Simulated [Micera et al, 2021] and observed [Berčič et al, 2020] PSP eVDF. In both, suprathermal populations are visible in parallel and perpendicular cuts.

Kinetic physics, and the physics of electrons in particular, plays a fundamental role in the formation of the heliosphere. The solar wind heat flux controls the solar wind energy budget and determines the overall solar wind acceleration. Most of the solar wind heat flux is carried by the strahl, suprathermal electrons at coronal temperatures collimated by the magnetic field. During solar wind propagation away from the Sun, kinetic processes (among which, electron-scale instabilities, such as whistler-type instabilities) are believed to be the main drivers responsible for heat flux regulation. The efficiency of these wave-particle interaction processes depends on the solar-wind conditions and the heliocentric distance.

Within this project, we aim to further our understanding of the kinetic physics of solar wind energy flux through a balanced mix of observations (Parker Solar Probe – PSP, Solar Orbiter – SO, Magnetospheric MultiScale, Wind, but also legacy missions such as Ulysses and Helios), simulations and theoretical investigation.

We will break this challenge into three sub-problems.
First: the role of kinetic physics in solar wind heat flux regulation. We will investigate which kinetic processes contribute the most to heat flux evolution in the solar wind, how they vary under different solar wind regimes and with heliocentric distance, and how efficiently they regulate heat flux.
Second: how to address kinetic physics in solar wind modeling. We will provide analytical “heat flux operators” (or, at a minimum, limiting values for the heat flux), associated with the kinetic processes identified in step one. This will constitute an essential foundation towards efficient multiscale modeling of the solar wind, that partially retains kinetic effects.
Third, we will address the origin problem. We will reconstruct electron Velocity Distribution Functions (VDFs) in the low solar corona, well below the PSP distance of closest approach, using the novel PSP observations at our disposal as outer boundary. The reconstructed VDFs will bear signatures of the mechanisms behind the escape of high-energy electrons into the solar atmosphere, and will shed light on how the heliosphere itself is formed.
The outcome of this project will be a milestone in the integration of kinetic physics in the greater picture of heliospheric global processes.

Scientific rationale and objectives

The solar wind emerges from the Sun with an “energy endowment” consisting of a mix of kinetic and magnetic energy, the result of a conversion, through coronal processes still not fully understood [Klimchuk 2006], of the photospheric energy associated with solar granulation and super-granulation. The heliosphere is shaped by how this energy is transferred and dissipated, and hence by the evolution of the solar wind heat flux. In the solar wind, the heat flux is largely carried by the suprathermal components of the electron Velocity Distribution Function (VDF), strahl and halo [Feldman et al, 1975, Pilipp et al, 1987, Bale et al, 2013, Halekas et al, 2020 b]. The strahl is composed of field-aligned electrons that “evaporate away” along open magnetic field lines from the Sun into interplanetary space, due to their high energies [Meyer-Vernet, 2007]. The halo originates most probably from the scattering of the highly anisotropic strahl into a more isotropic population, due to wave/ particle interactions [Maksimovic et al, 2005, Štverák et al, 2015, Vasko et al, 2019, Verscharen et al, 2019, Micera et al 2020]. This process is one example of a kinetic process that contributes to regulating the heat flux in the solar wind. Understanding which kinetic processes regulate heat flux evolution in the solar wind, how efficient they are and how they vary under different solar wind regimes and with heliocentric distances will contribute to our understanding of what powers the heliosphere. This constitutes our first objective.

The heliosphere is fundamentally a multiscale environment. Kinetic processes develop at spatial and temporal scales of the order of meters and fractions of seconds, though some of them are triggered by macroscopic processes such as solar wind expansion. These small scale processes have fundamental large scale consequences. They not only constrain the evolution of solar wind observables [Štverák et al, 2008; Matteini et al, 2013; Berčič et al, 2019], but also contribute to heat flux regulation. Thus, they may play a fundamental role in determining the structure of the whole heliosphere. This is reflected by the fact that global heliospheric models rely on information on heat sinks and sources (see Štverák et al, 2015) in the wind. Providing an analytic description of heat flux evolution as a result of different kinetic processes is a necessary first step towards including key kinetic processes into large scale heliospheric models, and our second objective.

Electron VDFs in the low corona are more than just the “starting point” for solar wind evolution. They also carry signatures of the mechanisms behind the escape of high-energy electrons into the extended solar atmosphere and the acceleration of particles. This is the reason why, over the years, reconstruction of low corona electron VDFs has been attempted, e.g. in Ko et al, 1996, Pierrard et al, 1999: observed solar wind variables or electron VDFs at different vantage points were used to infer properties of this “primordial electron VDF”. Now, we are in a very privileged position: not only we can build on decades of existing work. We can also start our reconstruction from the new Parker Solar Probe (PSP) [Fox et al, 2016] and Solar Orbiter (SO) [Mueller et al, 2013] observations: our vantage points have moved dramatically closer to the corona, and the resolution of our observations has increased dramatically in both time and velocity space. Reconstructing electron VDFs in the low corona (lower than the PSP distance of closest approach) by leveraging PSP and SO observations is our third objective.

Our scientific objectives will be achieved via an integrated theoretical/ numerical and observational effort. We will make use of state-of-the-art Particle-In-Cell codes that have recently been modified to include solar wind expansion effects, and of state-of-the-art exospheric models. Observations from PSP, Helios, Ulysses, Wind, MMS, Artemis and now SO will allow us to investigate observed properties at various radial distances from the Sun.

Heat flux regulation is key to understanding heat transport also in astrophysical environments outside the solar system, such as the intracluster medium in galaxy clusters [Narayan & Medvedev, 2001] and stellar winds. Characterizing transport in collisionless plasmas is arguably a fundamental issue of basic plasma physics. Thus, the applicability of our results will go well beyond the solar wind and will grant insight into plasma processes governing remote astrophysical objects.