A new role for solar wind plasma expansion in heat flux regulation

Abstract:

Ulysses observations showed that neither collisional processes nor solar wind expansion, alone, can explain the observed heat flux evolution with heliocentric distance [Scime et al, 1994]: collisionless processes, namely heat-flux instabilities, must contribute to heat flux regulation in the solar wind [Gary et al, 1975].

In previous work, we have introduced the fully kinetic, semi-implicit code EB-iPic3D [Innocenti et al, 2019a]. EB-iPic3D introduced the Expanding Box Model, EBM [Velli et al, 1992], into the semi-implicit, fully kinetic code iPic3D [Markidis et al, 2010], for self-consistent simulations of fully kinetic dynamics at scales intermediate between the electron and the ion characteristic scales.

Then, we have shown that solar wind expansion can trigger the onset of kinetic instabilities [Innocenti et al, 2019b].

Now, we examine the role of plasma expansion in regulating heat flux. We demonstrate through fully kinetic EBM simulations that this role is indirect: expansion affects heat flux by triggering, or modifying the evolution, of kinetic instabilities, which in turn regulate the heat flux [Innocenti et al, 2020, in press].

Heat flux instabilities affect the electron Velocity Distribution Function (eVDF) in several ways: they reduce the drift velocity between the electron species, alter the skewness of the eVDF, scatter one population into another. We show that, at least in our simulations, the first process chiefly determines heat flux evolution.

Our work intends to provide an interpretation framework for coordinated Parker Solar Probe/ Solar Orbiter/ Earth observations of magnetically connected plasma parcels at large heliocentric distances.