arXiv subjects

Hamid Saleem

Publications and source records attributed to Hamid Saleem.

14 recordsLinked to original sources

arXiv2026

Nonlinear Equation for Dust Drift Waves

A nonlinear equation for dust drift waves is derived assuming two-dimensional propagation and ignoring the role of dust acoustic waves. Both the nonlinear dust density term and the dust vorticity term are taken into account. If vorticity term is ignored, the equation gives two dimensional solitary waves and if density nonlinearity effect is discarded, then it reduces to Hasegawa-Mima equation for dust drift waves which admits dipole vortex solutions.

physics.plasm-ph↗

arXiv2026

Pressure gradient-driven plasma flows and magnetogenesis

We present a self-consistent two-fluid theory demonstrating that pressure gradients simultaneously generate plasma flows and magnetic fields. We show that compatibility between ion momentum balance and mass conservation imposes a previously unrecognized constraint on plasma evolution: the total pressure must satisfy the Laplace equation, $\nabla^2 p = 0$. This condition yields a class of exact analytical solutions in which pressure-driven flows and Biermann-type magnetic fields emerge together. Application of the model to a galactic gas clump reveals that, under thermal pressure, electrons and ions move almost together, giving rise to weak currents and consequently very small seed magnetic fields. Ion dynamics are also important for determining the seed magnetic-field generation time $\tau_B$ and for estimating the ion flow velocity. The model is further applied to laser-produced plasma to describe its short-time evolution. The present theory provides a unified, self-consistent description of pressure-driven flow generation and magnetogenesis in both astrophysical and laboratory plasmas.

physics.plasm-ph↗

arXiv2025

Solar Wind Penetration into Dusty Magnetospheres creates Electrostatic Waves and Structures

The low frequency electrostatic perturbations have been investigated in a bi ion plasma in the background of static dust. It is shown that the field aligned shear flow of both the ions produce low frequency electrostatic instabilities and create nonlinear structures, the double layers and the solitons. The general theoretical model is applied to the magnetospheres of Jupiter (with positively charged dust) and Saturn (with negatively charged dust) which have oxygen ions in addition to protons. This model predicts the existence of extremely low frequency electrostatic waves with real frequencies of the order of a milli Hertz (mHz) to several mHz and this range of frequencies have been reported in literature for these plasma environments. The estimated width of the nonlinear structures vary from a few hundred meters to a few kilometers. These structures are similar to that observed in the oxygen and oxygen hydrogen plasmas in Earth's upper ionosphere which is free from dust.

physics.plasm-ph↗

arXiv2025

Exploring Nonlinear Drift Waves: Limiting Cases and Dynamics

A general equation for drift waves is derived incorporating both nonlinear electron density perturbation and ion vorticity effects. It is emphasized that the well-known Hasegawa-Mima (HM) equation for drift waves [A. Hasegawa and K. Mima, Phys. Fluids 21, 87 (1978)] includes only the ion vorticity term and neglects nonlinear electron density contribution that naturally arises from the electrons Boltzmann response. If ion vorticity term is ignored, then the general nonlinear equation reduces to an equation which can give two-dimensional soliton solution under an appropriate coordinate transformation. Furthermore, under the assumption that the normalized electrostatic potential depends only on one spatial coordinate along the predominant propagation direction, i.e. $\Phi = \Phi(y)$, the equation reduces to one-dimensional KdV equation [H. Saleem, Phys. Plasmas 31, 112102 (2024)]. Conversely, if the nonlinear electron density term is artificially suppressed and a two-dimensional potential $\Phi = \Phi(x, y)$ is considered, the equation reduces to Hasegawa-Mima equation supporting dipolar vortex solution. Because the HM equation ignores nonlinear electron density term, it cannot support one- or two-dimensional soliton solutions. Finally, the limiting forms of the general nonlinear equation are also briefly discussed using the reductive perturbation method (RPM).

