W. Masood

Terrestrial lion roars and non-Maxwellian distribution

Lion roars are low-frequency (∼100 Hz) whistler waves frequently observed in the Earths magnetosheath. By analyzing both wave and electron data from the Cluster spacecraft, and comparing with linear Vlasov kinetic theory, Masood et al. (2006) investigated the underlying cause of the lion roar generation. However, the analysis based upon the bi-Maxwellian distribution function did not adequately explain the observations qualitatively as well as quantitatively. This outstanding problem is revisited in the present paper, and a resolution is put forth in which, the flat-top non-Maxwellian distribution function with a velocity power law energetic tail, known as the (r,q) distribution, or the generalized kappa distribution is employed. Upon carrying out the linear stability analysis of the (r,q) distribution against the whistler wave perturbation, and upon comparison with the Cluster data, good qualitative and quantitative agreements are found between theory and data.

Linear and nonlinear coupling of electromagnetic and electrostatic fluctuations with one dimensional trapping of electrons using product bi (r,q) distribution

In the present paper, we have investigated the ramifications of adiabatic trapping of electrons using a bi product (r,q) distribution function on obliquely propagating Alfven waves in a low β plasma. In this regard, we have analyzed the linear and nonlinear dispersion characteristics of finite amplitude coupled kinetic Alfven-acoustic solitary waves using the two-potential theory and employing Sagdeev potential approach. We have deliberated upon the results of the present inquest and highlighted its importance by citing works that have reported the simultaneous presence of electromagnetic pulses and flat-topped distribution of electrons.

Nonlinear kinetic Alfvén waves with non‐Maxwellian electron population in space plasmas

The present work discusses the effects of non-Maxwellian electron distributions on kinetic Alfven waves in low-beta plasmas. Making use of the two-potential theory and employing the Sagdeev potential approach, the existence of solitary kinetic Alfven waves having arbitrary amplitude is investigated. It is found that the use of non-Maxwellian population of electrons in the study of kinetic Alfven waves leads to solutions corresponding to solitary structures that do not exist for Maxwellian electrons. The present investigation solves the riddle of plasma density fluctuations associated with strong electromagnetic perturbations observed by the Freja satellite. The present findings can also be applied to regions of space where various satellite missions have observed the presence of suprathermal populations of plasma species and where the low β assumption is valid.

Electron acoustic nonlinear structures in planetary magnetospheres

In this paper, we have studied linear and nonlinear propagation of electron acoustic waves (EAWs) comprising cold and hot populations in which the ions form the neutralizing background. The hot electrons have been assumed to follow the generalized ( r , q ) distribution which has the advantage that it mimics most of the distribution functions observed in space plasmas. Interestingly, it has been found that unlike Maxwellian and kappa distributions, the electron acoustic waves admit not only rarefactive structures but also allow the formation of compressive solitary structures for generalized ( r , q ) distribution. It has been found that the flatness parameter r, tail parameter q, and the nonlinear propagation velocity u affect the propagation characteristics of nonlinear EAWs. Using the plasmas parameters, typically found in Saturns magnetosphere and the Earths auroral region, where two populations of electrons and electron acoustic solitary waves (EASWs) have been observed, we have given an estimate o...

Whistler waves with electron temperature anisotropy and non-Maxwellian distribution functions

The previous works on whistler waves with electron temperature anisotropy narrated the dependence on plasma parameters, however, they did not explore the reasons behind the observed differences. A comparative analysis of the whistler waves with different electron distributions has not been made to date. This paper attempts to address both these issues in detail by making a detailed comparison of the dispersion relations and growth rates of whistler waves with electron temperature anisotropy for Maxwellian, Cairns, kappa and generalized (r, q) distributions by varying the key plasma parameters for the problem under consideration. It has been found that the growth rate of whistler instability is maximum for flat-topped distribution whereas it is minimum for the Maxwellian distribution. This work not only summarizes and complements the previous work done on the whistler waves with electron temperature anisotropy but also provides a general framework to understand the linear propagation of whistler waves with electron temperature anisotropy that is applicable in all regions of space plasmas where the satellite missions have indicated their presence.The previous works on whistler waves with electron temperature anisotropy narrated the dependence on plasma parameters, however, they did not explore the reasons behind the observed differences. A comparative analysis of the whistler waves with different electron distributions has not been made to date. This paper attempts to address both these issues in detail by making a detailed comparison of the dispersion relations and growth rates of whistler waves with electron temperature anisotropy for Maxwellian, Cairns, kappa and generalized (r, q) distributions by varying the key plasma parameters for the problem under consideration. It has been found that the growth rate of whistler instability is maximum for flat-topped distribution whereas it is minimum for the Maxwellian distribution. This work not only summarizes and complements the previous work done on the whistler waves with electron temperature anisotropy but also provides a general framework to understand the linear propagation of whistler waves with...

