Generation and Evolution of Fluctuations in a Non-equilibrium Solar Wind Plasma

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Long-term surveys of the solar wind plasma microstates have revealed a direct correlation with observed fluctuations, pointing to a local generation and amplification of the latter by the kinetic instabilities. The low-energy (core) populations are close to an energy equipartition and constrained within the limits of temperature anisotropy shaped by the thresholds of the kinetic instabilities. However, this puzzle is still unresolved for the suprathermal populations, although they comprise a significant kinetic energy density and maintain the space plasmas out of thermal equilibrium even in the absence of kinetic anisotropies. Within the proposed 3-year project we will investigate in unprecedented detail the interplay of the core and halo populations that appears to be decisive in the generation and evolution of plasma fluctuations. The main goal is the formulation of a theoretical basis for a realistic prediction and characterization of plasma fluctuations induced by the kinetic anisotropies in the solar wind. This will be achieved by modeling the anisotropic plasma states on the kinetic level using generalized Kappa distribution functions, which represent an advanced powerful tool to self-consistently describe the solar wind plasma dynamics. The instability conditions and the maximum growth rates determined within the framework of the linear theory are particularly important and will unveil the fastest relaxation process of the kinetic anisotropy. Special attention will be given both to the mutual effects of electrons and protons as these can significantly change the instability conditions, and to the weakly propagating and aperiodic fluctuations which are usually ignored but appear to be more efficient than periodic modes. The proposed research programme comprises the first analytical and numerical investigation of the quasilinear (weakly-nonlinear) evolution of the self-generated microinstabilites in anisotropic Kappa distributed plasmas. This enables a comprehensive study of the linear and nonlinear evolution of fluctuations, including their back reaction on the velocity distributions and on their moments, i.e. on the macroscopic plasma parameters. This project is very timely in view of the present general interest in physics of non-equilibrium plasmas, as well as in view of the opportunities to advance our understanding of such plasmas offered by a large amount of observational data from passed, contemporary and, particularly, of forthcoming spacecraft missions.