Turbulence in the Solar Wind: from Fluid to Kinetic Scales

Almost six decades have passed since the first in-situ observations by Mariner II unraveled the complex nature of solar wind fluctuations. Successively, in-situ and remote sensing observations and numerical simulations have considerably augmented our capability to understand the solar wind dynamics. The heliophysics community devoted particular attention to studying the role that plasma waves/turbulence play in accelerating and heating the wind, fundamental problems of high relevance to astrophysics. Moreover, solar-heliospheric missions like Parker Solar Probe (PSP) and the future Solar Orbiter (SO) will provide further insights based on unprecedented observations from their orbital vantage points.

In fact, the solar wind is a turbulent magneto-fluid in which the frequency range of variability of plasma and magnetic field fluctuations extends from the inverse of the solar rotation period to the smallest scales of the order of the ion and electron gyro-frequency. Some of these fluctuations are propagating modes but others are simply coherent structures advected by the wind across the observer.

I will provide a short overview of our understanding of such a complex phenomenon, going through a phenomenological description of turbulence within different frequency ranges and for different wind speed regimes. I will focus on specific aspects of turbulence fluctuations, like intermittency and anisotropy, particularly relevant at kinetic scales. Finally, I will discuss perspectives related to the future observations provided by PSP and SO.