Publications

2011

• Turbulence intensity pulse propagation with self-consistent nonlinear noise
  
  • Wang Z.H.
  • Diamond P.H.
  • Gürcan Özgür D.
  • Garbet X.
  • Wang X.G.
  Physics of Plasmas, American Institute of Physics, 2011, 18, pp.032306. A model of turbulence intensity spreading with self-consistent nonlinear noise is derived systematically for the simple dynamical model of resistivity gradient driven turbulence. Local effective drive, thermal conduction damping, nonlinear coupling, and spatial scattering effects are included. As a consequence of nonlinear mode coupling processes (i.e., triad mode interactions), turbulence energy can be spatially scattered, leading to turbulence propagation and spreading. However, the range of any nonlinear mode interactions of the background with a test mode is restricted to within a few mode scale widths from the test mode rational surface. The speed of a turbulent spreading front is calculated. This front speed is effectively constant on macroscopic scales. We show that the effect of self-consistent nonlinear noise on the intensity front speed is modest, as a consequence of the ordering &#916;c<Lf, where &#916;c is the turbulence correlation length and Lf is the scale length of the fronts leading edge. The implications of these results for turbulence spreading models and the important differences between self-consistent mode coupling noise and ad hoc external noise are discussed. The broader implications of these results for turbulence front propagation are identified and explained. (10.1063/1.3567142)
  DOI : 10.1063/1.3567142
• 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
• 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