Thermalization of unmagnetized multicomponent collision-poor plasmas by noncollective fluctuations

The recently proposed mechanism for the efficient thermalization of collision-poor, unmagnetized plasmas is applied to fully ionized multicomponent electron–positron and electron–proton systems. The balance between particle momentum losses due to spontaneous emission of high-frequency non-collective fluctuations and momentum diffusion in the self-generated fluctuating electric fields leads to Maxwellian particle distributions with equal temperatures for all plasma species. Two key integrals that determine the time-averaged particle loss rate and the momentum–diffusion coefficient are evaluated more accurately. Compared with the approximate treatment of Paper I [Schlickeiser and Kröger, Phys. Rev. E 112, 025209 (2025)], the formation of Maxwellian distributions remains unchanged, but the self-consistent temperature equation is significantly modified. Analytical temperature–density relations are derived for pure electron, electron–positron, and electron–proton plasmas, yielding nonrelativistic temperatures that depend only weakly on electron density over a wide density range. These relations are then used to compute the associated non-collective electric and magnetic field fluctuations, as well as density fluctuations. For electron–proton plasmas, the total random fields are found to be |δE|≃4×10−13ne3/4 G and |δB|≃2×10−13ne3/4 G. The relative density fluctuation (|δne|+|δnp|)/ne≃6.4×10−9ne1/4 is well below unity, validating the equilibrium assumption based on a reduced transport equation. These results demonstrate that non-collective electromagnetic fluctuations provide a robust relaxation mechanism, resolving the long-standing problem of how thermal particle distributions are formed in collision-poor space plasmas where the timescale for elastic two-particle collisions is many orders of magnitude longer than the interaction of plasma particles with noncollective electromagnetic fluctuations.