arXiv subjects

N. S. Saini

Publications and source records attributed to N. S. Saini.

9 recordsLinked to original sources

arXiv2026

A Self-Consistent Model of Kinetic Alfven Solitons in Pulsar Wind Plasma: Linking Soliton Characteristics to Pulsar Observables

A self-consistent model is presented for the formation and propagation of kinetic Alfvén (KA) solitons in mass-loaded filaments within the pulsar wind, where a magnetized electron--positron--ion plasma flows along open magnetic field lines beyond the light cylinder. Using a reductive perturbation approach, we derive a Korteweg--de Vries (KdV) equation governing the nonlinear evolution of KA solitons in this environment. The soliton amplitude and width depend sensitively on key pulsar observables, including spin period, spin-down rate, and pair multiplicity, as well as on plasma composition and suprathermal particle distributions. Heavy ion species such as Fe$^{26+}$ produce significantly broader solitons through enhanced inertia and dispersion, while increasing pair multiplicity leads to smaller solitons through stronger screening. More oblique propagation (larger $θ$) yields wider but lower-amplitude solitons, whereas more thermalized pair plasmas (higher $κ$) support taller and broader structures. A population-level analysis of 1174 pulsars quantifies the physical scales of these nonlinear structures, showing that millisecond pulsars host the most compact solitons, whereas slower pulsars support broader structures. Within the adopted admissible finite-$β$ regime, this work links soliton properties to measurable pulsar parameters and provides a self-consistent framework for characterizing localized nonlinear plasma structures in finite-magnetization regions of pulsar winds and for assessing their role in modulating the local plasma environment.

physics.plasm-ph↗

arXiv2026

Morphological Evolution of Higher Order Nonlinear Kinetic Alfvén Waves in Structured Galactic Environments

Kinetic Alfven waves (KAWs) are fundamental to energy transport and small-scale structure formation in the turbulent, magnetized interstellar medium (ISM). While first-order Korteweg--de Vries (KdV) models describe weakly nonlinear KAW solitons, they fail in strongly inhomogeneous environments where higher-order effects become significant. We investigate higher-order "dressed" kinetic Alfven (KA) solitons in a structured ISM (warm ionized medium, H II regions, stellar-wind bubbles, supernova remnants). Using a multi-component fluid model with superthermal electrons, we derive an inhomogeneous KdV-type equation with cubic nonlinearity, nonlinear-dispersive cross terms, and fifth-order dispersion. The dressed soliton has a $\operatorname{sech}^2$ core decorated by higher-order corrections. We classify soliton morphologies across the Galactic plane as a function of electron suprathermality $κ_e$. Five classes ($ψ_{\rm I}$--$ψ_{\rm V}$) evolve non-monotonically with $κ_e$: strongly suprathermal ($κ_e=1.6$) favour negative double-hump ($ψ_{\rm III}$); intermediate $κ_e$ produce layered sequences of $ψ_{\rm II}$, $ψ_{\rm I}$, $ψ_{\rm IV}$, $ψ_{\rm V}$; near-Maxwellian ($κ_e=3.1$) revert to KdV-like $ψ_{\rm I}$. Localised $ψ_{\rm V}$ appear as a red ring around the SWB shell and a red core inside the SNR, showing embedded structures actively generate distinct morphologies. First-order KdV theory is insufficient; dressed solitons are the natural nonlinear states. The ISM morphology selects soliton class by modulating leading vs. higher-order terms. $ψ_{\rm V}$ features link macroscopic ISM structures to kinetic-scale fluctuations, offering candidates for extreme scattering events and pulsar scintillation. The non-monotonic $κ_e$ dependence can constrain electron suprathermality from observations.

physics.plasm-ph↗

arXiv2026

Interstellar Medium Modulation of Nonlinear Kinetic Alfvén Morphology in Structured Galactic Environments

We present a spatially dependent framework for the existence and propagation of nonlinear kinetic Alfvén (KA) structures in the interstellar medium (ISM). Using a multi-component analytical model that incorporates the diffuse warm ionized medium together with localized H II regions, supernova remnants (SNR), and stellar-wind bubbles (SWB), we derive location-dependent coefficients governing KA dispersion and nonlinearity. The reductive perturbation method is applied to obtain Korteweg-de Vries (KdV) equations, enabling the characterization of solitons under realistic astrophysical conditions. Numerical analysis demonstrates how superthermality, plasma $β$, temperature, and density gradients modulate soliton amplitude, width, and stability. Our results reveal distinct exclusion zones (EZs) for KA solitons in high-$β$ HII regions and SWB/SNR interiors, as well as ultra low-$β$ regions near central pulsar wind nebulae. While H II regions exhibit simple Gaussian-driven depletions, the complex ``hole-and-shell" morphologies of SWBs and SNRs imprint sharp spatial variations and discontinuities on soliton properties. This study establishes a direct link between macroscopic ISM morphology, ion-kinetic scale dissipation, and the emergence of coherent Alfvénic activity, with implications for radio scattering, pulsar scintillation, and fine-scale signatures in astrophysical observations.

