The solar wind is usually considered a collisionless plasma, where collisions are too weak to produce significant effects on the plasma dynamics. The estimation of the plasma collisionality is usually based on the assumption that particle velocity distribution function (VDF) is close to the thermodynamical equilibrium, while in-situ spacecraft measurements and kinetic simulations show that the distribution function exhibits strong non-Maxwellian features, due to the energy cascade towards short, kinetic spatial scales. Therefore, since collisional effects are proportional to velocity gradients of the VDF, the collisionless hypothesis may fail locally in velocity space.
By modeling inter-particle collisions through the fully nonlinear Landau operator, here we show that fine velocity structures are dissipated faster compared to other global non-Maxwellian features, as temperature anisotropies. Indeed, several characteristic times are recovered in the entropy growth associated with the approach towards equilibrium of the initial VDF and these times are inversely proportional to the steepness of the velocity gradients in the VDF. We also provide evidence that the nonlinearities of the collisional operator are significant to properly compare collisions with other physical phenomena. Hence, the plasma collisionality can locally increase due to the velocity space deformation of the particle velocity distribution and collisions may be an efficient method to dissipate energy and heat the plasma.