Speaker
Description
Turbulence in plasmas involves a complex cross-scale coupling of fields and distortions of particle velocity distributions, with the generation of non-thermal features. How the energy contained in the large-scale fluctuations cascades all the way down to the kinetic scales, and how such turbulence interacts with particles, remains one of the major unsolved problems in plasma physics. Moreover, solar wind turbulence is not homogeneous but is highly space-localized and the degree of non-homogeneity increases as the spatial/time scales decrease (intermittency).
Here, by means of new measurements by Solar Orbiter, the radial nature of the turbulent magnetic fluctuations around ion scales during the expansion of the wind, has been investigated. The ion scales appear to be characterized by the presence of non-compressive coherent structures, such as current sheets, vortex-like structures, and wave packets identified as ion cyclotron modes, responsible for solar wind intermittency and strongly related to the energy dissipation. Particle energization, temperature anisotropy, and strong deviation from Maxwellian, have been observed in and near coherent structures, both in in-situ data and numerical simulations. Furthermore, kinetic effects for both protons and alpha particles have been studied in presence of switchbacks, large deflections of the magnetic field which occur simultaneously with a sudden increase in the radial solar wind velocity. Very interestingly we observe a clear correlation between switchbacks and alpha particle temperature, but not with proton temperature, suggesting a role of magnetic field deflections in preferentially heating heavy ions. Finally, we investigate kinetic features in a multi-component turbulent plasma by means of Vlasov-Maxwell simulations.
Understanding the physical mechanisms that produce coherent structures and how they contribute to dissipation in collisionless plasma will provide key insights into the general problem of solar wind heating.
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