PI: Luca Franci

Although particles are accelerated to high energy throughout the universe, the underlying mechanisms are still not fully understood. Uncovering particle energisation is key for solving open issues in the heliosphere (e.g., solar wind origin and acceleration) and in astrophysical systems (e.g., interactions of stellar winds with exoplanetary magnetospheres), and for forecasting extreme space weather events. However, the plasma particle dynamics has a highly multi-scale nature, so it is extremely difficult to model. In the solar wind, it spans about eight orders of magnitude in length (from fractions of the radius of the electron gyromotion to the Sun-Earth distance) and about five in energy. These ranges are way more extreme in astrophysical systems.

In this 3-year project, we are using our revolutionary approach —the multi-scale Box-in-Box (BIB) framework— to unveil the particle dynamics in turbulent plasmas, and thus their energisation, over a massive range of spatial scales by modelling their large–scale propagation while retaining a realistic and self–consistent description of the microphysics. First, we model turbulence from very large to very small scales coupling different models (Hall-Magnetohydrodynamics, hybrid, and fully kinetic), using a portion of a large simulation (mother) of fully developed turbulence as initial condition for another simulation with a smaller domain but higher resolution (daughter), and repeating this procedure multiple times in sequence. Then, we advance the trajectories of hundreds of millions of test particles on the evolving turbulent plasma background from small to large scales while their gyroradius (and energy) increases.

Figure: Basic principle and first results of our innovative BiB approach. Top: a portion of a Hall-MHD mother simulation of plasma turbulence is used as initial condition for a smaller but better resolved hybrid daughter one, which is then further evolved. Bottom left: the power spectrum of the magnetic field of the mother simulation is preserved and extended at small scales by the daughter one. Bottom right: first power spectrum of the magnetic field spanning 4 decades obtained by coupling three Hall-MHD simulations using BiB, in remarkable agreement with spacecraft measurements in the solar wind.