Introduction.

Thanks to recent space missions, we have the unique opportunity to analyze directly the dynamical behavior of natural plasmas, such as those in the heliosphere. These measurements also provide crucial insight into processes that are applicable to systems throughout the Universe (e.g. the interstellar medium, astrophysical shocks and jets, accretion disks, clusters of galaxies, etc). One important example of heliospheric plasmas is the solar wind, a continuous, but highly variable, weakly-collisional plasma outflow from the Sun, that travels at high speeds and interacts with the environment of the planets with (e.g. Mercury, Earth, Jupiter) and without (e.g. Venus, Mars) magnetic field. It has been well assessed that the solar wind temperature decreases with the distance from the Sun more slowly than expected for an adiabatic expanding gas [Marsch et al., 1982, Hellinger et al., 2013, Perrone et al., 2019a,b], meaning that locally some heating mechanisms should be at work [Marsch et al., 1983, Marsch and Richter, 1987]. In-situ spacecraft observations show that the solar-wind plasma is in a state of fully developed turbulence [Coleman, 1968, Bruno and Carbone, 2013]. The energy, stored in the electromagnetic and velocity fields, is injected at the Sun into the heliosphere and is channeled towards smaller time/spatial scales through a turbulent cascade until it is eventually dissipated. The power spectral density of magnetic and velocity field fluctuations at intermediate scales, within the so-called inertial range, shows a power-law decay similar to the one observed in fluid turbulence [Kolmogorov, 1941, Tu and Marsch, 1995]. It has also been observed, in fast solar wind, that the low-frequency spectral break, which separates the fluid-like range of turbulence from larger scales (i.e. lower frequencies), is shifted towards larger and larger scales as the wind expands and turbulence evolves, as shown in Figure 1 [Bruno and Carbone, 2013]. At scales smaller than intermediate scales, ions become unmagnetized and the plasma dynamics is governed by particle kinetic properties. Here, different processes come into play, leading to both changes in the spectral shape [Leamon et al., 1998, Bale et al., 2005] and departures of the particle velocity distributions from the thermodynamic equilibrium [Marsch, 2006, Maksimovic et al., 1997, 2000, Servidio et al., 2015, Graham et al., 2017]. Moreover, the high-frequency spectral break, separating the inertial range from the kinetic range, is shifted towards smaller scales (higher frequencies), as the radial distance decreases [Perri et al., 2010, Bruno and Trenchi, 2014]. Furthermore, 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) [Veltri and Mangeney, 1999, Bruno et al., 2003]. 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 [Bruno et al., 2003]. These structures can be described as strong non-homogeneities of the magnetic field [Retinò et al., 2007, Perri et al., 2012, Perrone et al., 2016, 2017] over a broad range of scales [Greco et al., 2016]. 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 (Figure 2) [Matteini et al., 2010, Servidio et al., 2012, 2015, 2017, Wu et al., 2013, Perrone et al., 2013, 2014a,b]. 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.

The actions of this international Team can be summarized in 3 main work packages that are detailed below.

I. Evolution, heating and energy transfer mechanisms in the young solar wind.

A topic of great interest in space plasma physics is to investigate the mechanisms of energy dissipation in solar wind turbulence and how they evolve in different regions of the heliosphere. Complex processes in a turbulent, chaotic, and weakly collisional flow, such as wave-particle interactions or magnetic reconnection occurring at kinetic plasma scales, are believed to cause the energy transport and conversion in the solar wind. Thanks to the innovative Parker Solar Probe (PSP) mission [Fox et al., 2016], launched in August 2018, the Team will investigate the problem of energy transfer and dissipation at heliocentric distances never reached in any past exploration. PSP is the first mission to travel directly into the Sun’s atmosphere, enabling the in-situ study of the dynamics of the young solar wind. First encounters between 36 and 54 solar radii have shown that solar wind is highly structured, much more than at 1 AU, with impulsive magnetic field reversals [Bale et al., 2019] and a net rotational flow [Kasper et al., 2019]. Phan et al., [2020] showed that the observed B field reversals at 30 – 40 solar radii are in general Alfvénic and can generate non-reconnecting current sheets. The experiments onboard PSP ensure the unique capability of studying particle acceleration, energy dissipation and so on in the solar wind, near its source region.

The Team will analyze data of the upcoming PSP encounters, at distances down to 20 solar radii. In particular, we will study events featuring energetic particles, plasma waves, reconnection exhausts and so on and stablish their generation conditions and mechanisms, and discuss their consequences for the evolution of solar wind.

