Event Details

Selected collisionless dynamics in the expanding solar wind

Abstract: Signatures of kinetic processes are ubiquitous is the solar wind. Electron and ion Velocity Distribution Functions are far from what one would expect from a fluid description [Marsch et al, 2012; Pilipp et al, 1987], and their evolution with heliocentric distance can confidently be attributed to wave/ particle interaction [Maksimovic et al, 2005]. Kinetic instabilities appear to constrain the bulk parameters of both ions and electrons in observations at different heliocentric distances [Matteini et al, 2013; Stverak et al, 2008; Bercic et al, 2019]. Parker Solar Probe observations reveal a young solar wind where electron beams and electron and ion scale instabilities are the norm [Bale et al, 2019; Halekas et al, 2019]. First principle modelling of these phenomena then becomes a priority in the Parker Solar Probe and Solar Orbit era. Such modelling cannot ignore an underlying process that couples non-trivially with both fluid and kinetic processes: solar wind expansion.

Solar wind expansion operates on scales larger and longer than the typical kinetic scales. However, it has been proved, observationally and numerically, that solar wind expansion triggers and modifies the evolution of several kinetic instabilities, with significant consequences on solar wind observables [Matteini et al, 2006, 2013; Hellinger et al, 2013, Bercic et al, 2019, Innocenti et al, 2019b].

Here, we describe a fully kinetic, semi-implicit, Expanding Box Model method that can simulate from first principle electron and ion dynamics in the expanding solar wind plasma [Innocenti et al, 2019a].We then show that collisionless dynamics alone can produce a trajectory in the beta_par vs T_perp/ T_par plane which is compatible with observations (in contrast with recent works that attribute such dynamics to collisional processes).Finally, we comment on a scenario where solar wind expansion indirectly participate in heat flux regulation by triggering kinetic instabilities, which are, in turn, responsible for heat flux regulation [Innocenti et al, under review]