A measurement of the effective mean free path of solar wind protons

Weakly collisional plasmas are subject to nonlinear relaxation processes, which can operate at rates much faster than the particle collision frequencies. This causes plasma respond like a magnetised fluid despite having long mean-free-paths. In this Letter effective mechanisms modelled in kinetic equation produce density, pressure and magnetic field responses compare with spacecraft measurements of solar wind compressive fluctuations 1 AU. enables measurement mean-free-path protons, found be 4.35 $\times 10^5$ km, is $\sim 10^3$ times shorter mean-free-path. These shown support behavior scales above proton gyroradius demonstrate that processes alter thermodynamics transport weakly plasmas.