Blog
Articles about computational science and data science, neuroscience, and open source solutions. Personal stories are filed under Weekend Stories. Browse all topics here. All posts are CC BY-NC-SA licensed unless otherwise stated. Feel free to share, remix, and adapt the content as long as you give appropriate credit and distribute your contributions under the same license.
tags · RSS · Mastodon · simple view · page 18/19
Bi-Maxwellian distributions and anisotropic pressure
In many space and laboratory plasmas, velocity distributions are anisotropic with respect to the magnetic field direction. This leads to anisotropic pressure components that cannot be captured by fluid models assuming a scalar pressure. In this post, we construct bi-Maxwellian velocity distributions with prescribed temperature anisotropies and demonstrate explicitly how they give rise to anisotropic pressure tensors. The numerical results reproduce known analytic relations while providing an intuitive geometric interpretation in velocity space.
What velocity moments miss: Core plus beam distributions
In this post, we explore how different initial velocity distributions lead to qualitatively distinct macroscopic dynamics, illustrating the inherently kinetic nature of these processes.
Vlasov–Poisson dynamics: Landau damping and the two-stream instability
The Vlasov–Poisson system provides a minimal kinetic framework to illustrate fundamental plasma processes such as Landau damping and the two-stream instability. By varying the initial velocity distribution, we explore in this post how these inherently kinetic phenomena arise from the same collisionless equations yet lead to qualitatively different macroscopic dynamics.
Kinetic plasma theory: From distribution functions to the Vlasov equation
Kinetic plasma theory describes a plasma as an ensemble of particles represented by a distribution function in phase space. This framework captures velocity space structure, non Maxwellian populations, temperature anisotropies, drifts, beams, loss cones, weak collisionality, resonances, and wave particle interactions. In collisionless space plasmas, the Vlasov equation governs the evolution of the distribution function, providing a self consistent description of plasma dynamics beyond fluid models like magnetohydrodynamics (MHD). In this post, we explore the foundations of kinetic plasma theory, starting from the definition of the distribution function and leading to the Vlasov equation.
Plasma instabilities as dynamical departures from equilibrium
Plasma instabilities mark the transition from passive wave propagation to active energy conversion. While plasma waves describe small amplitude perturbations of a stable equilibrium, instabilities arise when the equilibrium itself is unable to support certain perturbations, leading to exponential growth in time. In mathematical terms, this corresponds to dispersion relations whose solutions acquire a positive imaginary part of the frequency. Physically, instabilities tap free energy stored in gradients, relative flows, or anisotropic particle distributions and convert it into electromagnetic fields and particle motion.
The Alfvén wave as a fundamental mode of magnetized plasmas
Among all plasma waves, the Alfvén wave occupies a special conceptual position. It is the simplest genuinely magnetized plasma mode, arising directly from the coupling between magnetic field line tension and plasma inertia. At the same time, it provides a concrete and analytically tractable example of how collective plasma dynamics interpolate between fluid and kinetic descriptions. In space plasmas, Alfvén waves dominate large scale fluctuations in the solar wind, control energy transport along magnetic field lines, and form the backbone of magnetosphere ionosphere coupling. In this post, we derive the Alfvén wave from first principles, explore its fundamental properties, and discuss its physical interpretation.
Plasma waves in space plasmas
Space plasmas support a rich spectrum of collective wave phenomena that have no direct analogue in neutral fluids. These waves arise from the self consistent coupling between charged particles and electromagnetic fields and therefore occupy a conceptual boundary between fluid descriptions and fully kinetic theory. On sufficiently large spatial and temporal scales, plasma waves can often be understood as perturbations of a conducting fluid governed by magnetohydrodynamics. On smaller scales, or whenever resonant interactions between particles and fields become important, a kinetic description in terms of distribution functions and phase space dynamics is unavoidable. Plasma waves thus provide a natural bridge between macroscopic magnetofluid behavior and microscopic particle physics. In this post, we introduce the fundamental concepts of plasma wave theory, including linear dispersion relations, common wave modes in magnetized plasmas, and the transition from fluid to kinetic descriptions.
Planetary aurorae
Planetary aurorae are luminous phenomena that occur in the upper atmospheres of magnetized planets, resulting from the interaction between energetic charged particles and atmospheric constituents. They serve as visible manifestations of complex plasma processes within planetary magnetospheres, linking solar wind dynamics, magnetospheric circulation, and atmospheric excitation. In this post, we explore their physical origin, mathematical description, and diverse manifestations across the Solar System.
Space Physics: A definitional perspective
Space physics is more than plasma physics. It is an extension of geophysics into space, applying physical thinking to coupled plasma–field systems around Earth and other planetary bodies. In this post, I outline how my training in space physics shaped my perspective on physical systems more generally.
Magnetic reconnection via X-point collapse
In this post, we explore a complementary toy model of magnetic reconnection based on the collapse of an X-point under a prescribed stagnation flow. This model highlights how different geometries and driving conditions can shape reconnection dynamics, while still governed by the same underlying resistive induction physics.