Speaker
Description
Non-thermal power-law distribution functions are ubiquitous in astrophysical, space, and laboratory kinetic plasmas, but their origin remains unclear. A related puzzle is the temperature inversion of the solar corona. We show that these phenomena are deeply connected by developing a self-consistent quasilinear theory for electromagnetically driven, unmagnetized kinetic plasmas. The theory yields a multi-species Fokker--Planck equation containing drive-induced diffusion, due to direct acceleration by broad-band turbulent or narrow-band wave-like fields and indirect acceleration by the waves excited, together with Balescu--Lenard diffusion and drag from self-generated Debye-scale fluctuations and Coulomb collisions. For a super-Debye turbulent electric-field spectrum, $|{\bf E}_{\bf k}|^2\propto k^{-\alpha}$, both electron and ion distributions relax toward a universal $f(v)\propto v^{-5}$, or $N(E)\propto E^{-2}$, attractor, equivalent to the high-energy fall-off of a $\kappa=1.5$ distribution, provided $\alpha\ge5$. This universality follows from Debye screening: large-scale fields accelerate unscreened fast particles but not screened slow ones. For shallower spectra, $\alpha<5$, the tail scales as $v^{-\alpha}$; incomplete relaxation and anisotropy can further break universality. Anisotropic wave drives produce branch- and spectrum-dependent exponents. Collisions do not efficiently decelerate suprathermal particles, so high-velocity tails resist Maxwellianization. In the solar atmosphere, such tails may be generated by chromospheric convection or nanoflares despite collisional and radiative losses. Direct wave heating preferentially energizes electrons through Landau-resonant interactions with whistler and electron-cyclotron waves, while ions may be accelerated by turbulent ambipolar fields. The resulting $\kappa\simeq1.5$--$3$ distributions naturally produce an abrupt upper-chromosphere/lower-corona temperature transition and velocity-filtration-driven inverted profiles, yielding coronal temperatures $\sim10^6\,{\rm K}$.