Interaction and resonance of fast magnetoacoustic solitary waves in cylindrical geometry for dense astrophysical plasmas

The cylindrical Kadomtsev-Petviashvili (CKP) equation, also known as Johnsons equation, is derived for dense electron ion plasmas in the small amplitude limit to study the interaction and resonance of two magnetoacoustic solitons in cylindrical geometry. The exact analytical solutions of the CKP equation are derived using Hirotas direct method and a novel gauge transformation. It is observed that cylindrical geometry transforms the line solitons to the horseshoe-like solitary structures. For multi-solitons, it is shown that the interaction parameter for the CKP solitary wave depends upon the plasma parameters of the system. For the present investigation, plasma parameters are chosen that are customarily found in the white dwarf stars. The interaction of magnetoacoustic solitons are shown to change their trajectory and introduce a phase shift. The resonance condition is developed for the CKP equation which shows that the amplitude of resultant solitons becomes four times the amplitude of initial solitons.The cylindrical Kadomtsev-Petviashvili (CKP) equation, also known as Johnsons equation, is derived for dense electron ion plasmas in the small amplitude limit to study the interaction and resonance of two magnetoacoustic solitons in cylindrical geometry. The exact analytical solutions of the CKP equation are derived using Hirotas direct method and a novel gauge transformation. It is observed that cylindrical geometry transforms the line solitons to the horseshoe-like solitary structures. For multi-solitons, it is shown that the interaction parameter for the CKP solitary wave depends upon the plasma parameters of the system. For the present investigation, plasma parameters are chosen that are customarily found in the white dwarf stars. The interaction of magnetoacoustic solitons are shown to change their trajectory and introduce a phase shift. The resonance condition is developed for the CKP equation which shows that the amplitude of resultant solitons becomes four times the amplitude of initial solitons.

Electrons in regions of high phase density exhibit uncanny traits in the study of linear and nonlinear drift waves in spatially non-uniform magnetoplasmas

Linear and nonlinear waves are examined on the ion time scale in a spatially inhomogeneous plasma having electrons that follow product bi (r,q) distribution. It has been shown that the linear dispersion relation for product bi (r,q) distribution undergoes appreciable changes as opposed to the one for the Maxwellian electrons. It has been found that the drift wave frequency is highest for flat-topped distribution, whereas it is lowest for the spiky distribution. It has been found that the drift solitary wave with flat-topped distribution (i.e., r > 0) is one of a kind and exhibits peculiar characteristics. It has been shown that Maxwellian and kappa-like electrons cannot alter the nature of the electrostatic drift waves under consideration; however, the spiky electrons can. The results obtained here are general and can be applied to many regions of space plasmas where the satellite missions have reported the presence of electron distribution functions that show deviation from the Gaussian behavior.

Analytical and numerical study of perpendicularly propagating kinetic mode in magnetized plasmas with Vasyliunas-Cairns distribution

In this paper, we have investigated the perpendicularly propagating kinetic mode employing the Vasyliunas-Cairns (VC) distribution. We have obtained the linear dispersion relation for Bernstein waves for two electron species and explored it both analytically and numerically. We have shown that Bernstein waves with the Vasyliunas-Cairns distribution exhibit a significant departure from the results reported earlier with Maxwellian and dual kappa distributions especially in the long wavelength regime. We have found that the parameters Λc and Λh, that represent the nonthermal percentage of cold and hot electrons in Cairns distribution, play a unique and vital role in determining the behavior of Bernstein modes in the VC distribution which has no parallel in Maxwellian and dual kappa distribution functions. Our study may be beneficial to understand the propagation characteristics of Bernstein waves in space plasmas where departure from Maxwellian behavior has been divulged by many expeditions in space.In this paper, we have investigated the perpendicularly propagating kinetic mode employing the Vasyliunas-Cairns (VC) distribution. We have obtained the linear dispersion relation for Bernstein waves for two electron species and explored it both analytically and numerically. We have shown that Bernstein waves with the Vasyliunas-Cairns distribution exhibit a significant departure from the results reported earlier with Maxwellian and dual kappa distributions especially in the long wavelength regime. We have found that the parameters Λc and Λh, that represent the nonthermal percentage of cold and hot electrons in Cairns distribution, play a unique and vital role in determining the behavior of Bernstein modes in the VC distribution which has no parallel in Maxwellian and dual kappa distribution functions. Our study may be beneficial to understand the propagation characteristics of Bernstein waves in space plasmas where departure from Maxwellian behavior has been divulged by many expeditions in space.

Investigation of colliding nonlinear structures in a relativistically degenerate plasma

In this paper, we have investigated the head-on collision of quantum ion acoustic solitons with relativistically degenerate electrons by deriving a set of Korteweg–de Vries equations. Using the refined Poincare-Lighthill-Kuo method, we have derived the expressions for phase shifts of colliding solitons. It has been found that the system under consideration admits only compressive electrostatic solitary structures. The solitons have also been found to form only for the sub-acoustic velocities of the nonlinear structures. It has been observed that the soliton interaction happens over long spatial and temporal scales in the non-relativistic limit whereas the converse happens in the ultrarelativistic regime. It has been found that the phase shift of the solitons after interaction is more pronounced in the ultra-relativistic limit by comparison with its non-relativistic counterpart. It has also been found that the phase shift settles to a constant value in the nonrelativistic limit. Using the parameters that are customarily found in white dwarf stars, we have given an estimate of the spatial scales over which the nonlinear structures are expected to interact with each other both in the non-relativistic and in the ultra-relativistic regimes.

An alternative explanation for the density depletions observed by Freja and Viking satellites

In this paper, we have studied the linear and nonlinear propagation of ion acoustic waves in the presence of electrons that follow the generalized (r,q) distribution. It has been shown that for positive values of r, which correspond to a flat-topped electron velocity distribution, the nonlinear ion acoustic waves admit rarefactive solitary structures or density depletions. It has been shown that the generalized (r,q) distribution function provides another way to explicate the density depletions observed by Freja and Viking satellites previously explained by proposing Cairns distribution function.In this paper, we have studied the linear and nonlinear propagation of ion acoustic waves in the presence of electrons that follow the generalized (r,q) distribution. It has been shown that for positive values of r, which correspond to a flat-topped electron velocity distribution, the nonlinear ion acoustic waves admit rarefactive solitary structures or density depletions. It has been shown that the generalized (r,q) distribution function provides another way to explicate the density depletions observed by Freja and Viking satellites previously explained by proposing Cairns distribution function.