Publications

2016

• Controlling the shape of the ion energy distribution at constant ion flux and constant mean ion energy with tailored voltage waveforms
  
  • Bruneau Bastien
  • Lafleur Trevor
  • Booth Jean-Paul
  • Johnson Erik
  Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (2), pp.025006. In this paper, we investigate the excitation of a capacitively coupled plasma using a non-sinusoidal voltage waveform whose amplitude- and slope-asymmetry varies continuously with a period which is a multiple of the fundamental RF period. We call this period the ?beating? period. Through particle-in-cell (PIC) simulations, we show that such waveforms cause oscillation of the self-bias at this beating frequency, corresponding to the charging and discharging of the external capacitor. The amplitude of this self-bias oscillation depends on the beating period, the value of the external capacitor, and the ion flux to the electrodes. This self-bias oscillation causes temporal modulation of the ion flux distribution function (IFDF), albeit at a constant ion flux and constant mean ion energy, and allows the energy width of the IFDF (averaged over the beating period) to be varied in a controlled fashion. (10.1088/0963-0252/25/2/025006)
  DOI : 10.1088/0963-0252/25/2/025006
• Observations of turbulence in a Kelvin-Helmholtz event on 8 September 2015 by the Magnetospheric Multiscale mission
  
  • Stawarz J. E.
  • Eriksson S.
  • Wilder F. D.
  • Ergun R. E.
  • Schwartz S. J.
  • Pouquet A.
  • Burch J. L.
  • Giles B. L.
  • Khotyaintsev Y. V.
  • Le Contel Olivier
  • Lindqvist P.-A.
  • Magnes W.
  • Pollock C. J.
  • Russell C. T.
  • Strangeway R. J.
  • Torbert R. B.
  • Avanov L. A.
  • Dorelli J. C.
  • Eastwood Jonathan P.
  • Gershman D. J.
  • Goodrich K. A.
  • Malaspina D. M.
  • Marklund G. T.
  • Mirioni Laurent
  • Sturner A. P.
  Journal of Geophysical Research Space Physics, American Geophysical Union/Wiley, 2016, 121 (11), pp.11,021-11,034. Spatial and high-time-resolution properties of the velocities, magnetic field, and 3-D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin-Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high-frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two-dimensional turbulence in association with the KHI. The behavior of kinetic-scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI. (10.1002/2016JA023458)
  DOI : 10.1002/2016JA023458
• Cold ion demagnetization near the X-line of magnetic reconnection
  
  • Toledo-Redondo Sergio
  • André M.
  • Khotyaintsev Y. V.
  • Vaivads A.
  • Walsh Andrew P.
  • Li Wenya
  • Graham Daniel B.
  • Lavraud Benoit
  • Masson A.
  • Aunai Nicolas
  • Divin A. V.
  • Dargent Jérémy
  • Fuselier Stephen
  • Gershman D. J.
  • Dorelli J. C.
  • Giles B. L.
  • Avanov L.
  • Pollock Craig
  • Saito Y.
  • Moore T. E.
  • Coffey Victoria
  • Chandler Michael O.
  • Lindqvist Per-Arne
  • Torbert Roy
  • Russell Christopher T.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (13), pp.6759-6767. Although the effects of magnetic reconnection in magnetospheres can be observed at planetary scales, reconnection is initiated at electron scales in a plasma. Surrounding the electron diffusion region, there is an Ion-Decoupling Region (IDR) of the size of the ion length scales (inertial length and gyroradius). Reconnection at the Earth's magnetopause often includes cold magnetospheric (few tens of eV), hot magnetospheric (10 keV), and magnetosheath (1 keV) ions, with different gyroradius length scales. We report observations of a subregion inside the IDR of the size of the cold ion population gyroradius (15 km) where the cold ions are demagnetized and accelerated parallel to the Hall electric field. Outside the subregion, cold ions follow the E × B motion together with electrons, while hot ions are demagnetized. We observe a sharp cold ion density gradient separating the two regions, which we identify as the cold and hot IDRs. (10.1002/2016GL069877)
  DOI : 10.1002/2016GL069877
• Currents and associated electron scattering and bouncing near the diffusion region at Earth's magnetopause
  
