arXiv subjects

Kirit Makwana

Publications and source records attributed to Kirit Makwana.

6 recordsLinked to original sources

arXiv2022

Cosmic ray Transport in Magnetohydrodynamic turbulence

This paper studies cosmic ray (CR) transport in magneto hydrodynamic (MHD) turbulence. CR transport is strongly dependent on the properties of the magnetic turbulence. We perform test particle simulations to study the interactions of CR with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. Our results confirm that the fast modes dominate the CR propagation, whereas Alfvén and slow modes are much less efficient and have shown similar pitch angle scattering rates. We investigate the cross field transport on large and small scales. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by $M_A^ζ$ compared to the parallel diffusion coefficients, with $ζ$ closer to 4 in Alfvén modes than that in total turbulence as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale, both the local and global magnetic reference frames are adopted. Super diffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfvén modes show clear Richardson diffusion in the local reference frame. Our results have broad applications to CRs in various astrophysical environments.

astro-ph.HE↗

arXiv2021

Role of Parallel Solenoidal Electric Field on Energy Conversion in 2.5D Decaying Turbulence with a Guide Magnetic Field

We perform 2.5D particle-in-cell simulations of decaying turbulence in the presence of a guide (out-of-plane) background magnetic field. The fluctuating magnetic field initially consists of Fourier modes at low wavenumbers (long wavelengths). With time, the electromagnetic energy is converted to plasma kinetic energy (bulk flow+thermal energy) at the rate per unit volume of ${\pp J}\cdot{\pp E}$ for current density ${\pp J}$ and electric field ${\pp E}$. Such decaying turbulence is well known to evolve toward a state with strongly intermittent plasma current. Here we decompose the electric field into components that are irrotational, ${\pp E}_{\rm ir}$, and solenoidal (divergence-free), ${\pp E}_{\rm so}$. ${\pp E}_{\rm ir}$ is associated with charge separation, and ${\pp J}\cdot{\pp E}_{\rm ir}$ is a rate of energy transfer between ions and electrons with little net change in plasma kinetic energy. Therefore, the net rate of conversion of electromagnetic energy to plasma kinetic energy is strongly dominated by ${\pp J}\cdot{\pp E}_{\rm so}$, and for a strong guide magnetic field, this mainly involves the component ${\pp E}_{\rm so,\parallel}$ parallel to the total magnetic field ${\pp B}$. We examine various indicators of the spatial distribution of the energy transfer rate {\bf J$_\parallel\cdot$E$_{so,\parallel}$}, which relates to magnetic reconnection, the best of which are 1) the ratio of the out-of-plane electric field to the in-plane magnetic field, 2) the out-of-plane component of the non-ideal electric field, and 3) the magnitude of the estimate of current helicity.

astro-ph.SR↗

arXiv2021

Identification of plasma modes in Galactic turbulence with synchrotron polarization

Magneto-hydrodynamic (MHD) turbulence is ubiquitous and a fundamental ingredient underlying many astrophysical phenomena. The multiphase nature of interstellar medium and diversity of driving mechanisms give rise to spatial variation of turbulence properties, particularly their plasma properties. There has been no observational diagnosis of the plasma modes so far beyond the solar system. Here we report the identification of different plasma modes in various Galactic environments, including active star forming zones and supernova remnants, based on our synchrotron polarization analysis. The observed high degree of consistency between the $γ-$ray excess in Cygnus cocoon and the location of magnetosonic modes provides strong observational evidence for the long-advocated theory that magnetosonic modes dominate the cosmic ray (CR) scattering and acceleration. Our results open up a new avenue for the study of interstellar turbulence and demonstrate the indispensability to account for their plasma property in all the relevant processes including CR transport and star formation.

physics.plasm-ph↗

arXiv2019

Ideal MHD instabilities for coronal mass ejections

We review and discuss insights on ideal magnetohydrodynamic (MHD) instabilities that can play a role in destabilizing solar coronal flux rope structures. For single flux ropes, failed or actual eruptions may result from internal or external kink evolutions, or from torus unstable configurations. We highlight recent findings from 3D magnetic field reconstructions and simulations where kink and torus instabilities play a prominent role. For interacting current systems, we critically discuss different routes to coronal dynamics and global eruptions, due to current channel coalescence or to tilt-kink scenarios. These scenarios involve the presence of two nearby current channels and are clearly distinct from the popular kink or torus instability. Since the solar corona is pervaded with myriads of magnetic loops -- creating interacting flux ropes typified by parallel or antiparallel current channels as exemplified in various recent observational studies -- coalescence or tilt-kink evolutions must be very common for destabilizing adjacent flux rope systems. Since they also evolve on ideal MHD timescales, they may well drive many sympathetic eruptions witnessed in the solar corona. Moreover, they necessarily lead to thin current sheets that are liable to reconnection. We review findings from 2D and 3D MHD simulations for tilt and coalescence evolutions, as well as on particle acceleration aspects derived from computed charged particle motions in tilt-kink disruptions and coalescing flux ropes. The latter were recently studied in two-way coupled kinetic-fluid models, where the complete phase-space information of reconnection is incorporated.

astro-ph.SR↗

arXiv2017

Two-way coupling of magnetohydrodynamic simulations with embedded particle-in-cell simulations

We describe a method for coupling an embedded domain in a magnetohydrodynamic (MHD) simulation with a particle-in-cell (PIC) method. In this two-way coupling we follow the work of Daldorff et al. in which the PIC domain receives its initial and boundary conditions from MHD variables (MHD to PIC coupling) while the MHD simulation is updated based on the PIC variables (PIC to MHD coupling). This method can be useful for simulating large plasma systems, where kinetic effects captured by particle-in-cell simulations are localized but affect global dynamics. We describe the numerical implementation of this coupling, its time-stepping algorithm, and its parallelization strategy, emphasizing the novel aspects of it. We test the stability and energy/momentum conservation of this method by simulating a steady-state plasma. We test the dynamics of this coupling by propagating plasma waves through the embedded PIC domain. Coupling with MHD shows satisfactory results for the fast magnetosonic wave, but significant distortion for the circularly polarized Alfvén wave. Coupling with Hall-MHD shows excellent coupling for the whistler wave. We also apply this methodology to simulate a Geospace Environmental Modeling (GEM) challenge type of reconnection with the diffusion region simulated by PIC coupled to larger scales with MHD and Hall-MHD. In both these cases we see the expected signatures of kinetic reconnection in the PIC domain, implying that this method can be used for reconnection studies.

physics.comp-ph↗

arXiv2016

Dissipation in PIC simulations of moderate to low \b{eta} plasma turbulence

We simulate decaying turbulence in electron-positron pair plasmas using a fully- kinetic particle-in-cell (PIC) code. We run two simulations with moderate-to-low plasma beta. The energy decay rate is found to be similar in both the cases. The perpendicular wave-number spectrum of magnetic energy shows a slope of k^-1.3 in both the cases. The particle energy distribution function shows the formation of a non-thermal feature in the case of lower plasma beta, with a slope close to E^-1. The role of thin turbulent current sheets in this process is investigated. The heating by E_{\parallel}.J_{\parallel} term dominates the E_{\perp}.J_{\perp} term. Regions of strong E_{\parallel}.J_{\parallel} are spatially well-correlated with regions of intense current sheets, which also appear correlated with regions of strong E_{\parallel} in the low beta simulation, suggesting an important role of magnetic reconnection in the dissipation of low beta plasma turbulence.

physics.plasm-ph↗