arXiv subjects

Parth Pavaskar

Publications and source records attributed to Parth Pavaskar.

5 recordsLinked to original sources

arXiv2026

Non-Markovian Cosmic-Ray Pitch-Angle Transport from Mirror Interactions

Cosmic-ray pitch-angle transport in magnetohydrodynamic (MHD) turbulence is governed by the interplay between magnetic mirroring and gyroresonant scattering. We develop a guiding-center (GC) Langevin model with explicit mirror drift and gyroresonant diffusion to describe the pitch angle evolution. This model accurately captures our test-particle simulation results in three-dimensional MHD turbulence, driven both solenoidally and compressively. We find that magnetic mirroring can drive anomalous pitch-angle diffusion at large pitch angles (including $90^\circ$) with non-Markovian memory effects, which arises from trapping of particles in magnetic wells. Gyroresonant scattering controls the escape rate from these wells. Across $M_{\rm A}$, large-pitch-angle particles are jointly regulated by mirror trapping and gyroresonant escape, exhibiting a transition from anomalous to normal diffusive pitch-angle transport as scattering strengthens, whereas small-pitch-angle particles remain gyroresonance-dominated and diffusive throughout. The pitch angle transport is found to be dominated by the compressible perturbations with marginal influence from Alfv\'en modes. In compressible turbulence with realistic damping accounted for, transit time damping (TTD) treatment fully recovers mirror interactions.

astro-ph.HE↗

arXiv2025

Energy Cascade and Damping in Fast-Mode Compressible Turbulence

Compressible turbulence governs energy transfer across scales in space and astrophysical systems. Capturing both the turbulence cascade and damping is therefore crucial for models of energy conversion, plasma heating, and particle transport in diverse plasma environments, but remains challenging. Progress is constrained by two unresolved fundamental questions: the persistence of the turbulence cascade in the presence of shocks and discontinuities, and the validity of classical wave theories under strong nonlinearity. In particular, it remains unclear whether meaningful cascade dynamics can be defined in compressible turbulence with phase steepening, and whether frameworks developed for monochromatic waves remain applicable to complex, broadband fluctuations. Using large-scale, high-resolution kinetic simulations, we analyze turbulence-particle interactions, which are beyond the capability of standard magnetohydrodynamic (MHD) simulations. We show that compressible turbulence damping at MHD scales in quantitative agreement with transit-time damping theory, even in fully developed nonlinear states. Moreover, the cascade persists despite the generation of shocks and discontinuities due to phase steepening, revealing a surprising robustness of cross-scale energy transfer under extreme conditions. We further provide the spectral expression of compressible turbulence. These results close a long-standing gap in the physics of compressible turbulence and establish a robust foundation for turbulence modeling from the heliosphere to galaxies.

astro-ph.SR↗

arXiv2024

Diagnostics of magnetohydrodynamic modes in the ISM through synchrotron polarization statistics

One of the biggest challenges in understanding Magnetohydrodynamic (MHD) turbulence is identifying the plasma mode components from observational data. Previous studies on synchrotron polarization from the interstellar medium (ISM) suggest that the dominant MHD modes can be identified via statistics of Stokes parameters, which would be crucial for studying various ISM processes such as the scattering and acceleration of cosmic rays, star formation, dynamo. In this paper, we present a numerical study of the Synchrotron Polarization Analysis (SPA) method through systematic investigation of the statistical properties of the Stokes parameters. We derive the theoretical basis for our method from the fundamental statistics of MHD turbulence, recognizing that the projection of the MHD modes allows us to identify the modes dominating the energy fraction from synchrotron observations. Based on the discovery, we revise the SPA method using synthetic synchrotron polarization observations obtained from 3D ideal MHD simulations with a wide range of plasma parameters and driving mechanisms, and present a modified recipe for mode identification. We propose a classification criterion based on a new SPA+ fitting procedure, which allows us to distinguish between Alfv\'en mode and compressible/slow mode dominated turbulence. We further propose a new method to identify fast modes by analyzing the asymmetry of the SPA+ signature and establish a new asymmetry parameter to detect the presence of fast mode turbulence. Additionally, we confirm through numerical tests that the identification of the compressible and fast modes is not affected by Faraday rotation in both the emitting plasma and the foreground.

astro-ph.GA↗

arXiv2023

Magnetic field measurement from the Davis-Chandrasekhar-Fermi method employed with Atomic Alignment

The Davis-Chandrasekhar-Fermi (DCF) method is widely employed to estimate the mean magnetic field strength in astrophysical plasmas. In this study, we present a numerical investigation using the DCF method in conjunction with a promising new diagnostic tool for studying magnetic fields: the polarization of spectral lines resulting from the atomic alignment effect. We obtain synthetic spectro-polarimetry observations from 3D magnetohydrodynamic (MHD) turbulence simulations and estimate the mean magnetic field projected onto the plane of the sky using the DCF method with GSA polarization maps and a modification to account for the driving scale of turbulence. We also compare the method to the classical DCF approach using dust polarization observations. Our observations indicate that the modified DCF method correctly estimates the plane-of-sky projected magnetic field strengths for sub-Alfv\'enic turbulence using a newly proposed correction factor of $\xi' \in 0.35 - 0.75$. We find that the field strengths are accurately obtained for all magnetic field inclination and azimuth angles. We also observe a minimum threshold for the mean magnetic field inclination angle with respect to the line of sight, $\theta_B \sim 16^\circ$, for the method. The magnetic field dispersion traced by the polarization from the spectral lines is comparable in accuracy to dust polarization, while mitigating some of the uncertainties associated with dust observations. The measurements of the DCF observables from the same atomic/ionic line targets ensure the same origin for the magnetic field and velocity fluctuations and offer a possibility of tracing the 3D direction of the magnetic field.

astro-ph.IM↗

arXiv2022

Diffuse gamma-ray emission in Cygnus X: Comments to Fornieri & Zhang 2022

High energy emissions near particle accelerators provide unique windows to probe the particle acceleration and ensuing escape process determined by local medium properties, particularly the turbulence properties. It has been demonstrated both theoretically and observationally that particle diffusion in local environment can differ from the averaged values inferred from the CR global propagation in the Galaxy determined by local medium, particularly magnetic field and turbulence. A recent publication by Fornieri & Zhang (2022) computed particle transport employing the formalism of fast modes scattering calculation from Yan & Lazarian (2008) and the MHD modes composition results from Makwana & Yan (2020) and Zhang et al. (2020). The authors claim that the Cygnus X observations from HAWC and Ferm-LAT can be reproduced (Abeysekara et al., 2021; Ackermann et al., 2011). We clarify in this paper that the particle diffusion coefficients they obtained do not correspond to their adopted turbulence and medium properties and are incorrect by an order of magnitude. We also point out that the injection process also plays an indispensable role in determining the high energy particle distribution and the resulting gamma-ray emission.

astro-ph.HE↗