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Zahida Ehsan

Publications and source records attributed to Zahida Ehsan.

17 recordsLinked to original sources
arXiv2023

Three-dimensional vortex dipole solitons in self-gravitating systems

We derive the nonlinear equations governing the dynamics of three-dimensional (3D) disturbances in a nonuniform rotating self-gravitating fluid under the assumption that the characteristic frequencies of disturbances are small compared to the rotation frequency. Analytical solutions of these equations are found in the form of the 3D vortex dipole solitons. The method for obtaining these solutions is based on the well-known Larichev-Reznik procedure for finding two-dimensional nonlinear dipole vortex solutions in the physics of atmospheres of rotating planets. In addition to the basic 3D x-antisymmetric part (carrier), the solution may also contain radially symmetric (monopole) or/and antisymmetric along the rotation axis (z-axis) parts with arbitrary amplitudes, but these superimposed parts cannot exist without the basic part. The 3D vortex soliton without the superimposed parts is extremely stable. It moves without distortion and retains its shape even in the presence of an initial noise disturbance. The solitons with parts that are radially symmetric or/and z-antisymmetric turn out to be unstable, although at sufficiently small amplitudes of these superimposed parts, the soliton retains its shape for a very long time.

nlin.PS↗
arXiv2022

Coupling of acoustic and drift modes, harmonic modons in astrophysical dusty plasma and a new mode in the Comet Halley

This manuscript, presents a theoretical study of the linear and nonlinear characteristics of acoustic and drift waves in a bounded inhomogenious dusty plasma which has potential applications in space and lab environments. In this analysis, flow of all plasma particles is assumed to be along the axial direction whereas gradients in velocity are considered to be along the radial direction. First we study coupling of dust modified ion acoustic (DMA) and drift (DMD) waves (linear analysis) at fast time when dynamics of dust remain inactive, later formation of nonlinear vortex like structures are examined. In the second case at slow time when dust participates in dynamics, coupling between ultra low frequency dust acoustic (UDA) and dust drift (ULD) waves is studied and modons like solutions are obtained in the nonlinear analysis. Existence of the vortex solution with azimuthal harmonics higher than the dipole vortex has been studied both analytically and numerically. Later we extend our analysis to consider applications of dust particle's presence in the comet Halley and tropical mesospheric dusty plasma where in earlier case we report that that unexpectedly comet Halley plasma admits a new mode where presence of dust in background does not have any affect, this mode is similar to the convective cell mode where electrostatic drift (ExB) cancels out, density gradients also vanish due to the presence of negative ions and only polarization drift contributes to the mode. For the linear analysis of this mode, there is no coupling observed but this the convective cell mode evolves whereas nonlinear dynamics of this mode are found to obey the vortex-like solution.

physics.plasm-ph↗
arXiv2022

Linear and nonlinear analysis of Ion-Temperature-Gradient (ITG) Driven mode in the asymmetric Pair-Ion Magnetoplasma

We have investigated linear and nonlinear dynamics of ion-temperature-gradient driven drift mode for Maxwellian and non Maxwellian pair-ion plasma embedded in an inhomogeneous magnetic field having gradients in ion's temperature and number density. Linear dispersion relations are derived and analyzed analytically as well as numerically for different cases. It has been found that growth rate of instability increases with increasing eta. By using the transport equations of Braginskii, model, a set of nonlinear equations are derived. In the nonlinear regime, soliton structures are found to exist. Our numerical analysis shows that amplitude of solitary waves increases by increasing ion to electron number density ratio. These solitary structures are also found to be sensitive to non thermal kappa and Cairns distributed electrons. Our present work may contribute a good illustration of the observation of nonlinear solitary waves driven by the ITG mode in magnetically confined pair-ion plasmas and space pair-ion plasmas as the formation of localized structures along drift modes is one of the striking reasons for L-H transition in the region of improved confinements in magnetically confined devices like tokamaks.

physics.plasm-ph↗
arXiv2020

Formation of global vortices in a dusty plasma cylinder

The ultra low frequency modes associated to the activated dust specie in a bounded cylinder orientation plasma has been investigated here. Here flow of all plasma particles is assumed along the axial coordinates where gradients in velocity are set to be along radial direction. This analysis has been carried out for both Maxwellian and non-Maxwellian (Kappa and Carins) dusty plasma where coupling of electrostatic dust acoustic and drift modes drives global vortex formations. The coefficients of the vortex solution has been affected strongly by the presence of superthermal particles. Significance of this analysis can be viewed from the lab experiments and for the PK-4 mission perspectives. Additionally this may be fruitful for the astrophysical settings that can be mimicked as a plasma in a cylindrical column on macroscopic scale.

