arXiv subjects

Stephen Majeski

Publications and source records attributed to Stephen Majeski.

6 recordsLinked to original sources

arXiv2026

Reduced Effective Viscosity from Anisotropic Transport and Plasma Instabilities in the Sloshing Cores of Galaxy Clusters

The $\sim \mu$G magnetic field in the intracluster medium (ICM) introduces a pressure anisotropy with respect to the magnetic field's direction that manifests as an anisotropic viscous stress. Plasma instabilities arising from the pressure anisotropy crossing certain thresholds force it to marginally stable values, reducing viscous transport. Additionally, the feedback of this anisotropic pressure on the velocity field has been predicted to lead to a form of self-organization that also can reduce viscous dissipation without affecting the collisionality. In this work, we present high-resolution Braginskii-MHD simulations of a galaxy cluster core with sloshing gas motions and turbulence, including the effects of anisotropic viscous stress and different simple prescriptions for limiting the pressure anisotropy due to plasma instabilities. Braginskii viscosity has an expected, though modest, effect on suppressing Kelvin-Helmholtz instabilities at sloshing cold front surfaces, dependent on how the pressure anisotropy is limited. Due to the sloshing motions, the magnetic field's strength can become high enough in places that the pressure anisotropy need not be limited. Nevertheless, the combined effect of the limiters and the turbulent structure of the magnetic field in all simulations is that the effective viscosity is much lower than the isotropic Spitzer value in a significant fraction of the core region. However, we find that this reduced viscosity is capable of steepening the velocity-amplitude spectrum and transferring a small fraction of the turbulent kinetic energy into heat. Finally, we present evidence for magneto-immutable dynamics in our simulations.

astro-ph.GA↗

arXiv2025

Cosmic-ray transport in inhomogeneous media

A theory of cosmic-ray transport in multi-phase diffusive media is developed, with the specific application to cases in which the cosmic-ray diffusion coefficient has large spatial fluctuations that may be inherently multi-scale. We demonstrate that the resulting transport of cosmic rays is diffusive in the long-time limit, with an average diffusion coefficient equal to the harmonic mean of the spatially varying diffusion coefficient. Thus, cosmic-ray transport is dominated by areas of low diffusion even if these areas occupy a relatively small, but not infinitesimal, fraction of the volume. On intermediate time scales, the cosmic rays experience transient effective sub-diffusion, as a result of low-diffusion regions interrupting long flights through high-diffusion regions. In the simplified case of a two-phase medium, we show that the extent and extremity of the sub-diffusivity of cosmic-ray transport is controlled by the spectral exponent of the distribution of patch sizes of each of the phases. We finally show that, despite strongly influencing the confinement times, the multi-phase medium is only capable of altering the energy dependence of cosmic-ray transport when there is a moderate (but not excessive) level of perpendicular diffusion across magnetic-field lines.

astro-ph.HE↗

arXiv2023

On hydromagnetic wave interactions in collisionless, high-$β$ plasmas

We describe the interaction of parallel-propagating Alfvén waves with ion-acoustic waves and other Alfvén waves, in magnetized, high-$β$ collisionless plasmas. This is accomplished through a combination of analytical theory and numerical fluid simulations of the Chew-Goldberger-Low (CGL) magnetohydrodynamic (MHD) equations closed by Landau-fluid heat fluxes. An asymptotic ordering is employed to simplify the CGL-MHD equations and derive solutions for the deformation of an Alfvén wave that results from its interaction with the pressure anisotropy generated either by an ion-acoustic wave or another, larger-amplitude Alfvén wave. The difference in timescales of acoustic and Alfvénic fluctuations at high-$β$ means that interactions that are local in wavenumber space yield little modification to either mode within the time it takes the acoustic wave to Landau damp away. Instead, order-unity changes in the amplitude of Alfvénic fluctuations can result after interacting with frequency-matched acoustic waves. Additionally, we show that the propagation speed of an Alfvén-wave packet in an otherwise homogeneous background is a function of its self-generated pressure anisotropy. This allows for the eventual interaction of separate co-propagating Alfvén-wave packets of differing amplitudes. The results of the CGL-MHD simulations agree well with these predictions, suggesting that theoretical models relying on the interaction of these modes should be reconsidered in certain astrophysical environments. Applications of these results to weak Alfvénic turbulence and to the interaction between the compressive and Alfvénic cascades in strong, collisionless turbulence are also discussed.

