nonMaxwellian

Non-Maxwellian describes velocity or energy distributions of particles that depart from the Maxwell-Boltzmann form, which characterizes thermodynamic equilibrium for classical, dilute gases. The term is used broadly in plasma physics, astrophysics, and related fields to denote any distribution with significant deviations from the standard Maxwellian shape.

Non-Maxwellian features arise in systems that are not in local thermodynamic equilibrium or are influenced by external drivers. Examples include low-collisionality plasmas, strong wave-particle interactions, rapid heating or acceleration, magnetic fields, and turbulent or transient processes. In such contexts, populations may contain suprathermal tails, anisotropies, or multiple components with distinct temperatures or bulk flows.

Several common non-Maxwellian forms are used in modeling. The kappa distribution introduces a power-law tail to account for high-energy particles observed in space plasmas. Bi-Maxwellian distributions allow different temperatures along and across magnetic field lines. Drifted or shifted Maxwellians describe bulk flows superimposed on a thermal core. Core-halo or nonthermal distributions capture a dense thermal core with a separate high-energy population. Non-extensive statistical frameworks, such as Tsallis distributions, provide alternative fits in some systems.

The presence of non-Maxwellian distributions affects transport coefficients, reaction rates, radiative properties, and energy partitioning. Accurate description often requires kinetic models or hybrid approaches rather than fluid approximations. Data from in situ measurements, spectroscopy, and imaging are used to infer deviations and constrain models, typically by fitting observed spectra or particle distributions with the appropriate non-Maxwellian function.

In summary, non-Maxwellian is a broad descriptor for distributions that deviate from equilibrium Maxwellian form, reflecting non-equilibrium dynamics, external forcing, and non-thermal particle populations across various physical settings.