Nov. 8, 2019,
3:30 p.m. - 5 p.m.
IAPS, Istituto Nazionale di Astrofisica, Rome, Italy
The solar wind provides an ideal laboratory for the experimental study of astrophysical plasma turbulence. As the solar wind expands, it carries fluctuations in the plasma quantities, namely velocity, magnetic field and density, that in many ways resemble the well-known hydrodynamic turbulence described by Kolmogorov. However, the presence of a large-scale magnetic field implies that, at scales significantly larger than the ion skin depth or Larmor radius, a magnetohydrodynamic (MHD) framework is needed to understand the evolution. Solar wind fluctuations often display what is called “Alfvénic” turbulence: a turbulence that, together with a well developed power spectrum, also shows the strong correlation between velocity and magnetic field fluctuations typical of Alfvén waves propagating away from the Sun. Because the nonlinear interactions in MHD are mediated by Alfvén waves propagating in opposite directions along a mean field, the presence of Alfvénic correlations in the turbulence is crucial to the evolution of the turbulence. Turbulence in the solar wind also depends on the large scale properties of the wind in which it is embedded: the solar wind comes in at least two distinct ‘flavours’, fast and slow, which are characterized by a a different Alfvénic content. Schematically, Alfvénic fluctuations dominate in high speed wind, while the slow wind turbulence contains more standard, evolved fluctuations in which no prevalence of outwardly propagating modes may be found. However, recent studies have shown the existence of peculiar slow wind streams that are similar from many points of view to the fast wind, especially regarding the Alfvénic content of the fluctuations. This presentation will give an overview on turbulence in the solar wind highlighting the role that Parker Solar Probe and Solar Orbiter will have in allowing a better understanding of solar wind origins, of the correlation between wind speed and Alfvénicity, and the processes responsible for the observed evolution with distance from the Sun.