Collisional relaxation of fine velocity structures in plasmas.
Pezzi O., Valentini F., Veltri P.
In a weakly collisional plasma, such as the solar wind, collisions are usually considered far too weak to produce any significant effect on the plasma dynamics. The estimation of collisionality is often based on the restrictive assumption that the particle velocity distribution function (VDF) shape is close to Maxwellian, while in situ measurements and kinetic numerical experiments indicate that the VDFs develop marked non-Maxwellian features (temperature anisotropies, particle beams). Here we show that the collisional relaxation of fine velocity structures occur on several characteristic times. Therefore the plasma collisionality can increase locally due to the velocity space deformation of the particle velocity distribution, thus indicating that the collisionless hypothesis may locally fail. Moreover fine velocity structures are dissipated by collisions in a time much shorter than global non-Maxwellian features as temperature anisotropies, thus suggesting that that high-resolution measurements of the particle VDFs are crucial for an accurate description of weakly collisional systems, as the solar wind.