Nonthermal

Nonthermal describes processes, emissions, or particle populations that do not arise from thermodynamic equilibrium at a single temperature. In a thermal system, many degrees of freedom share a common temperature and radiation can be described by blackbody or Planck distributions. Nonthermal systems exhibit energy distributions that deviate from Maxwellian and may feature excess-high-energy tails, often described by power laws or kappa distributions. Such systems can involve ongoing energy input, rapid acceleration, or weak collisional relaxation.

In radiation and high-energy astrophysics, nonthermal emission results from accelerated particles rather than thermal heating. Common mechanisms include synchrotron radiation from relativistic electrons in magnetic fields, inverse Compton scattering, and nonthermal bremsstrahlung. Observations of nonthermal radio, X-ray, or gamma-ray spectra appear in sources such as supernova remnants, active galactic nuclei, pulsar wind nebulae, and solar flares, where particle acceleration processes dominate the emission.

In plasma physics and laboratory contexts, nonthermal plasmas have electron temperatures that differ from ion or neutral species and often contain non-Maxwellian electron distributions. Cold or nonthermal (often called cold atmospheric) plasmas can have high electron temperatures while maintaining a near-room-temperature bulk gas, enabling surface treatment, sterilization, or biomedical applications without bulk heating.

Overall, the term highlights departures from equilibrium thermodynamics, emphasizing acceleration, nonradiative processes, or distributions that cannot be characterized by a single temperature.