arXiv subjects

B. P. Pandey

Publications and source records attributed to B. P. Pandey.

At least 19 recordsLinked to original sources

arXiv2024

Viscous Heating and Instabilities in the Partially Ionized Solar Atmosphere

In weak magnetic fields ($\lesssim 50 \,\mbox{G}$), parallel and perpendicular viscosities, mainly from neutrals, may exceed magnetic diffusivities (Ohm, Hall, ambipolar) in the middle and upper chromosphere. Ion-driven gyroviscosity may dominate in the upper chromosphere and transition region. In strong fields ($\gtrsim 100\, \mbox{G}$), viscosities primarily exceed diffusivities in the upper chromosphere and transition region. Parallel and perpendicular viscosities, being similar in magnitude, dampen waves and potentially compete with ambipolar diffusion in plasma heating, potentially inhibiting Hall and ambipolar instabilities when equal. The perpendicular viscosity tensor has two components, $ν_1$ and $ν_2$, which differ slightly and show weak dependence on ion magnetization. Their differences, combined with shear, may destabilize waves, though magnetic diffusion introduces a cutoff for this instability. In configurations with a magnetic field $\bf{B}$ having vertical ($b_z=B_z/|\bf{B}|$) and azimuthal ($b_y=B_y/|\bf{B}|$) components, and a wavevector $\bf{k}$ with radial ($\kx=k_x/|\bf{k}|$) and vertical ($\kz=k_z/|\bf{k}|$) components, parallel viscosity and Hall diffusion can generate the viscous-Hall instability. Gyroviscosity further destabilizes waves in the upper regions. These findings indicate that the solar atmosphere may experience various viscous instabilities, revealing complex interactions between viscosity, magnetic fields, and plasma dynamics across different atmospheric regions.

astro-ph.SR↗

arXiv2022

The non-ideal finite Larmor radius effect in the solar atmosphere

The dynamics of the partially ionized solar atmosphere is controlled by the frequent collision and charge exchange between the predominant neutral Hydrogen atoms and charged ions. At signal frequencies below or of the order of either of the collision or charge exchange frequencies the magnetic stress is {\it felt} by both the charged and neutral particles simultaneously. The resulting neutral-mass loading of the ions leads to the rescaling of the effective ion-cyclotron frequency-it becomes the Hall frequency, and the resultant effective Larmor radius becomes of the order of few kms. Thus the finite Larmor radius (FLR) effect which manifests as the ion and neutral pressure stress tensors operates over macroscopic scales. Whereas parallel and perpendicular (with respect to the magnetic field) viscous momentum transport competes with the Ohm and Hall diffusion of the magnetic field in the photosphere-chromosphre, the gyroviscous effect becomes important only in the transition region between the chromosphere and corona, where it competes with the ambipolar diffusion. The wave propagation in the gyroviscous effect dominated medium depends on the plasma $β$ (a ratio of the thermal and magnetic energies). The abundance of free energy makes gyro waves unstable with the onset condition exactly opposite of the Hall instability. However, the maximum growth rate is identical to the Hall instability. For a flow gradient $\sim 0.1 \,\mbox{s}^{-1}$ the instability growth time is one minute. Thus, the transition region may become subject to this fast growing, gyroviscous instability.

astro-ph.SR↗

arXiv2019

Charged grains and Kelvin Helmholtz instability in molecular clouds

The presence of dust grains profoundly affects the diffusion of the magnetic field in molecular clouds. When the electrons and ions are well coupled to the magnetic field and charged grains are only indirectly coupled, emergent Hall diffusion may dominate over all the other non ideal magnetohydrodynamic (MHD) effects in a partially ionized dusty cloud. The low frequency, long wavelength (0.01 t0 1 pc) dispersive MHD waves will propagate in such a medium with the polarization of the waves determined by the dust charge density or the dust size distribution. In the presence of shear flows, these waves may become Kelvin Helmholtz (KH) unstable with the dust charge density or the grain size distribution operating as a switch to the instability. When Hall diffusion time is long (compared to the time over which waves are sheared), the growth rate of the instability in the presence of sub Alfvenic flow increases with the charge number Z on the grain, while it is quenched in the presence of Alfvenic or super Alfvenic flows. However, when Hall diffusion is fast, the growth rate of the instability depends on the dust charge only indirectly.

