The observed Maxwellian velocity distribution functions in plasmas and the fact that the rate of elastic electron-electron collisions is many orders of magnitude smaller than the electron plasma frequency have been a long-standing puzzle. Here, we present a mechanism for efficient thermalization in collision-poor unmagnetized plasmas, which resolves this puzzle. The competition between the momentum losses of plasma particles by spontaneously emitting high-frequency non-collective fluctuations and the momentum diffusion of these particles in their self-generated fluctuating electric field fluctuations provides the Maxwellian particle distribution function. The mechanism is self-regulating, providing electron temperatures of about ${10}^7\mathrm{K}$, and it is applicable to fully ionized plasmas with electron densities below ${10}^{27}{\mathrm{cm}}^{-3}$.

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• Plasma ionization
• Plasma stability
• Plasma turbulence
• Space & astrophysical plasma