Browsing by Author "Ganguli, Gurudas"

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    Early time evolution of a chemically produced electron depletion
    Scales, Wayne A.; Bernhardt, P. A.; Ganguli, Gurudas (American Geophysical Union, 1995-01-01)
    The early time evolution of an ionospheric electron depletion produced by a radially expanding electron attachment chemical release is studied with a two-dimensional simulation model. The model includes electron attachment chemistry, incorporates fluid electrons, particle ions and neutrals, and considers the evolution in a plane perpendicular to the geomagnetic field for a low beta plasma. Timescales considered are of the order of or less than the cyclotron period of the negative ions that result as a by-product of the electron attachment reaction. This corresponds to time periods of tenths of seconds during recent experiments. Simulation results show that a highly sheared azimuthal electron flow velocity develops in the radially expanding depletion boundary. This sheared electron flow velocity and the steep density gradients in the boundary give rise to small-scale irregularities in the form of electron density cavities and spikes. The nonlinear evolution of these irregularities results in trapping and ultimately turbulent heating of the negative ions.
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    Early time evolution of negative-ion clouds and electron-density depletions produced during electron-attachment chemical-release experiments
    Scales, Wayne A.; Bernhardt, P. A.; Ganguli, Gurudas (American Geophysical Union, 1994-01-01)
    Two-dimensional electrostatic particle-in-cell simulations are used to study the early time evolution of electron depletions and negative ion clouds produced during electron attachment chemical releases in the ionosphere. The simulation model considers the evolution in the plane perpendicular to the magnetic field and a three-species plasma that contains electrons, positive ions, and also heavy negative ions that result as a by-product of the electron attachment reaction. The early time evolution (less than the negative ion cyclotron period) of the system shows that a negative charge surplus initially develops outside of the depletion boundary as the heavy negative ions move across the boundary. The electrons are initially restricted from moving into the depletion due to the magnetic field. An inhomogenous electric field develops across the boundary layer due to this charge separation. A highly sheared electron flow velocity develops in the depletion boundary due to E X B and del N x B drifts that result from electron density gradients and this inhomogenous electric field. Structure eventually develops in the depletion boundary layer due to low-frequency electrostatic waves that have growth times shorter than the negative ion cyclotron period. It is proposed that these waves are most likely produced by the electron-ion hybrid instability that results from sufficiently large shears in the electron flow velocity.
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    Low Frequency Oscillations in A Plasma with Spatially Variable Field-Aligned Flow
    Ganguli, Gurudas; Slinker, S.; Gavrishchaka, V.; Scales, Wayne A. (AIP Publishing, 2002-05-01)
    The effects of a transverse gradient in the plasma flow velocity parallel to the ambient magnetic field are analyzed. A transverse velocity gradient in the parallel ion flow, even in small magnitude, can increase the parallel phase speed of the ion-acoustic waves sufficiently to reduce ion Landau damping. This results in a significantly lower threshold current for the current driven ion acoustic instability. Ion flow gradients can also give rise to a new class of ion cyclotron waves via inverse cyclotron damping. A broadband wave spectrum with multiple cyclotron harmonics is possible. A combination of the multiple cyclotron harmonic waves can result in spiky electric field structures with their peaks separated by an ion cyclotron period. A spatial gradient in the parallel electron flow is also considered but it is found to play a minimal role in the low frequency regime. Relevance of these to natural plasma environments is discussed. (C) 2002 American Institute of Physics.
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    Model for nonlinear evolution of localized ion ring beam in magnetoplasma
    Scales, Wayne A.; Ganguli, Gurudas; Rudakov, Leonid; Mithaiwala, Manish (AIP Publishing, 2012-06-01)
    An electrostatic hybrid model, which investigates the nonlinear evolution of a localized ion ring beam in a magnetoplasma, is described and applied to the generation and evolution of turbulence in the very low frequency (VLF) (Omega(ci) < omega < Omega(ce)) range, where Omega(ci(e)) is the ion (electron) gyro frequency. Electrons are treated as a fluid and the ions with the particle-in-cell method. Although the model is electrostatic, it includes the effects of energy loss by convection of electromagnetic VLF waves out of the instability region by utilizing a phenomenological model for effective collisions with the fluid electrons. In comparison with a more conventional electrostatic hybrid model, the new model shows much more efficient extraction of energy from the ion ring beam and reduced background plasma heating over a range of parameters. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729330]
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    Three dimensional character of whistler turbulence
    Ganguli, Gurudas; Rudakov, Leonid; Scales, Wayne A.; Wang, Joseph J.; Mithaiwala, Manish (AIP Publishing, 2010-05-01)
    It is shown that the dominant nonlinear effect makes the evolution of whistler turbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a two-dimensional electromagnetic particle-in-cell model with the ambient magnetic field out of the simulation plane to generate the essential three-dimensional nonlinear effects. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3420245]