Anisotropic kinetic effects of photoelectrons on polar wind transport

There is increasing observational evidence that photoelectrons may affect polar wind dynamics. For example, suprathermal electron pitch-angle distributions in the photoelectron energy range have been observed in the high-altitude polar wind. These distributions contribute little to the polar wind density, but carry an appreciable outward heat flux. Evidence of such reflected photoelectron distributions at low altitudes have been attributed to field-aligned potential drop. More recently, measurements of day-night asymmetries in electron temperature and ion outflow provide further indications of the photoelectrons’ impact on the polar wind. Such non-thermal fluxes can be explained by a mechanism relying on the earth’s decreasing magnetic field, the field-aligned potential drop, and the energy dependence of the Coulomb collisional cross-sections. The description of this mechanism requires a kinetic approach. Such an approach was used in a testparticle simulation of this mechanism, in agreement with the measured suprathermal fluxes. However, the effects of these fluxes on the polar wind itself require a self-consistent description. Unfortunately, a fully kinetic self-consistent description is at present not achievable. Instead, we suggest a hybrid approach, in which the background features of the polar wind are described by well-established fluid models, while the suprathermal features are described using a kinetic model. This approach retains the expediency of fluid theory while in effect extending its applicability. In this paper, we will review the physics underlying the mechanism mentioned earlier, discuss how the kinetic-fluid synthesis can best be achieved, and present our latest results. Our initial calculations show, for example, that the suprathermal electrons carry much of the polar wind heat flux, and may significantly increase the ambipolar electric field. This increase in the electric field can change the dynamics of the polar wind outflow.