physics.plasm-ph↗
arXiv2020

Shock waves in a rotating non-Maxwellian magnetized dusty plasma

A theoretical model is presented to study characteristics of dust acoustic shock in a viscous, magnetized and rotating dusty plasma at both fast and slow time scales. By employing reductive perturbation technique the nonlinear Zakharov--Kuznetsov (ZK) equation has been derived for both cases when dust is inactive and dynamic (fast and slow time scales). Both electrons and ions are considered to follow kappa/Cairns distribution. It is observed that the viscosity in both cases when dust is in background and active plays as a key role in dissipation for the propagation of acoustic shock. Magnetic field and rotation are responsible for the dispersive term. Superthermality has been found to affect significantly on the formation of shock wave along with viscous nature of plasma. The present investigation may be beneficial to understanding the rotating plasma in particular experiments being carried out.

physics.plasm-ph↗
arXiv2020

Shocklets in the Comet Halley Plasma

Dust acoustic (DA) waves evolving into shocklets are investigated in the Comet Halley plasma system relaxing to Maxwellian, Kappa and Cairns distributions. Here dynamics of dust is described by the fully nonlinear continuity and momentum equations. A set of two characteristic wave nonlinear equations is obtained and numerically solved to examine the DA solitary pulse which develops into oscillatory shocklets with the course of time such as at time τ=0, symmetric solitary pulses are formed, which develop into oscillatory shocklets. It has been observed that variation in superthermality strongly affects the profiles of nonlinear DA structures in terms of negative potential, dust velocity and density.

physics.plasm-ph↗
arXiv2018

Gravitational Instability Analysis in Multi-Ion Dense Quantum Magnetoplasma

Electrostatic Gravitational or Rayleigh-Taylor (RT) instability in an inhomogeneous magnetized multi-ions plasma with some fraction of quantum mechanical electrons. The effect of Bohm potential, temperature degeneracy and magnetic field are carried out. A generalized dispersion relation is deduced under the drift approximation. The presence of negative ions with their different streaming velocities make the dispersion relation a cubic equation. Different roots of both real and imaginary parts of the RT mode are studied by using the Cardano method of solving the cubic equation. The growth rates of RT instability are examined analytically and numerically. It is shown that the basic features of these waves are significantly modified by the positive and negative ions drift speed as well as by the magnetic field and density. Relevance of the work regarding to dense astrophysical plasmas is pointed out.

physics.plasm-ph↗
arXiv2018

Weibel instability in relativistic asymmetric electron positron plasma

We consider a situation in when the interaction of relativistically intense EM waves with an isotropic electron positron plasma takes place, i.e.,we consider short pulse lasers with intensity up to 1021 W/cm2, in which the photon density is of the order of 1030cm3 and the strength of electric field E = 10^9 statvolt/cm. Such a situation is possible in astrophysical and laboratory plasma which are subject to intense laser radiation, thus leading to non thermal equilibrium field radiations. Such interaction of the super-strong laser radiation with an isotropic pair plasma leads to the generation of low frequency electromagnetic EM waves and in particular a quasistationary magnetic field. When the relativistic circularly polarized transverse EM wave propagates along z- axis, it creates a ponderomotive force, which affects the motion of particles along the direction of its propagation. On the other hand, motion of the particles across the direction of propagation is defined by the ponderomotive potential. Moreover dispersion relation for the transverse EM wave using a special distribution function, which has an anisotropic form, is derived and is subsequently investigated for a number of special cases. In general, it is shown that the growth rate of the EM wave strongly depends upon its intensity.

physics.plasm-ph↗
arXiv2018

Bernstein Waves in Symmetric and Asymmetric Pair Ions Plasma

Positive and negative ions forming so-called pair plasma differing in sign of their charge and asymmetric in mass and temperature support a new electrostatic mode. Bernstein mode for a pair ions and pair ions with contribution of electrons in pair plasma both cases are investigated. By solving the linearized Vlasov equation along Maxwell equations, a generalized expression for the Bernstein waves is derived by employing the Maxwell distribution function. In paper we discuss the different types of ions Bernstein waves and comparison of the symmetry and asymmetry on these ions Bernstein waves. We also apply the fluid limit on these Bernstein waves and we different fluid results from kinetic theory.

physics.plasm-ph↗
arXiv2017

Solar wind driven electrostatic instabilities with generalized r,q distribution function

Using Boltzmann Vlasov kinetic model with the flat-top non-Maxwellian distributed electrons and ions, a velocity power law energetic tail, known as the generalized r, q distribution, a current-less electrostatic instability namely ion acoustic which is driven by a stream of solar wind plasma is studied here. The instability threshold is affected and depends upon the spectral indices r and q. It is found that the growth rate increases with the decrease of spectral index. Moreover, such kinetic instability has also been discussed for a three species electron-ion-dust plasma using the generalized r, q distribution function. Such case is of interest when the solar wind is streaming through the cometary plasma in the presence of interstellar dust and excites electrostatic instabilities. In the limits of phase velocity of the waves larger and smaller than the thermal velocity of dust particles, the dispersion properties and growth rate of dust-acoustic mode are calculated analytically and has been plotted for different values of the spectral indices.

physics.plasm-ph↗
arXiv2017

Modulation instability of lower hybrid waves leading to cusp solitons in electron-positron-ion Thomas Fermi plasma

