics, IOP Publishing, 2011, 44, pp.445202. This paper presents a global model of micro-hollow cathode discharges working in argon gas. Inspired by experiments and two-dimensional numerical simulations, the discharge is decomposed into two main regions, labelled the cathodic region and the positive column region. The first is composed of a cathode sheath (surrounding the cathode) and a plasma in the centre, while the positive column region is essentially filled with plasma. A cathode sheath model is developed to calculate the sheath size and the power dissipated by ions and electrons in the sheath. Charged-particle transport equations are solved in one-dimensional cylindrical coordinates, in the quasi-neutral plasma region, in order to determine the radial density profiles and the electron temperature in the microhole. A global power balance is then performed in the two distinct regions in order to determine the absolute electron density. We found that the electron density is one order of magnitude higher in the cathodic region than in the positive column, showing that the power dissipation is mainly located in the cathode sheath. The calculated electron density in the cathodic region is in reasonable agreement with experiments. (10.1088/0022-3727/44/44/445202)
  DOI : 10.1088/0022-3727/44/44/445202
• Modelling of the beam-plasma interaction in a strongly inhomogeneous plasma
  
  • Volokitin A.
  • Krasnoselskikh V.
  • Krafft C.
  • Kuznetsov Evgenii
  AIP Conference Proceedings, American Institute of Physics, 2013, 1539, pp.78-81. The interaction of a beam propagating in a inhomogeneous solar wind plasma is considered. The properties of the plasma waves are described by the one dimensional Zakharov's equation, the beam is modelled by means of particles moving in the electric fields of the Langmuir waves. We take into account the presence of high level density fluctuations that are known to be present in the solar wind. It is shown that when the level of density fluctuations is low, δn/n0≪3k2λ2d, the regime of beam relaxation is very similar to that occurring in a homogeneous plasma and can be described by the quasilinear (QL) equations. In this case, the relaxation length is very short and corresponds to that obtained using the QL approximation. On the contrary, when the level of density fluctuations overcomes some limit, i.e. δn/n0≥3k2λ2d, where δn/n0≪1, the plasma inhomogeneities crucially influence the process of relaxation. First, the linear wave growth becomes localized and clearly identifiable wave packets/clumps dominate the wave spectrum; this is associated with the kinematic properties of the waves' propagation and the wave-particle resonant interactions. Most of the wave packets grow in the regions of density gradients. The second important feature revealed by the beam relaxation consists in the generation of a tail of accelerated electrons with velocities V > Vb exceeding the beam drift. The beam widens in both directions, toward lower velocities as well as higher velocities; the density of the accelerated electrons can reach more than 10 ÷ 20 % of the beam density and the energy flux carried by this population can become as large as 40% of the initial energy flux of the beam. (10.1063/1.4810994)
  DOI : 10.1063/1.4810994
• Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions
  
  • Nishimura Y.
  • Bortnik J.
  • Li W.
  • Thorne R. M.
  • Chen Lin
  • Lyons L.R.
  • Angelopoulos V.
  • Mende S. B.
  • Bonnell J. W.
  • Le Contel Olivier
  • Cully C. M.
  • Ergun R.
  • Auster U.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116, pp.11221. A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of ~5000 km diameter for corresponding azimuthally elongated patches with ~100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region. (10.1029/2011JA016876)
  DOI : 10.1029/2011JA016876
• Suprathermal electron acceleration during reconnection onset in the magnetotail
  
  • Vaivads A.
  • Retinò Alessandro
  • Khotyaintsev Y. V.
  • André M.
  Annales Geophysicae, European Geosciences Union, 2011, 29, pp.1917-1925. We study one event of reconnection onset associated to a small substorm on 27 September 2006 by using Cluster observations at inter-spacecraft separation of about 10 000 km. We focus on the acceleration of suprathermal electrons during different stages of reconnection. We show that several distinct stages of acceleration occur: (1) moderate acceleration during reconnection of pre-existing plasma sheet flux tubes, (2) stronger acceleration during reconnection of lobe flux tubes, (3) production of the most energetic electrons within dipolarization fronts (magnetic pile-up regions). The strongest acceleration is reached at the location of B<SUB>z</SUB> maxima inside the magnetic pile-up region where the reconnection jet stops. Very strong localized dawn-dusk electric field are observed within the magnetic pile-up regions and are associated to most of the magnetic flux transport. (10.5194/angeo-29-1917-2011)
  DOI : 10.5194/angeo-29-1917-2011
• Small-scale filamentary structures recorded in the auroral regions connected to the plasma sheet boundary layer
  
  • Pottelette Raymond
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2011, 116. We use high time resolution data from the FAST spacecraft during a moderate substorm and concentrate on the auroral regions that are magnetically linked to the plasma sheet boundary layer (PSBL). The crossing of these regions is characterized by the presence of a pair of oppositely directed, field-aligned current sheets. The more poleward of the two current sheets is directed earthward. Strong turbulent fluctuations are detected in association with the field-aligned currents. The low-frequency component (1 Hz) of the electric field fluctuations consists of localized structures whose characteristics are not entirely consistent with the presence of Alfvén waves. They may represent a possible example of the coupling of Alfvén waves with electron acoustic waves on small length scales. In the high-frequency range (1-10 kHz), the turbulent fluctuations are dominated by large amplitude (500 mV/m peak to peak) bipolar electric field structures whose polarity depends on the direction of the field-aligned currents. These latter structures are moving earthward in the upward current region and antiearthward in the downward current region. The generation of both low- and high-frequency filamentary nonlinear structures appears as a natural consequence of the disturbances imposed during substorms on the auroral regions connected to the PSBL when hot and cold plasmas interact. (10.1029/2010JA016192)
  DOI : 10.1029/2010JA016192
• Typical properties of rising and falling tone chorus waves
  
  • Li W.
  • Thorne R. M.
  • Bortnik J.
  • Shprits Y. Y.
  • Nishimura Y.
  • Angelopoulos V.
  • Chaston C.
  • Le Contel Olivier
  • Bonnell J. W.
  Geophysical Research Letters, American Geophysical Union, 2011, 38, pp.14103. Chorus waves, which have received intense attention recently due to their significant role in radiation belt electron dynamics, frequently exhibit rising and falling tones. Lower-band chorus waves, observed using THEMIS wave burst data, are analyzed to obtain the typical properties of either class of chorus emissions. Our results show that rising tones are more likely to be quasi field-aligned, whereas falling tones are typically very oblique, close to the resonance cone. Furthermore, rising tones occur significantly more often than falling tones, and magnetic amplitudes of rising tones are generally much larger than those of falling tones. We also show the preferential regions of rising and falling tones dependent on MLT and magnetic latitude. Our new findings suggest that two separate mechanisms may be responsible for the generation and nonlinear evolution of rising and falling tone chorus. (10.1029/2011GL047925)
  DOI : 10.1029/2011GL047925