physics.plasm-ph↗

arXiv2024

Generation and Life Cycle of Solar Spicules

Physical mechanism for the creation of solar spicules is proposed with three stages of their life cycle. It is assumed that at stage-I, the density hump is formed locally in the xy-plane in lower chromosphere in the presence of temperature gradients of electrons and ions along z-axis (the vertical direction). In this region, the density structure of quasi-neutral $(n_i\simeq n_e=n)$ plasma after taking birth is accelerated in the vertical direction due to the thermal force ${\bf F}_{th} \propto \nabla n(x,y,t) \times (\nabla T_e + \nabla T_i)$. The exact time-dependent analytical solution of two fluid plasma equations is presented assuming that density is maximum at the center and decays away from it gradually. The two dimensional (2D) density structure is created as a step function $H(t)$ in time at bottom of the chromosphere and consequently the vertical plasma velocity turns out to be the ramp function of time $R(t)=t H(t)$ whereas the source term $S(x,y,t)$ for the density follows the delta function $δ(t)$ form. The upward acceleration ${\bf a}=a(x,y)\hat{z}$ produced in this density structure is greater than the downward constant solar acceleration $-{\bf g}_\odot$ in the chromosphere. In the transition region (TR), the temperature gradients are steeper; therefore, the upward acceleration increases in magnitude $g_\odot << a$ and the density hump spends lesser time there. This is stage-II of its life cycle. In stage-III, the density structure enters into corona where the gradients of temperatures vanish and the structure decelerates to zero velocity under the action of the solar gravitational force.

astro-ph.SR↗

arXiv2022

3-D exact analytical solutions of two fluid plasma, MHD and neutral fluid equations for the creation of ordered structures as well as jet-like flows

The 3-D exact analytical solutions of ideal two fluid plasma, single fluid plasma (MHD) and neutral fluid equations have been found using physically justifiable assumptions. Surprisingly these solutions satisfy all non-linearities in the systems. It is pointed out that these solutions explain the fundamental mechanism behind the creation of vast variety of ordered structures in plasmas and fluids. In the limiting case of two dimensional (2-D) dependence of fields, the theoretical model for plasma is applied to explain the formation of spicules in solar chromosphere. It is pointed out that the main contribution of electron (ion) baro clinic vectors is to produce vorticity in the plasma and that magnetic field generation is coupled with the flow of both electrons and ions.

physics.plasm-ph↗

arXiv2019

Exact solution of two fluid plasma equations for the creation of jet-like flows and seed magnetic fields in cylindrical geometry

An exact solution of two fluid ideal classical plasma equations is presented which shows that the jet-like outflow and magnetic field are generated simultaneously by the density and temperature gradients of both electrons and ions. Particular profiles of density function $ψ=\ln \bar{n}$ (where $\bar{n}$ is normalized by some constant density $N_0$) and temperatures $T_j$ (for $j=e,i)$ are chosen which reduce the set of nonlinear partial differential equations to two simple linear equations generating longitudinally uniform axial outflow and magnetic field in cylindrical geometry in several astrophysical objects. This mechanism also seems to be operative for producing short scale plasma jets in the solar atmosphere in the form of spicules and flares. The presented solution requires particular profiles of density and temperatures, but it is a natural solution of the two fluid ideal classical plasma equations. Similar jet-like outflows can be generated by the density and temperature gradients in neutral fluids as well.

physics.plasm-ph↗

arXiv2015

Extraordinary Drift Wave in Space and Cylindrically Bounded Heavy Ion Plasmas

It is pointed out that electrostatic and electromagnetic waves propagating in perpendicular direction to the density gradient $\nabla n_0=\hat{x} |\frac{dn_0}{dx}|$ and external magnetic field $\vec{B}_0=B_0 \hat{z}$ in hybrid frequency range $\Omega_i \ll \omega \ll \Omega_e$ (where $\Omega_j=\frac{eB_0}{m_j c}$ and $j=e,i$) have several applications in space and laboratory plasmas. The electron plasma waves are important for the future experiments on heavier ion plasmas as well as the results are applicable to the experiments performed to create pure pair ion fullerene plasmas. The electrostatic modes are shown to exist in upper F-region of terrestrial ionosphere and electromagnetic waves are applied to cylindrically bounded heavy ion plasmas.

