Turbulence and HEating in the SOlar Wind – THESOW

Turbulence is ubiquitous in space and astrophysical plasmas. It is indeed involved in all physical phenomena such as mass transport, energy transfers and dissipation and magnetic reconnection. Unlike neutral fluids, where energy dissipation occurs via viscous effects originating from particle collisions at the microscopic level, most of space plasmas are collisionless. Most of the relevant interactions are therefore in the form of field-particle interactions between the charged particles and the electromagnetic fields.
The solar wind (SW) is certainly the most accessible astrophysical plasma to in-situ measurements. Observations of SW turbulence have usually emphasized magnetohydrodynamic (MHD) scales where the Kolmogorov scaling ~f-5/3 is frequently observed. A general agreement exists that this spectrum results as a consequence of strongly nonlinear interacting Alfvén waves. However, the spatial properties of the turbulence, for instance the wavenumber spectra and anisotropy, are still hotly debated. Other longstanding questions are how turbulence of the MHD scales terminates its cascade at smaller scales? What are the relevant processes of its dissipation (cyclotron and/or Landau resonances, magnetic reconnection, etc)? Answering these questions is very crucial to understanding problems of particle accelerations and heating in the SW and in other astrophysical plasmas (e.g., cosmic rays, coronal heating).
The project that we are proposing here aims at answering these challenging questions. For this purpose we will study the turbulence cascade and itsdissipation in the SW from MHD scales down to electron scales using a multiple approach combining: i) in-situ observations available from the multispacecraft missions; ii) numerical simulations to model the complex behavior of turbulence cascade from MHD to kinetic scales where it is dissipated; iii) and theoretical modeling to make realistic predictions that can be tested in the data.
The observations will be made using mainly data from the four Cluster satellites, which offer a unique chance to identify and to characterize three-dimensional (3D) spatial plasma structures. Moreover, its high time resolution fields measurements (E and B) makes it possible to probe into the smallest scales ever explored in the SW (waveforms are available up to ~100 Hz in the spacecraft frame). Throughout this work we will be using novel signal processing techniques, which take full advantage of the available of the multi-spacecraft data. We will also extend the existing techniques to address the new problems targeted in this proposal. The expected observations will be systematically compared to existing theoretical predictions, to the new ones that we will make within the timeline of the proposal, and to simulations results. The simulations will be performed using 3D Landau-Fluid (LF) code. The LF model is an extension of compressible MHD that incorporates the linear kinetic effects that play an important role at ion scales, i.e. Landau damping and finite Larmor radius corrections. This makes it very appropriate to address the problems of energy cascade and dissipation by kinetic effects.
The results that will be obtained throughout this proposal will have a profound impact on the current knowledge of SW turbulence and heating, and will have potential applications to the study of other astrophysical objects less accessible to direct in-situ measurements.