Kinetic physics in the solar wind: local processes and global consequences*

Parker Solar Probe and Solar Orbiter observations have confirmed that kinetic scale processes are ubiquitous in the solar wind. The spatial and temporal scales of kinetic instabilities are smaller and shorter than system scales by several orders of magnitudes. However, they contribute to shape large-scale solar wind dynamics. 

Recent PSP observations [e.g., Cattell et al, 2021; Jagarlamudi et al, 2021] have focused on whistler waves generated by whistler-type instabilities, and on their role in scattering electrons from the strahl to the halo and in heat flux regulation. 

The contribution of collisionless kinetic instabilities in heat flux regulation is supported by simulations [e.g., only in the last two years, Kuzichev et al, 2019; Lopez et al, 2019 & 2020; Vasko et al, 2019, Verscharen et al, 2019; Innocenti et al, 2020; Micera et al, 2020] and observations, even quite close to the Sun [Halekas et al, 2020]. Given the role of heat flux in the solar wind energy balance, one could argue that, through heat flux regulation, kinetic processes significantly affect global heliospheric dynamics.

It is well known from simulations and observations that solar wind plasma expansion influences the onset and evolution of a number of kinetic instabilities, at the ion [Hellinger et al, 2003, 2008, 2013; Matteini et al, 2006] and electron [Innocenti et al, 2019b] scale. 

In this talk, we will review the role of kinetic physics in large scale heliospheric dynamics. We will focus in particular on the modeling of small-scale, fast kinetic processes against the backdrop of (slow, large scale) solar wind plasma expansion. With the support of simulations performed with the fully kinetic, Expanding Box Model code EB-iPic3D [Innocenti et al, 2019a], we will then show how solar wind expansion can indirectly contribute to heat flux regulation by affecting the evolution of heat flux regulating instabilities [Innocenti at al, 2020; Micera et al, accepted].