Gurudas Ganguli

Interactions between dust grains in a dusty plasma

Dust grains in plasma acquire a large negative charge, and can constitute a strongly coupled system. If the plasma is stationary, the plasma-mediated electrostatic potential around a single grain can be calculated by orbital-motion-limited (OML) theory, including ion absorption at the grain surface. This potential is repulsive at all ranges, and falls off as r−2 at long range. Nonlinear modifications occur when there are several grains, but the interaction is still repulsive. If the plasma is streaming by the grains, each grain generates a wake field potential which can be calculated via linear response theory, and which attracts other grains to stationary points behind the grain. There is in addition an attractive force between grains, due to ion-impact momentum deposition. In certain parameter regimes, this “shadowing” force can yield a weak net attraction at long range. Trapped-ion effects are significant at high plasma density, but have not yet been calculated.

Trapped ion effect on shielding, current flow, and charging of a small object in a plasma

The problem of electrostatic shielding around a small spherical collector immersed in nonflowing plasma, and the related problem of electron and ion flow to the collector, date to the origins of plasma physics. Calculations have typically neglected collisions, on the grounds that the mean free path is long compared to the Debye length. However, it has long been suspected that negative-energy trapped ions, created by occasional collisions, could be important. This paper presents self-consistent analytic calculations of the density and distribution function of trapped and untrapped ions, the potential profile, the ion and electron current to the collector, and the floating potential and charge of the collector. Under typical conditions for dust grains immersed in a discharge plasma, trapped ions are found to dominate the shielding near the grain, substantially increase the ion current to the grain, and suppress the floating potential and grain charge, even when the mean free path is much greater than the Debye length.

Dispersive properties of a magnetized plasma with a field‐aligned drift and inhomogeneous transverse flow

Electrostatic fluctuations driven by the combination of a magnetic‐field‐aligned electron current and a localized transverse electric field are investigated. Characteristic parameters, such as scale length and magnitude of the sheared E×B velocity, magnitude of the magnetic‐field‐aligned current, and temperature ratio τ≡Ti/Te are varied to include conditions associated with electrostatic waves driven entirely by magnetic‐field‐aligned current, driven entirely by transverse electric field, and driven by a combination of magnetic‐field‐aligned current and transverse electric field. It is shown that, in contrast to the homogeneous case of current‐driven modes, the modes in the presence of a transverse‐velocity shear can be unstable in a wider range of temperature ratio τ and they are broadband in frequency. Using a simplified model, numerical solutions of the nonlocal dispersion relation, and physical arguments, cases of stabilization and destabilization due to the inhomogeneous energy‐density driven instabi...

Low Frequency Oscillations in A Plasma with Spatially Variable Field-Aligned Flow

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.

Linear and nonlinear Landau resonance of kinetic Alfvén waves: Consequences for electron distribution and wave spectrum in the solar wind

Kinetic Alfven wave turbulence in solar wind is considered and it is shown that non-Maxwellian electron distribution function has a significant effect on the dynamics of solar wind plasmas. Linear Landau damping leads to the formation of a plateau in the parallel electron distribution function which diminishes the Landau damping rate significantly. Nonlinear scattering of waves by plasma particles is generalized to short wavelengths and it is found that for the solar wind parameters this scattering is the dominant process as compared to three-wave decay and coalescence in the wave vector range 1/ρi<k<ωpe/c. Incorporation of these effects leads to the steepening of the wave spectrum between the inertial and the dissipation ranges with a spectral index between 2 and 3. This region can be labeled as the scattering range. Such steepening has been observed in the solar wind plasmas.

Structure and dynamics of dust in streaming plasma: dust molecules, strings, and Crystals

In a plasma with ions streaming at a uniform velocity /spl sim/c/sub s/, dust grains can be accurately modeled as particles interacting via the dynamically-screened Coulomb interaction, calculated from linear response theory for the plasma. This force is nonreciprocal, i.e., action does not equal reaction, which has remarkable dynamical consequences. We show that up to four grains can form a stable self-bound molecule, which propels itself upstream against the ion flow. Stable equilibria are also found for pairs of grains confined in harmonic or quartic external potentials. For two grains in an anharmonic potential, or for three or more grains in any potential, there is no conserved quantity and self-excited oscillations can occur. In general, there are multiple equilibria, hysteresis occurs as parameters are varied, and it is not possible to distinguish ground and excited states. We show how the organizational and dynamical principles that govern the behavior of few-grain and low-dimensional systems also elucidate the more complex dynamics of crystals.

Three dimensional character of whistler turbulence

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.

Generation of ELF electromagnetic waves in the ionosphere by localized transverse dc electric fields: Subcyclotron frequency regime

It is commonly believed that Alfvenic waves observed in the ionosphere originated at trans ionospheric altitudes. However, recent observations of waves localized over skin depth scales, which are prone to Landau damping, and upward going Poynting flux suggest an ionospheric source. This theoretical study establishes the generation of electromagnetic waves at subcyclotron frequencies by localized static electric fields of the type commonly observed in the auroral ionosphere. The problem is formulated in terms of an eigenvalue system of equations which can describe all cold plasma normal modes, ion acoustic waves, and the various mode couplings induced by nonuniform electric fields. It is found that the velocity shear associated with the background electric field can significantly affect the observable properties of Alfven waves. In particular, Alfven waves can be destabilized when the magnitude of the velocity shear frequency exceeds the ion cyclotron frequency. Velocity shear can also significantly modify the ratio of E/B for these waves. The analysis further reveals electromagnetic Kelvin-Helmholtz instabilities, which are non-Alfvenic, with lower velocity shear thresholds. We model conditions encountered by several rockets and satellites, for example, FAST, Freja, AMICIST, AT2, to analyze wave properties and find that for typical ionospheric parameters the Kelvin-Helmholtz instabilities can generate electromagnetic waves with broadbanded spectra and other physical characteristics similar to observations. It is also shown that these waves are capable of resonant interaction with electrons over localized regions and hence may be important to understanding the generation of suprathermal electron bursts.

Velocity-shear-induced ion-cyclotron turbulence: Laboratory identification and space applications

Laboratory measurements are reported that identify a new class of plasma oscillation driven by the inhomogeneity in wave energy density caused by transverse‐velocity shear [Ganguli et al., Phys. Fluids 31, 823 (1988)]. The experiments concentrate on a branch of oscillation in the ion‐cyclotron range of frequencies that results from the coupling of the magnetic‐field‐aligned current and the inhomogeneous dc electric field localized in a layer thicker than the ion gyroradius. The observed transition between the well‐known current‐driven electrostatic ion‐cyclotron mode and this inhomogeneous energy–density‐driven mode is related to the ratio of the azimuthal and axial Doppler shifts. The mode characteristics associated with the two instabilities have significantly different properties. For conditions of large transverse‐velocity shear, turbulence is generated with a broadband, spiky spectrum around the ion‐cyclotron frequency at small values of the magnetic‐field‐aligned current. The experimental identification is reinforced with numerical results from a nonlocal eigenvalue condition.

Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study

We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region ...