Turbulence evolution of coronal hole solar wind in the inner heliosphere: Solar Orbiter and Parker Solar Probe combined observations

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). Such an intermittent nature has also been found to evolve with distance from the Sun, possible due to the emergence of strong non-homogeneities over a broad range of scales.

Here, by means of new measurements by both Solar Orbiter and Parker Solar Probe, the radial evolution of a homogeneous recurrent fast wind, coming from the same source on the Sun (namely a coronal hole), has been studied from global properties and large-scale features to kinetic structures as it expands in the inner heliosphere from 0.1 out to 1 AU [Perrone et al., 2022]. In particular, the nature of the turbulent magnetic fluctuations around ion scales during the expansion of the wind, has been investigated and the observed coherent events both close to the Sun and to the Earth are statistically studied. 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.

 

Perrone, D., et al. (2022) Astronomy & Astrophysics (Special Issue: Solar Orbiter First Results – Nominal Mission Phase) 668, A189