Following the idea of three wave resonant interactions of lower hybrid waves it is shown that quantum -modified lower hybrid (QLH) wave in electron positron ion plasma with spatial dispersion can decay into another QLH wave ( where electron and positrons are activated whereas ions remain in the background) and another ultra low frequency QULH (where ions are mobile). Quantum effects like Bohm potential, exchange correlation and Fermi pressure on the lower hybrid wave significantly reshaped the dispersion properties of lower hybrid waves. Later a set of nonlinear Zakharov equations have been derived to consider the formation of QLH wave solitons with the nonlinear contribution coming from the QLH waves. Further, modulational instability of the lower hybrid wave solitons is investigated and consequently it's growth rates are examined for different limiting cases. Since the growth rate associated with the three-wave resonant interaction are generally smaller than the growth associated with the modulational instability, therefore only latter have been investigated. Soliton solutions from the set of coupled Zakharov and NLS equations in the quasi-stationary regime have been studied. Ordinary solitons are attribute of nonlinearity whereas a cusp soliton solution featured by nonlocal nonlinearity have also studied. Such an approach to lower hybrid waves and cusp solitons study in Fermi gas comprising electron positron and ions is new and important. The general results obtained in this quantum plasma theory will have widespread applicability, particularly for processes in high energy plasma-laser interactions set for laboratory astrophysics and solid state plasmas.

physics.plasm-ph↗
arXiv2017

Effects of quantum statistical pressure and exchange correlation on the low frequency electromagnetic waves in degenerate Fermi-Dirac pair-ion plasma

The low frequency, long wavelength electromagnetic waves, viz, shear Alfven wave in quantum electron-positron-ion magneto plasmas, have been examined using quantum magneto hydrodynamic model. In this model, we have considered electrons and positrons are to be magnetized as well as degenerate whereas ions are magnetized but classical. We have also included the effects of exchange correlation terms which appear entirely the dynamic equations of electrons and positrons. The whole treatment is done using multi-fluid model. Our object is to study the shear Alfvén waves propagating in above said system of plasma. For that we have derived the modified dispersion relation of the shear Alfvén waves. Results are relevant to the terrestrial laboratory astrophysics.

physics.plasm-ph↗
arXiv2016

New longitudinal mode and compression of pair ions in plasma

Positive and negative ions forming so-called pair plasma differing in sign of their charge but asymmetric in mass and temperature support a new acoustic mode where damping of heavier ion dominates. The condition for the excitation of ion sound wave through electron beam induced Cherenkov instability is also investigated. This beam can generate a perturbation in the pair ion plasmas in the presence of electrons when there is number density, temperature and mass difference in the two species of ions. Basic emphasis is on the focusing of ion sound waves and we show how, in the area of localization of wave energy, the density of pair particles increases while electrons are pushed away from that region. Further, this localization of wave is dependent on the shape of the pulse. Considering the example of pancake and bullet shaped pulses, we find that only the former leads to compression of pair ions in the supersonic regime of the focusing region. Here possible existence of regions where pure pair particles can exist may also be speculated which is not only useful from academic point of view but also to mimic the situation of plasma (electro positron asymmetric and symmetric) observed in astrophysical environment.

physics.plasm-ph↗
arXiv2013

Lattice modes in a dusty plasma crystal

A model is presented to explain the normal mode features of dust particles in a planar zigzag crystal chain for the first and second neighbors. The degrees of freedom of particles are the longitudinal and transverse displacements in plane coupled by the first and second neighbor harmonic forces in two-dimensions (2D). The constant electric force duded to the electrodes to keep the zigzag structure is calculated. The coupling between transverse and longitudinal dust-lattice (DL) modes is derived. The latter is considered due to the energy of the electrostatic (Yukawa) potential. Moreover coupled (acoustic and optical) and decoupled (longitudinal and transverse) branches of dust lattice modes for different lattice parameters and structures are studied. Propagation of the longitudinal and acoustic modes is found to be strictly dependent on the value of the distance between the two chains; below that value mode may not propagate Finally it is shown that the frequencies of the acoustic (optical) branches increase (decrease) with increasing the distance between the two chains.

physics.plasm-ph↗
arXiv2011

A modified orbital motion limited (OML) theory

The validity of the orbital motion limited (OML) theory is reviewed with reference to the floating potential acquired by a spherical object immersed in a plasma. A new and perhaps more realistic approach for obtaining the floating potential is introduced by including the current outward from the spherical object and the current coming from infinity. This novel approach is also valid for cases where the standard OML theory ceases to apply.

physics.plasm-ph↗
arXiv2009

Acceleration of dust particles by vortex ring

It is shown that nonlinear interaction between large amplitude circularly polarized EM wave and dusty plasma leads to a nonstationary ponderomotive force which in turn produces a vortex ring, and magnetic field. Then the ensuing vortex ring in the direction of propagation of the pump wave can accelerate the micron-size dust particles which are initially at rest and eventually form a non relativistic dust jet. This effect is purely nonstationary and unlike linear vortices, dust particles do not rotate here. Specifically, it is pointed out that the vortex ring or closed filament can become potential candidate for the acceleration of dust in tokamak plasmas.

physics.plasm-ph↗