No seminar

The fast solar wind is a collisionless magnetised plasma. Therefore, deviations from thermodynamic equilibrium can develop, and a kinetic description of the plasma becomes necessary. I will discuss the properties of the dispersion relations and polarisation properties of kinetic slow modes and magnetohydrodynamic slow modes. I compare theoretical predictions for the fluctuations in the three lowest velocity moments (density, bulk velocity, and pressure) of the proton distribution function associated with large-scale compressive slow modes to solar-wind observations at 1 au. Surprisingly, the predictions from magnetohydrodynamics (MHD) are in a better agreement with the data than the kinetic predictions, even at times when the collisionality of the plasma is low. This unexpected result suggests that some unknown kinetic process assumes the role of collisions in suppressing fluctuations in heat flux. Possible candidates for such a process include fluctuating-moment effects, wave-particle scattering on small-scale fluctuations, and anti-phase-mixing.