Kinetic turbulence in plasmas: space data analysis and numerical simulations
Intervenant : Denise Perrone
European Space Agency / ESAC
Living on Earth, and thanks to the support of many space missions, we have the unique opportunity to analyze directly the features of the dynamical behavior of a natural plasma: the solar wind. A puzzling aspect of solar-wind dynamics consists in the empirical evidence that it is hotter than expected for an adiabatic expanding gas. The cooling of the expanding solar wind is less efficient than it should be, then a key question is how does the solar-wind energy turn into heat and keep it hot. Understanding the physical mechanisms of dissipation, and the related heating, in such turbulent collisionless plasma represents nowadays one of the key issues of plasma physics.
In this scenario, the use of both spacecraft measurements and kinetic numerical simulations becomes crucial.
Here, the nature of the turbulent fluctuations close to the ion scales, in both slow and fast solar wind streams, is investigated by using high-time resolution magnetic field data of multi-point measurements of Cluster spacecraft. The ion scales are characterized by the presence of coherent structures responsible for solar wind intermittency. Moreover, close to coherent structures the proton distribution function appears strongly deformed and far from the thermodynamic equilibrium.
However, due to the limitation on the particle measurements, a support from self-consistent fully nonlinear Vlasov models is needed and a crucial tool is represented by kinetic numerical simulations. Here, hybrid Vlasov-Maxwell simulations of multi-component turbulent plasma, such as the solar wind, show that the response of different ion species to the fluctuating electromagnetic fields appears to be different. In particular, a significant differential heating of alpha particles with respect to protons is observed, localized nearby the peaks of ion vorticity and where strong deviations from thermodynamic equilibrium are recovered.
The understanding of the complex process of particle heating results strongly related to the study of the non-Maxwellian features on the three-dimensional ion velocity distributions. These numerical results highlight the importance for the future space missions to provide detailed ion measures to make a significant step forward in the problem of heating in turbulent space plasmas.