Thermal distributions in stellar plasmas, nuclear reactions and solar neutrinos

The physics of nuclear reactions in stellar plasma is reviewed with: specia l emphasis on the importance of the velocity distribution of ions. Then the properties (density and temperature) of the weak-coupled solar plasma are analysed, showing that, the ion velocities should deviate from the Maxwelli an distribution and could be better described bai a weakly-nonexstensive (\ q - 1\ < 0.02) Tsallis' distribution. We discuss concrete physical framewor ks for calculating this deviation: the introduction of higher-order correct ions to the diffusion and friction coefficients in the Fokker-Planck equati on, the influence of the electric-microfield stochastic distribution on the particle dynamics, a velocity correlation function with long-time memory a rising from the coupling of the collective and individual degrees of freedo m. Finally, we study the effects of such de deviations on stellar nuclear r ates, on the solar neutrino fluxes, and on the pp neutrino energy spectrum, and analyse the consequences for the solar neutrino problem.