On the role of solar wind expansion as a source of whistler waves: scattering of suprathermal electrons and heat flux regulation in the inner heliosphere.*

Results obtained by adopting an electron velocity distribution function that faithfully reproduces those recently observed during the PSP Encounter 1, and following its evolution with heliocentric distance, will be presented. They were obtained with the purpose of shedding light on the kinetic processes that influence the electron dynamics during solar wind expansion from the inner heliosphere onward. In order to do so, we carried out a two spatial dimensions, three velocity components (2D3V) fully kinetic Expanding Box Model simulation through which we could monitor the nonlinear development of kinetic instabilities and study their effects on the solar wind electrons. We observe that the expansion drives the solar wind into successive regimes where whistler heat flux instabilities are triggered. These instabilities produce sunward whistler waves initially characterized by predominantly oblique propagation with respect to the interplanetary magnetic field. The excited waves interact with the electrons via resonant scattering processes. As a consequence, strahl electrons are scattered in the direction perpendicular to the magnetic field, and an electron halo is formed. Wave-particle interaction processes are accompanied by a substantial reduction of the solar wind heat flux.