  • Lavraud B.
  • Zhang Y. C.
  • Vernisse Y.
  • Gershman D. J.
  • Dorelli J. C.
  • Cassak P. A.
  • Dargent Jérémy
  • Pollock C.
  • Giles B. L.
  • Aunai Nicolas
  • Argall M.
  • Avanov L.
  • Barrie A.
  • Burch J. L.
  • Chandler Michael O.
  • Chen L.-J.
  • Clark G.
  • Cohen I.
  • Coffey Victoria
  • Eastwood Jonathan P.
  • Egedal J.
  • Eriksson S.
  • Ergun R.
  • Farrugia C. J.
  • Fuselier S. A.
  • Génot V.
  • Graham Daniel B.
  • Grigorenko E. E.
  • Hasegawa H.
  • Jacquey C.
  • Kacem I.
  • Khotyaintsev Y. V.
  • Macdonald E.
  • Magnes W.
  • Marchaudon A.
  • Mauk B.
  • Moore T. E.
  • Mukai Toshifumi
  • Nakamura R.
  • Paterson W. R.
  • Penou E.
  • Phan T. D.
  • Rager A.
  • Retinò Alessandro
  • Rong Z. J.
  • Russell C. T.
  • Saito Y.
  • Sauvaud J.-A.
  • Schwartz S. J.
  • Shen C.
  • Smith S.
  • Strangeway R. J.
  • Toledo-Redondo Sergio
  • Torbert R.
  • Turner D. L.
  • Wang S.
  • Yokota S.
  Geophysical Research Letters, American Geophysical Union, 2016, 43 (7), pp.3042-3050. Based on high-resolution measurements from NASA's Magnetospheric Multiscale mission, we present the dynamics of electrons associated with current systems observed near the diffusion region of magnetic reconnection at Earth's magnetopause. Using pitch angle distributions (PAD) and magnetic curvature analysis, we demonstrate the occurrence of electron scattering in the curved magnetic field of the diffusion region down to energies of 20 eV. We show that scattering occurs closer to the current sheet as the electron energy decreases. The scattering of inflowing electrons, associated with field-aligned electrostatic potentials and Hall currents, produces a new population of scattered electrons with broader PAD which bounce back and forth in the exhaust. Except at the center of the diffusion region the two populations are collocated and appear to behave adiabatically: the inflowing electron PAD focuses inward (toward lower magnetic field), while the bouncing population PAD gradually peaks at 90° away from the center (where it mirrors owing to higher magnetic field and probable field-aligned potentials). (10.1002/2016GL068359)
  DOI : 10.1002/2016GL068359
• On the Existence of the Kolmogorov Inertial Range in the Terrestrial Magnetosheath Turbulence
  
  • Huang S. Y.
  • Sahraoui Fouad
  • Hadid L. Z.
  • Yuan Z. G.
  , 2016, 18, pp.EPSC2016-6911. In solar wind, it is well established that at MHD scales the magnetic energy spectra generally follow the so-called Kolmogorov inertial range spectrum f-5/3 in solar wind. In this study, we used three years data from the Cluster mission to statistically investigate the existence of the Kolmogorov inertial range in the whole magnetosheath, including flanks and subsolar regions. Statistical results show that Kolmogorov inertial range is not ubiquitous in the magnetosheath. Indeed, most spectra were found to be shallower than the Kolmogorov one and have a scaling f-1recalling the energy containing scales of solarwind turbulence. The Kolmogorov scaling is observed only faraway from the bow shock and in the flanks region. These results suggest that random-like fluctuations are generated behind the shock, which reach a fully developed turbulence state only after some time corresponding to their propagation (or advection) away from the shock. These fluctuations are shown to be essentially compressible and not Alfvenic as in the case of the solar wind. Implications on the new theoretical modeling of space plasma turbulence will be discussed.
• Electron Acceleration by Langmuir Waves Produced by a Decay Cascade
  
  • Krafft C.
  • Volokitin A. S.
  The Astrophysical Journal, American Astronomical Society, 2016, 821 (2), pp.99. It was recently reported that a significant part of the Langmuir waveforms observed by the STEREO satellite during type III solar radio bursts are likely consistent with the occurrence of electrostatic decay instabilities, when a Langmuir wave L resonantly interacts with another Langmuir wave L <SUP>\prime</SUP> and an ion sound wave S <SUP>\prime</SUP> through the decay channel L \to L <SUP>\prime</SUP> S <SUP>\prime</SUP> . Usually such wave-wave interactions occur in regions of the solar wind where the presence of electron beams can drive Langmuir turbulence to levels allowing waves L to decay. Moreover, such solar wind plasmas can present long-wavelength, randomly fluctuating density inhomogeneities or monotonic density gradients which can significantly modify the development of such resonant instabilities. If some conditions are met, the waves can encounter a second decay cascade (SDC) according to L <SUP>\prime</SUP> \to L <SUP>\prime\prime</SUP> S <SUP>\prime\prime</SUP> . Analytical estimates and observations based on numerical simulations show that the Langmuir waves L <SUP>\prime\prime</SUP> produced by this SDC can accelerate beam particles up to velocities and kinetic energies exceeding two times the beam drift velocity v<SUB>b</SUB> and half the initial beam energy, respectively. Moreover, this process can be particularly efficient if the scattering effects of waves on the background plasma inhomogeneities have already accelerated a sufficient amount of beam electrons up to the velocity range where the phase velocities of the L <SUP>\prime\prime</SUP> waves are lying. The paper shows that the conditions necessary for such process to occur can be easily met in solar wind plasmas if the beam velocities do not exceed around 35 times the plasma thermal velocity. (10.3847/0004-637X/821/2/99)
  DOI : 10.3847/0004-637X/821/2/99