arXiv subjects

Britan Singh

Publications and source records attributed to Britan Singh.

5 recordsLinked to original sources

arXiv2026

Ion Channel Dynamics in Temperature-Dependent Weibel Instability Saturation

We present 1X2V continuum Vlasov-Maxwell simulations of interpenetrating plasma beams with mobile ions. While the early-time evolution is similar to the stationary-ion case, the late-time dynamics are dominated by the ion-Weibel instability. As ion channels merge, the magnetic energy increases and the magnetic structures extend further along the beam direction. Electrons rapidly reach thermal equilibrium, whereas ions retain distinct bulk velocities for much longer and thermalize more slowly. These results are relevant to collisionless shock formation in astrophysical compact objects and laser-plasma experiments. Wind/SWE observations place all four simulated cases in the firehose/Weibel-unstable region of the proton temperature anisotropy diagram, and MMS1 observations of a quasi-perpendicular bow shock ($\theta_{Bn}\approx83^\circ$, $M_A\approx27$) show a qualitatively similar electron-ion thermalization disparity.

physics.plasm-ph↗

arXiv2026

Linear-wave bound on electromagnetic energy equipartition at sub-electron scales in non-relativistic plasmas

Recent Magnetospheric Multiscale (MMS) observations report approximate equality between electric and magnetic field energy spectral densities, $\varepsilon_{0} P[\delta E]/2 \approx P[\delta B]/(2\mu_{0})$, at sub-electron scales in reconnection-driven magnetotail turbulence, interpreted as relaxation toward thermodynamic equilibrium. We derive the electric-to-magnetic energy ratio from the linear polarization of kinetic Alfv\'en waves and whistler-mode waves in the two-fluid framework and show that it saturates at $\mathcal{R}_{\infty}=(V_{A}/c)^{2}(m_{i}/m_{e})(\beta_{e}/2)$ deep in the sub-electron regime. Setting $\mathcal{R}_{\infty}=1$ yields the universal threshold $V_{A}/c \gtrsim \sqrt{2/[(m_{i}/m_{e})\beta_{e}]}$, which no non-relativistic space plasma satisfies. For typical magnetotail parameters, $\mathcal{R}_{\infty}\approx 2\times 10^{-3}$, approximately 500 times below the observed value, a discrepancy rooted in the non-relativistic ordering $(V_{A}/c)^{2}\ll 1$. Noise-floor estimates show that Search Coil Magnetometer and Electric Double Probe sensitivity convergence produces a spurious apparent equipartition throughout this regime. The observed equality likely reflects nonlinear dynamics, incoherent superposition of electromagnetic and electrostatic fluctuations, or instrumental noise contamination.

physics.plasm-ph↗

arXiv2026

Inertial-Range Suppression and Ponderomotive Density Cavitation in Broadband Sub-Alfv\'{e}nic Turbulence under Plasma Sheet Boundary Layer Conditions

Kinetic Alfv\'{e}n waves (KAWs) are among the most pervasive electromagnetic fluctuations in magnetized astrophysical plasmas, from Earth's magnetosphere to galaxy clusters. Their ponderomotive coupling to compressive density fluctuations is poorly understood in the broadband turbulent regime. We present two-dimensional pseudospectral simulations of the modified nonlinear Schr\"{o}dinger--magnetosonic (MNLS--MS) system governing KAW envelopes, initialized with a broadband power-law spectrum ($|\psi(\mathbf{k})|^2\propto k^{-5/6}$) spanning many interacting modes, at $\beta \sim 0.1$--$0.3$ representative of plasma sheet boundary layer (PSBL) conditions. A fourth-order Runge--Kutta scheme on a $256\times 256$ grid integrates the system to $t = 40$ (normalized), with total energy conserved to within $0.085\%$ in the undamped run; a damped run with dissipation loses $\sim 4\%$ of the magnetic energy over the same interval. The nonlinearity parameter $\chi_\mathrm{NL} \approx 0.25$ confirms broadband sub-Alfv\'{e}nic turbulence throughout. Magnetic field intensity and plasma density develop spatially intermittent, filamentary structures within the first few wave periods, consistent with ponderomotive density cavitation and plasma expulsion from wave-intense regions. The magnetic energy spectra show inertial-range suppression, with a rapid transition from injection ($k < 0.3$) to dissipation without an extended power-law cascade, in agreement with the moderate magnetic Reynolds number ($\mathrm{R_m} \sim 250$--$370$) of the simulation and the observationally constrained range for PSBL turbulence. These results provide numerical evidence that broadband KAW turbulence self-organizes into coherent density structures at kinetic scales, and that the spectral character of such turbulence is governed primarily by moderate-Reynolds-number constraints rather than by the wave physics alone.