II. Radial evolution of turbulence and dissipation down to electron scales.

Another important aspect the Team will address is the study of turbulence evolution as a function of heliocentric distance. PSP is providing data at very short distances from the Sun since 2018, Solar Orbiter (SolO) [Muller et al., 2013] is gathering data along its cruise phase at various heliocentric distances since 2020, and MMS spends large periods in the solar wind at 1 AU since its apogee increase in 2017, including a dedicated campaign carried out in 2019, where the four spacecraft were in string of pearls configuration, the particle instruments were adapted and the spacecraft were tilted to improve electric field measurements in the solar wind. We will combine measurements by PSP, SolO and MMS, to build a comprehensive catalogue of observations with varying heliocentric distances, from tens of solar radii to 1 AU. Changes in properties like, e.g., the plasma beta impact how the dissipative processes behave, and possibly which ones dominate in different heliospheric regions. Magnetic reconnection and waves are known to play a major role in particle energization in collisionless plasmas [e.g., Vaivads et al., 2004a, b, Phan et al., 2013, 2014, Lavraud et al, 2016, Toledo-Redondo et al., 2016, 2017]. Furthermore, recent studies of turbulence in the Earth’s magnetosheath have provided new insights into the interplay between turbulence and magnetic reconnection occurring at electron scales [Phan et al., 2018, Stawarz et al., 2019].

The Team will study the evolution of solar wind turbulence, using both electromagnetic and particle data, at different heliocentric distances. Previous measurements allowed to study both the inertial range and the ion kinetic spectral break (Figure 1) [e.g., Bruno and Carbone, 2013]. The new datasets will allow us to perform detailed statistics as a function of radial distance and solar wind conditions, resolve down to electron scales using the MMS datasets, and study the role of magnetic reconnection and wave-particle interactions as dissipation mechanisms.

III. Multiple ion species in the solar wind and their effects at kinetic scales.

High-resolution particle distribution functions data are crucial in order to understand the particle energization mechanisms in collisionless plasmas [Vaivads et al., 2016]. The understanding of the interplay at kinetic scales of multiple ion populations has recently been advanced owing to the high-resolution measurements made by MMS in the Earth’s magnetosphere [e.g., Toledo-Redondo et al. 2016b, 2018, Graham et al., 2017]. Owing to its composition, the solar wind needs to be studied from a multi-scale perspective [Raines et al., 2005, Tracy et al., 2016]. Energization processes occurring at various scales may have different weights in terms of energy dissipation at different heliocentric distances. SolO allows us to study the kinetic features of the major ion solar wind constituent (proton and alpha particles) with an unprecedented time resolution. Moreover, for the first time, it is possible to have access to the three- dimensional velocity distribution of the minor ions and, finally, measure their temperature anisotropy in the inner heliosphere. In the last decade, a Eulerian hybrid Vlasov-Maxwell (HVM) algorithm [Valentini et al., 2007, Perrone et al., 2011] has been applied on a turbulent system, in typical condition of the solar wind [Valentini et al., 2008, 2009, 2010, 2011, 2014, 2016, Perrone et al., 2013, 2014a, 2014b, 2018, Servidio et al., 2012, 2014, 2015].

Nowadays, this code represents an indispensable tool to investigate kinetic effects in turbulent collisionless plasmas, due to its low computational noise. These simulations show that kinetic effects produce, close to the reconnection regions, a distortion of the ion velocity distribution functions, through the generation of accelerated field-aligned beams, temperature anisotropy and trapped particle populations (Figure 3). Moreover, departures from Maxwellian are more pronounced for heavy ions than for protons [Perrone et al., 2011, 2013, 2014a, 2014b, Valentini et al., 2016], pointing out that the role of secondary ions must be considered for understanding energy partition.

The Team will perform direct comparisons between kinetic Vlasov simulations and SolO in-situ measurements of the multiple ion distribution functions and electromagnetic field data in the solar wind at various heliocentric distances, with the aim of understanding which kinetic processes (magnetic reconnection, instabilities and wave particle interactions) occurring at ion scales are responsible for ion energization and cause the spectral break of turbulence. We expect the relative importance of these processes to change as a function of heliocentric distance.

2.2 Main scientific goals

The main scientific goals of this Team consist in answering the following key questions:

I Evolution, heating and energy transfer mechanisms in the young solar wind

• Which mechanisms heat and accelerate the young solar wind?
• What is the nature and the origin of waves and small-scale structures in this region?
• Does magnetic reconnection play a major role in the early stages of the solar wind?

II. Radial evolution of turbulence and dissipation down to electron scales

• How does turbulence evolve as a function of radial distance within the heliosphere?
• Which are the dominant dissipation mechanisms near the solar corona vs at 1 AU?
• Which mechanisms are responsible for turbulent energy dissipation in collisionless plasmas?

III. Multiple ion species in the solar wind and their effects at kinetic scales

• How does the energy partition between protons and alpha particles evolve with distance to the Sun?
• How can the heavy ions (including minor species) be introduced into our understanding of themicrophysical processes leading to dissipation?
• Which kinetic ion processes dominate the ion spectral break?