astro-ph.GA↗

arXiv2018

Dust modification of the plasma conductivity in the mesosphere

Relative transverse drift (with respect to the ambient magnetic field) between the weakly magnetized electrons and the unmagnetized ions at the lower altitude (80 km) and between the weakly magnetized ions and unmagnetized dust at the higher altitude (90 km) gives rise to the finite Hall conductivity in the Earth's mesosphere. If, on the other hand, the number of free electrons is sparse in the mesosphere and most of the negative charge resides on the weakly magnetized, fine, nanometre sized dust powder and positive charge on the more massive, micron sized, unmagnetized dust, the sign of the Hall conductivity due to their relative transverse drift will be opposite to the previous case. Thus the sign of the Hall effect not only depends on the direction of the local magnetic field but also on the nature of the charge carrier in the partially ionized dusty medium. As the Hall and the Ohm diffusion are comparable below 80 km, the low frequency long wavelength waves will be damped at this altitude with the damping rate typically of the order of few minutes. Therefore, the ultra--low frequency magnetohydrodynamic waves can not originate below 80 km in the mesosphere. However, above 80 km since Hall effect dominates Ohm diffusion the mesosphere can host the ultra--low frequency waves which can propagate across the ionosphere with little or, no damping.

physics.plasm-ph↗

arXiv2015

Surface Wave Propagation in non--ideal plasmas

The properties of surface waves in a partially ionized, compressible magnetized plasma slab are investigated in this work. The waves are affected by the nonideal magnetohydrodynamic effects which causes finite drift of the magnetic field in the medium. When the magnetic field drift is ignored, the characteristics of the wave propagation in a partially ionized plasma fluid is similar to the fully ionized ideal MHD except now the propagation properties depend on the fractional ionization as well as on the compressibility of the medium. The phase velocity of the sausage and kink waves increases marginally (by a few percent) due to the compressibility of the medium in both ideal as well as Hall diffusion dominated regimes. However, unlike ideal regime, only waves below certain cut off frequency can propagate in the medium in Hall dominated regime. This cut off for a thin slab has a weak dependence on the plasma beta whereas for thick slab no such dependence exists. More importantly, since the cut off is introduced by the Hall diffusion, the fractional ionization of the medium is more important than the plasma compressibility in determining such a cut off. We discuss the relevance of these results in the context of solar photosphere-chromosphere.

astro-ph.SR↗

arXiv2013

Surface Waves in the paritally ionized solar plasma slab

The properties of surface waves in the partially ionized, incompressible magnetized plasma slab are investigated in the present work. The waves are affected by the non ideal MHD effects which causes the finite drift of the magnetic field in the medium. When the finite drift of the magnetic field is ignored, the characteristics of the wave propagation in the partially ionized plasma fluid is similar to the ideal MHD except now the propagation properties depend on the fractional ionization of the medium. In the presence of Hall diffusion, the propagation of the sausage and kink surface waves depends on the level of fractional ionization of the medium. When both the Hall and Pedersen diffusion are present in the medium, the waves undergoes damping. For typical solar parameters, waves may damp over few minutes.

astro-ph.SR↗

arXiv2013

Some unexplored features of the nonlinear compressive magnetoacoustic Alfvenic waves

The theory of nonlinear magnetoacoustic wave in the past has strictly been focused on purely compressive features of the mode. We show that a complete set of nonlinear equations necessarily includes both compressional and shear components of the magnetic field. These two turn out to be described by exactly the same nonlinear equations, which make the use of such a complete full set of equations far less complicated than expected. Present results should considerably enrich the theory of these waves by opening up new frontiers of investigation and providing some completely new types of nonlinear solutions.

physics.plasm-ph↗

arXiv2013

Magnetic diffusion driven shear instability of solar flux tubes

Macroscopic gas motions are widespread throughout the solar atmosphere and shearing motions couple to the non--ideal effects, destabilising low frequency fluctuations in the medium. The origin of this non-ideal magnetohydrodynamic instability lies in the collisional coupling of the neutral particles to the magnetized plasma in the presence of a sheared background flow. Unsurprisingly, the maximum growth rate and most unstable wavenumber depend on the flow gradient and ambient diffusivities. The orientation of the magnetic field, velocity shears and perturbation wave vector play a crucial role in assisting the instability. When the magnetic field and wave vector are both vertical, ambipolar and Ohm diffusion can be combined as Pedersen diffusion and cause only damping; in this case only Hall drift in tandem with shear flow drives the instability. However, for non-vertical fields and oblique wave vectors, both ambipolar diffusion and Hall drift are destabilizing. We investigate the stability of magnetic elements in the network and internetwork regions. The shear scale is not yet observationally determined, but assuming a typical shear flow gradient $\sim 0.1 \,\mbox{s}^{-1}$ we show that the magnetic diffusion shear instability grows on a time scale of one minute. Thus, it is plausible that network--internetwork magnetic elements are subject to this fast growing, diffusive shear instability, which could play an important role in driving low frequency turbulence in the plasma in the solar photosphere and chromosphere.

