Kinetic turbulence in the inner heliosphere

Turbulence in plasmas involves a complex cross-scale coupling of fields and distortions of particle velocity distributions, with the emergence 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. The heliosphere, characterized by nonlinear processes, represents the best natural laboratory to study in-situ plasma turbulence. Thanks to new solar missions, namely the NASA Parker Solar Probe and the ESA/NASA Solar Orbiter, it is finally possible to study the radial evolution of the solar wind as it expands in the inner heliosphere, from the solar corona out to 1 AU. 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). Such an intermittent nature has also been found to evolve with distance from the Sun in fast streams, possible due to the emergence of coherent structures, namely strong non-homogeneities of the magnetic field over a broad range of scales. Here, the nature of the turbulent fluctuations close to the ion scales, is investigated by using high-time resolution magnetic field data in different regions of the heliosphere and in different solar wind conditions. 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. 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.