physics.plasm-ph↗

arXiv2023

Evolution of ion acoustic solitary waves in pulsar wind

We have studied the evolution of ion-acoustic solitary waves (IASWs) in pulsar wind. The pulsar wind is modeled by considering a weakly relativistic unmagnetized collisionless plasma comprised of relativistic ions and superthermal electrons and positrons. Through fluid simulations, we have demonstrated that the localized ion density perturbations generated in the polar wind plasma can evolve the relativistic IASW pulses. It is found that the concentration of positrons, relativistic factor, superthermality of electrons, and positrons have a significant influence on the dynamical evolution of IASW pulses. Our results may provide insight to understand the evolution of IASW pulses and their role in astrophysical plasmas, especially in the relativistic pulsar winds with supernova outflow which is responsible for the production of superthermal particles and relativistic ions.

physics.plasm-ph↗

arXiv2017

Amplitude modulation of three-dimensional low frequency solitary waves in a magnetized dusty superthermal plasma

The amplitude modulation of three dimensional (3D) dust ion-acoustic wave (DIAW) packets is studied in a collisionless magnetized plasma with inertial positive ions, superthermal electrons and negatively charged immobile dust grains. By using the reductive perturbation technique, a 3D-nonlinear Schrödinger (NLS) equation is derived, which governs the slow modulation of DIAW packets. The latter are found to be stable in the low-frequency $(ω<ω_c)$ regime, whereas they are unstable for $ω>ω_c$, and the modulational instability (MI) is related to the modulational obliqueness $(θ)$. Here, $ω~(ω_c)$ is the nondimensional wave (ion-cyclotron) frequency. It is shown that the superthermal parameter $κ$, the frequency $ω_c$ as well as the charged dust impurity $(0<μ<1)$ shift the MI domains around the $ω-θ$ plane, where $μ$ is the ratio of electron to ion number densities. Furthermore, it is found that the decay rate of instability is quenched by the superthermal parameter $κ$ with cut-offs at lower wave number of modulation ($K$), however, it can be higher (lower) with increasing values of $μ$ ($ω_c$) having cut-offs at higher values of $K$.

physics.plasm-ph↗

arXiv2017

Constraints on the Growth and Spin of the Supermassive Black Hole in M32 From High Cadence Visible Light Observations

We present 1-second cadence observations of M32 (NGC221) with the CHIMERA instrument at the Hale 200-inch telescope of the Palomar Observatory. Using field stars as a baseline for relative photometry, we are able to construct a light curve of the nucleus in the g-prime and r-prime band with 1sigma=36 milli-mag photometric stability. We derive a temporal power spectrum for the nucleus and find no evidence for a time-variable signal above the noise as would be expected if the nuclear black hole were accreting gas. Thus, we are unable to constrain the spin of the black hole although future work will use this powerful instrument to target more actively accreting black holes. Given the black hole mass of (2.5+/-0.5)*10^6 Msun inferred from stellar kinematics, the absence of a contribution from a nuclear time-variable signal places an upper limit on the accretion rate which is 4.6*10^{-8} of the Eddington rate, a factor of two more stringent than past upper limits from HST. The low mass of the black hole despite the high stellar density suggests that the gas liberated by stellar interactions was primarily at early cosmic times when the low-mass black hole had a small Eddington luminosity. This is at least partly driven by a top-heavy stellar initial mass function at early cosmic times which is an efficient producer of stellar mass black holes. The implication is that supermassive black holes likely arise from seeds formed through the coalescence of 3-100 Msun mass black holes that then accrete gas produced through stellar interaction processes.

astro-ph.GA↗

arXiv2015

Modulation of ion-acoustic waves in a nonextensive plasma with two-temperature electrons

We study the amplitude modulation of ion-acoustic wave (IAW) packets in an unmagnetized electron-ion plasma with two-temperature (cool and hot) electrons in the context of the Tsallis' nonextensive statistics. Using the multiple-scale technique, a nonlinear Schr{ö}dinger (NLS) equation is derived which governs the dynamics of modulated wave packets. It is shown that in nonextensive plasmas, the IAW envelope is always stable for long-wavelength modes $(k\rightarrow0)$ and unstable for short-wavelengths with $k \gtrsim1$. However, the envelope can be unstable at an intermediate scale of perturbations with $0<k<1$. Thus, the modulated IAW packets can propagate in the form of bright envelope solitons or rogons (at small- and medium scale perturbations) as well as dark envelope solitons (at large scale). The stable and unstable regions are obtained for different values of temperature and density ratios as well as the nonextensive parameters $q_c$ and $q_h$ for cool and hot electrons. It is found that the more (less) the population of superthermal cool (hot) electrons, the smaller is the growth rate of instability with cutoffs at smaller wave numbers of modulation.

physics.plasm-ph↗

arXiv2011

Large-amplitude electron-acoustic solitons in a dusty plasma with kappa-distributed electrons

The Sagdeev pseudopotential method is used to investigate the occurrence and the dynamics of fully nonlinear electrostatic solitary structures in a plasma containing suprathermal hot electrons, in the presence of massive charged dust particles in the background. The soliton existence domain is delineated, and its parametric dependence on different physical parameters is clarified.

astro-ph.SR↗

arXiv2011

Effect of superthermality on nonlinear electrostatic modes in plasmas

The nonlinear propagation of electron-acoustic solitary structures is investigated in a plasma containing kappa-distributed (superthermal) electrons. Different types of localized structures are shown to exist. The occurrence of modulational instability is investigated.

physics.plasm-ph↗