astro-ph.SR↗

arXiv2012

Response to Comment of Shukla and Akbari-Moghanjoughi

Shukla and Akbari-Moghanjoughi have {\it corrected} their Comment (see their version 1 on `arXiv:1207.7029v1) to EPL on our work [1] after receiving our Response from the Editors of EPL. We have a pleasant duty at hand to present our second Response to their second version of the Comment. It is hoped that this response adds strength to our plea {\it for a common sense} [1] on quantum description of plasmas.

physics.plasm-ph↗

arXiv2012

On quantum plasma: a plea for a common sense

The quantum plasma theory has flourished in the past few years without much regard to the physical validity of the formulation or its connection to any real physical system. It is argued here that there is a very limited physical ground for the application of such a theory.

physics.plasm-ph↗

arXiv2012

Hall instability of solar flux tubes

The magnetic network which consists of vertical flux tubes located in intergranular lanes is dominated by Hall drift in the photosphere-lower chromosphere region ($\lesssim 1 Mm$). In the internetwork regions, Hall drift dominates above $0.25 Mm$ in the photosphere and below $2.5 Mm$ in the chromosphere. Although Hall drift does not cause any dissipation in the ambient plasma, it can destabilise the flux tubes and magnetic elements in the presence of azimuthal shear flow. The physical mechanism of this instability is quite simple: the shear flow twists the radial magnetic field and generates azimuthal field; torsional oscillations of the azimuthal field in turn generates the radial field completing feedback loop. The maximum growth rate of Hall instability is proportional to the absolute value of the shear gradient and is dependent on the ambient diffusivity. The diffusivity also determines the most unstable wavelength which is smaller for weaker fields. We apply the result of local stability analysis to the network and internetwork magnetic elements and show that the maximum growth rate for kilogauss field occurs around $0.5 Mm$ and decreases with increasing altitude. However, for a $120 G$ field, the maximum growth rate remains almost constant in the entire photosphere-lower chromosphere except in a small region of lower photosphere. For shear flow gradient $\sim 0.01 s^{-1}$, the Hall growth time is 10 minute near the footpoint. Therefore, network fields are likely to be unstable in the photosphere, whereas internetwork fields could be unstable in the entire photosphere-chromosphere. Thus the Hall instability can play an important role in generating low frequency turbulence which can heat the chromosphere.

astro-ph.SR↗

arXiv2012

Magnetorotational instability in magnetic diffusion dominated accretion discs

We investigate the stability of partially ionised, differentially rotating, diffusive disc threaded by both azimuthal and vertical magnetic field. The general stability criterion of such a disc in the presence of axisymmetric fluctuations can be stated purely in terms of ambipolar and Hall diffusivities. It is shown that the disc is magnetorotationally unstable if the sum of scaled ambipolar and Hall diffusivities are larger than some numerical constant determined by the rotation profile of the disc. This criterion suggests that the diffusive discs are always unstable to almost radial fluctuations. The field geometry and obliqueness of wavevector (encapsulated together in the topological factor $g$), plays dual role of not only assisting MRI in ambipolar--Ohm dominated disc but also making otherwise stable region in Hall--ambipolar diffusion plane unstable.

astro-ph.SR↗

arXiv2008

Ion thermal effects in oscillating multi-ion plasma sheath theory

The effects of ion temperature are discussed in a two-ion electron plasma and for a model applicable to the oscillating sheath theory that has recently been much in the focus of researchers. The differences between the fluid and kinetic models have been pointed out, as well as the differences between the approximative kinetic description (which involves the expansion of the plasma dispersion function), and the exact kinetic description. It is shown that the approximative kinetic description, first, can not describe the additional acoustic mode which naturally exists in the plasma with an additional ion population with a finite temperature, and, second, it yields an inaccurate Landau damping of the bulk ion acoustic mode. The reasons for these two failures are described. In addition to this, a fluid model is presented that is capable of capturing both of these features that are missing in the approximative kinetic description, i.e., two (fast and slow) ion acoustic modes, and the corresponding Landau damping of both modes.

physics.plasm-ph↗

arXiv2008

Energy flux of Alfven waves in weakly ionized plasma

The overshooting convective motions in the solar photosphere are frequently proposed as the source for the excitation of Alfvén waves. However, the photosphere is a) very weakly ionized, and, b) the dynamics of the plasma particles in this region is heavily influenced by the plasma-neutral collisions. The purpose of this work is to check the consequences of these two facts on the above scenario and their effects on the electromagnetic waves. It is shown that the ions and electrons in the photosphere are both un-magnetized; their collision frequency with neutrals is much larger than the gyro-frequency. This implies that eventual Alfvén-type electromagnetic perturbations must involve the neutrals as well. This has the following serious consequences: i) in the presence of perturbations, the whole fluid (plasma + neutrals) moves; ii) the Alfvén velocity includes the total (plasma + neutrals) density and is thus considerably smaller compared to the collision-less case; iii) the perturbed velocity of a unit volume, which now includes both plasma and neutrals, becomes much smaller compared to the ideal (collision-less) case; and iv) the corresponding wave energy flux for the given parameters becomes much smaller compared to the ideal case.