physics.plasm-ph↗

arXiv2011

Beltrami-like fields created by baroclinic effect in two-fluid plasmas

A theory of two-dimensional plasma evolution with Beltrami-like flow and field due to baroclinic effect has been presented. Particular solution of the nonlinear two-fluid equations is obtained. This simple model can explain the generation of magnetic field without assuming the presence of a seed in the system. Coupled field and flow naturally grow together. The theory has been applied to estimate B-field in laser-induced plasmas and the result is in good agreement with experimental values.

physics.plasm-ph↗

arXiv2011

Criteria to define a pair-ion plasma and the role of electrons in nonlinear dynamics

A criterion to define a pure pair-ion (PI) plasma is presented. It is suggested that the lighter elements (like H and He) are more suitable to produce PI plasmas. The observation of ion acoustic wave (IAW) in recent experiments with fullerene plasmas clearly indicates the presence of electrons in the system. A set of two coupled non-linear differential equations has been obtained for PI plasma dynamics. In moving frame, it can be reduced to a form similar to Hasegawa-Mima equation but it does not contain drift wave.

physics.plasm-ph↗

arXiv2011

Creation of Magnetic Fields by Electrostatic and Thermal Fluctuations

It is pointed out that the unmagnetized inhomogeneous plasmas can support a low frequency electromagnetic ion wave as a normal mode like Alfven wave of magnetized plasmas. But this is a coupled mode produced by the mixing of longitudinal and transverse components of perturbed electric field due to density inhomogeneity. The ion acoustic wave does not remain electrostatic in non-uniform plasmas. On the other hand, a low frequency electrostatic wave can also exist in the pure electron plasmas. But the magnetic field fluctuations in both electron as well as in electron-ion plasmas are coupled with the electrostatic perturbations in unmagnetized case. The main instability condition for these low frequency electrostatic and electromagnetic modes is the same ${2/3}κ_n < κ_T$ (where $κ_n$ and $κ_T$ are inverse of the scale lengths of density and electron temperature, respectively).

physics.plasm-ph↗

arXiv2011

Non-equilibrium two-fluid plasmas can generate magnetic fields and flows simultaneously

A new analytical solution of the set of highly nonlinear two-fluid equations is presented to explain the mechanism for the generation of "seed" magnetic field and plasma flow by assuming the density n to have a profile like an exponential in xy-plane and temperature profiles of electrons (ions) to be linear in yz-plane. {It is shown that the baroclinic vectors - $\nablaΨ\times\nabla T_{j}$ (where $Ψ= ln\bar{n}, \bar{n}$ is normalized density, and $T_{j}$ denote the temperatures of electrons and ions for j = e, i) can generate not only the magnetic field but the plasma flow as well.} It is also pointed out that the electron magnetohydrodynamics (EMHD) model has inconsistencies because it does not take into account the ion dynamics while the magnetic field is produced on slow time scale. The estimate of the magnitude of the magnetic field in a classical laser plasma using this model is in agreement with the experimental observations.

physics.plasm-ph↗

arXiv2009

Electrostatic and thermal fluctuations are the source of magnetic fields in unmagnetized inhomogeneous plasmas

It is pointed out that electron thermal fluctuations can couple with the ion acoustic mode in an inhomogeneous plasma to generate a low frequency ion time scale electromagnetic wave. This electromagnetic wave can become unstable if the temperature and density gradients are parallel to each other which can be the case in laser-plasmas similar to stellar cores. The comparisons of the present theoretical model with the previous investigations are also presented. The final result is applied to a classical laser induced plasma for illustration.

physics.plasm-ph↗

arXiv2005

Plasma Frequency Shift Due to a Slowly Rotating Compact Star

We investigate the effects of a slowly rotating compact gravitational source on electron oscillations in a homogeneous electrically neutral plasma in the absence of an external electric or magnetic field. Neglecting the random thermal motion of the electrons we assume the gravitoelectromagnetic approximation to the general theory of relativity for the gravitational field. It is shown that there is a shift in the plasma frequency and hence in the dielectric constant of the plasma due to the gravitomagnetic force. We also give estimates for the difference in the frequency of radially transmitted electromagnetic signals for typical compact star candidates.

astro-ph↗