Summary of Main Project Outcomes

• Multiple proton beams in the solar wind have been observed for decades. Using the very high cadence from PAS onboard Solar Orbiter we have identified, for the first time, magnetic reconnection as a mechanism capable of producing multiple proton beams in the solar wind, although our observations indicate that reconnection alone cannot account for all proton beams that are typically observed, suggesting that other mechanisms must be involved as well.
• We investigated electron-scale reconnection in the magnetosheath using MMS data and found the magnetic correlation length is a good indicator of the prevalence of electron-only reconnection in turbulent plasmas, with smaller correlation lengths being associated to higher number of electron-scale reconnection current sheets. On average, we find that ~10% of thin current sheets in magnetosheath have clear evidence of ongoing reconnection.
• We conducted a statistical survey of magnetic switchback observations made by PSP and found a systematic clockwise deviation of their orientation with respect to the Parker spiral model. This systematic deviation supports the hypothesis that interchange reconnection in the low corona may be the main mechanism for switchback. Using a subset of the switchback database during PSP encounter 5, we found that their length-scale may be associated to granulation and super-granulation scales at the Solar atmosphere.
• The radial evolution of a homogeneous fast stream has been studied, for the first time, between 0.1 and 1 AU, by using both measurements from Solar Orbiter and Parker Solar Probe, both connected to the same coronal hole. We observed a high level of intermittency, related to the presence of coherent magnetic events, which decreases as the distance from the Sun increases. No compressive coherent structures characterize the magnetic field fluctuations at ion scales, suggesting a significant role in plasma heating.
• The nature of turbulent magnetic fluctuations around ion scales has been studied in an Alfvénic slow interval observed by Solar Orbiter at 0.64 AU. Current sheets and vortices (which also appear in chain) have been investigated in detail. The presence of these intermittent events is strongly correlated to kinetics effects of both proton and alpha particle velocity distribution functions, which significantly deviate from thermodynamic equilibrium.

ISSI team project

We propose a Team of 12 scientists to advance our current understanding of the energy dissipation in the solar wind, one of the most compelling problems in space plasmas. The solar wind has been observed to be hotter than expected for an adiabatic expanding gas. Understanding the mechanisms of energy dissipation into heat from the Sun in such an almost collision-free system represents a top priority in space physics and will improve the understanding of other astrophysical plasmas. Moreover, explaining how irreversible heating is accomplished in such a weakly collisional environment represents a key challenge for thermodynamics in general, since any mechanism in which collisions are not present is lacking the part of the heating process related to the irreversible degradation of information.

The solar wind is observed to be turbulent. 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 of electromagnetic and velocity fields 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, with strong implications for space, astrophysical, and laboratory plasmas. The heliosphere, characterized by nonlinear processes, such as the generation of shocks, waves, coherent structures, magnetic reconnection and particle acceleration, represents the best natural laboratory to study in-situ plasma turbulence.

Thanks to new solar missions, namely the NASA Parker Solar Probe, launched on August 12th, 2018, and the ESA/NASA Solar Orbiter, successfully launched on February 10th, 2020, it will be finally possible to study, at unprecedented time scales, the radial evolution of the solar wind as it expands in the inner heliosphere, from the solar corona out to 1 AU. Moreover, the study of the microphysics in this turbulent medium will be significantly enhanced by knowledge acquired by near-Earth missions, such as the Magnetospheric Multiscale (MMS) mission, launched by NASA in 2015, which provides multi-spacecraft measurements of both the Earth’s magnetosphere and the solar wind at 1 AU, down to electron scales. These observations, focused on investigating the microphysics of energy transport and conversion in the solar wind at kinetic scales, will be supported by state-of-the-art Vlasov simulations, where the evolution of particle distribution functions can be studied to understand the nature of the energy dissipation and the corresponding energization of the plasma.

This Team will first select several intervals by radial distances and solar wind speed, collecting electromagnetic field and plasma measurements. Statistical analysis of turbulence in different environments, with the identification of coherent structures and magnetic reconnection events, will be conducted and the plasma energization will be studied through particle data. Then, the Team will use high-resolution measurements in conjunction with Vlasov simulations to study the microphysics of local energy dissipation. Further, simulations with different initial conditions, e.g. level of turbulence, ion to electron temperature ratio, and plasma beta (i.e., the ratio between kinetic and magnetic pressure) will be planned to ‘mimic’ the plasma at different heliocentric distances.

The Team proposed here includes 12 scientists (3 women, including the Team Leader) from 6 nationalities, involves 12 institutions and comprises experts of different domains, including in-situ observations and kinetic plasma simulations. Moreover, most of the team members have strong responsibilities in the key spacecraft missions or are directly involved in them. We plan to have three meetings in two years: (i) we ask ISSI Bern to fund two meetings, and (ii) a parallel proposal is submitted to the ESAC/ESA science faculty to support the organization of a third meeting in Madrid, Spain.