astro-ph.HE↗

arXiv2023

Microphysically modified magnetosonic modes in collisionless, high-$β$ plasmas

With the support of hybrid-kinetic simulations and analytic theory, we describe the nonlinear behaviour of long-wavelength non-propagating (NP) modes and fast magnetosonic waves in high-$β$ collisionless plasmas, with particular attention to their excitation of, and reaction to, kinetic micro-instabilities. The perpendicularly pressure balanced polarization of NP modes produces an excess of perpendicular pressure over parallel pressure in regions where the plasma $β$ is increased. For mode amplitudes $δB/B_0 \gtrsim 0.3$, this excess excites the mirror instability. Particle scattering off these micro-scale mirrors frustrates the nonlinear saturation of transit-time damping, ensuring that large-amplitude NP modes continue their decay to small amplitudes. At asymptotically large wavelengths, we predict that the mirror-induced scattering will be large enough to interrupt transit-time damping entirely, isotropizing the pressure perturbations and morphing the collisionless NP mode into the magnetohydrodynamic (MHD) entropy mode. In fast waves, a fluctuating pressure anisotropy drives both mirror and firehose instabilities when the wave amplitude satisfies $δB/B_0 \gtrsim 2β^{-1}$. The induced particle scattering leads to delayed shock formation and MHD-like wave dynamics. Taken alongside prior work on self-interrupting Alfvén waves and self-sustaining ion-acoustic waves, our results establish a foundation for new theories of electromagnetic turbulence in low-collisionality, high-$β$ plasmas such as the intracluster medium, radiatively inefficient accretion flows, and the near-Earth solar wind.

astro-ph.HE↗

arXiv2023

Super-Fermi Acceleration in Multiscale MHD Reconnection

We investigate the Fermi acceleration of charged particles in 2D MHD anti-parallel plasmoid reconnection, finding a drastic enhancement in energization rate $\dot{\varepsilon}$ over a standard Fermi model of $\dot{\varepsilon} \sim \varepsilon$. The shrinking particle orbit width around a magnetic island due to $\vec{E}\times\vec{B}$ drift produces a $\dot{\varepsilon}_\parallel \sim \varepsilon_\parallel^{1+1/2χ}$ power law with $χ\sim 0.75$. The increase in the maximum possible energy gain of a particle within a plasmoid due to the enhanced efficiency increases with the plasmoid size, and is by multiple factors of 10 in the case of solar flares and much more for larger plasmas. Including effects of the non-constant $\vec{E}\times\vec{B}$ drift rates leads to further variation of power law indices from $\gtrsim 2$ to $\lesssim 1$, decreasing with plasmoid size at the time of injection.

physics.plasm-ph↗

arXiv2021

Guide Field Effects on the Distribution of Plasmoids in Multiple Scale Reconnection

The effects of a finite guide field on the distribution of plasmoids in high-Lundquist-number current sheets undergoing magnetic reconnection in large plasmas are investigated with statistical models. Merging of plasmoids is taken into account either assuming that guide field flux is conserved resulting in non-force-free profiles in general, or that magnetic helicity is conserved and Taylor relaxation occurs to convert part of the summed guide field flux into reconnecting field flux towards minimum energy states resulting in force-free profiles. It is found that the plasmoid distribution in terms of reconnecting field flux follows a power law with index 7/4 or 1 depending on whether merger frequencies are independent of or dependent on their relative velocity to the outflow speed, respectively. This result is approximately the same for the force-free and non-force-free models, with non-force-free models exhibiting indices of 2 and 1 for the same velocity dependencies. Distributions in terms of guide field flux yield indices of 3/2 for the non-force-free model regardless of velocity dependence. This is notably distinct from the indices of 11/8 and 1 for the force-free models independent of and dependent on velocity, respectively. At low guide field fluxes the force-free models exhibit a second power law index of 1/2 due to non-constant flux growth rates. The velocity dependent force-free model predicts the production of slightly more rapidly moving large guide field flux plasmoids which is supported by observational evidence of flux ropes with strong core fields. Implications are discussed on particle acceleration via Fermi processes.

physics.plasm-ph↗