astro-ph↗

arXiv2008

On electrostatic modes in multi-ion and pair-ion collisional plasmas

The physics of plasmas containing positive and negative ions is discussed with special attention to the recently produced pair-ion plasma containing ions of equal mass and opposite charge. The effects of the density gradient in the direction perpendicular to the ambient magnetic field vector, observed in the experiment, are discussed. In addition, the possible presence of electrons is discussed in the context of plasma modes propagating at an angle with respect to the magnetic field vector. The electrons may either be added to the plasma or enter the plasma attached to negative ions and then become released from the ions as a result of collisions. It is shown that the electron plasma mode in a cold plasma may become a backward mode in the presence of a density gradient, and this behavior may be controlled either by the electron number density or the mode number in the perpendicular direction. In plasmas with hot electrons an instability may develop, driven by the combination of electron collisions and the density gradient, and in the regime when the ions' response is similar to a sound mode, i.e., for un-magnetized ions, implying mode frequencies much above the ion gyro-frequency or mode wave-lengths shorter than the ion gyro-radius. In the case of a pure pair-ion plasma, for lower frequencies and for parameters close to those used in the recent experiments, the perturbed ions may feel the effects of the magnetic field. In this case the plasma mode also becomes backward, resembling features of an experimentally observed but yet unexplained backward mode.

physics.plasm-ph↗

arXiv2008

Waves in the solar photosphere

The solar photosphere is a partially ionized medium with collisions between electrons, various metallic ions and neutral hydrogen playing an important role in the momentum and energy transport in the medium. Furthermore, the number of neutral hydrogen atom could be as large as $10^{4}$ times the number of plasma particles in the lower photosphere. The non-ideal MHD effects, namely Ohm, Ambipolar and Hall diffusion can play an important role in the photosphere. We demonstrate that Hall is an important non-ideal MHD effect in the solar photosphere and show that Hall effect can significantly affect the excitation and propagation of the waves in the medium. We also demonstrate that the non-ideal Hall dominated inhomogeneous medium can become parametrically unstable, and it could have important ramification for the photosphere and chromosphere of the sun. The analysis hints at the possibility of solar photosphere becoming parametrically unstable against the linear fluctuations.

astro-ph↗

arXiv2008

Hall magnetohydrodynamics of partially ionized plasmas

The Hall effect arises in a plasma when electrons are able to drift with the magnetic field but ions cannot. In a fully-ionized plasma this occurs for frequencies between the ion and electron cyclotron frequencies because of the larger ion inertia. Typically this frequency range lies well above the frequencies of interest (such as the dynamical frequency of the system under consideration) and can be ignored. In a weakly-ionized medium, however, the Hall effect arises through a different mechanism -- neutral collisions preferentially decouple ions from the magnetic field. This typically occurs at much lower frequencies and the Hall effect may play an important role in the dynamics of weakly-ionised systems such as the Earth's ionosphere and protoplanetary discs. To clarify the relationship between these mechanisms we develop an approximate single-fluid description of a partially ionized plasma that becomes exact in the fully-ionized and weakly-ionized limits. Our treatment includes the effects of ohmic, ambipolar, and Hall diffusion. We show that the Hall effect is relevant to the dynamics of a partially ionized medium when the dynamical frequency exceeds the ratio of ion to bulk mass density times the ion-cyclotron frequency, i.e. the Hall frequency. The corresponding length scale is inversely proportional to the ion to bulk mass density ratio as well as to the ion-Hall beta parameter.

astro-ph↗

arXiv2007

Parametric instability in dark molecular clouds

The present work investigates the parametric instability of parallel propagating circularly polarized Alfven(pump) waves in a weakly ionized molecular cloud. It is shown that the relative drift between the plasma particles gives rise to the Hall effect resulting in the modified pump wave characteristics. Although the linearized fluid equations with periodic coefficients are difficult to solve analytically, it is shown that a linear transformation can remove the periodic dependence. The resulting linearized equations with constant coefficients are used to derive an algebraic dispersion relation. The growth rate of the parametric instability is a sensitive function of the amplitude of the pump wave as well as to the ratio of the pump and the modified dust-cyclotron frequencies. The instability is insensitive to the plasma-beta The results are applied to the molecular clouds.

astro-ph↗