physics.plasm-ph↗

arXiv2026

Weibel Instability in Collisionless Plasmas Across Astrophysical and Laboratory Shocks

We present a cold-fluid analysis of the purely transverse Weibel (current-filamentation) instability across four regimes: non-relativistic (NR) single-species, NR multi-species, relativistic single-species, and relativistic multi-species (electron--positron and electron--proton). Beginning from linearized fluid equations, we derive the dispersion relations in each regime and extract scaling laws for the maximum growth rate $\gamma_{\rm max}$ and characteristic unstable wavenumber $k_{\rm max} = \omega_{pi}/c$. Relativistic corrections suppress $\gamma_{\rm max}$ by up to 40 per cent above $v_0 \approx 0.2c$, peaking near $v_0 \approx 0.9c$. Multi-species effects are significant only for $m_e/m_i \gtrsim 1/500$. For the tabletop laser experiment of Bai et al., Nat.Commun., 16, 3770 (2025), the cold-fluid prediction gives $d_i = c/\omega_{pi} \approx 31.7\,\mu{\rm m}$, within 2 per cent of the measured filament spacing $\lambda_F \approx 31\,\mu{\rm m}$. The saturation field estimate $B_{\rm sat} \approx 2.3\times10^4$ T is an upper bound, consistent with the measured $\approx 5000$ T under kinetic suppression. Two MMS burst-mode bow shock crossings (October 16, 2015 and November 25, 2017) confirm $k_{\rm max} d_i = 1$ from FGM/FPI data. A multi-environment scatter plot spans 21 orders of magnitude in $n_i$, with all points within a factor of 3 of the 1:1 line.

physics.plasm-ph↗

arXiv2025

Damped Kinetic Alfv\'en Waves in Earth's Magnetosheath: Numerical Simulations and MMS Observations

The Earth's magnetosheath provides a high $\beta$ (ratio of electron thermal pressure to magnetic pressure) plasma environment where kinetic Alfv\'en waves (KAWs) strongly influence turbulence and energy dissipation. This study investigates how Landau damping modifies the nonlinear evolution of KAWs by solving a modified nonlinear Schr\"odinger equation that captures both dispersive and nonlinear effects. Without Landau damping, modulational instability drives rapid self-focusing into intense magnetic filaments, producing a turbulent cascade with $k_\perp^{-5/3}$ scaling in the inertial range ($k_\perp\rho_i<1$) that transitions to $k_\perp^{-8/3}$ at sub-ion scales ($k_\perp\rho_i>1$), here $k_\perp$ is the wavevector component perpendicular to the background magnetic field and $\rho_i$ the ion thermal gyroradius. When Landau damping is included, magnetic structures are significantly suppressed, and the spectrum steepens to $k_\perp^{-11/3}$ in the sub-ion range while the inertial range maintains $k_\perp^{-5/3}$ scaling. The damping acts across all scales through resonant wave-particle interactions, efficiently transferring energy from waves to particles. Direct comparison with Magnetospheric Multiscale (MMS) spacecraft observations shows that the observed kinetic range spectral slope falls between our undamped and damped simulation limits, consistent with an intermediate damping regime in magnetosheath turbulence. This agreement confirms that Landau damping is one of the primary mechanisms controlling turbulent energy dissipation at kinetic scales in collisionless plasmas.

physics.plasm-ph↗