Solar wind acceleration obtained from kinetic models based on electron velocity distribution functions with suprathermal particles

Abstract:

Astrophysical and space plasmas are commonly found to be out of
thermal equilibrium, i.e., the velocity distribution functions (VDF)
of plasma particles cannot be described well enough by Maxwellian
distribution functions. The suprathermal populations are ubiquitous
enhancing the high-energy tail of the distribution. A kinetic model
has been developed to successfully
describe such plasmas with tails decreasing as a power law of the
velocity. In the present work, we show that a natural heating of
solar and stellar coronas automatically appears when an enhanced
population of suprathermal particles is present at low altitude in
the solar (or stellar) atmosphere. This is true not only for
electrons and protons, but also for the minor ions which exhibit a
temperature increase proportional to their mass. Moreover,
suprathermal electrons contribute to the acceleration of stellar
winds to high bulk velocities when Coulomb collisions are neglected.
These results are illustrated by using a global model of the solar
corona and solar wind based on VDF with suprathermal tails for the
different particle species. The energetic particles are non-collisional
(without Coulomb collisions) even when thermal
particles are submitted to collisions. In the presence of long-range
correlations supplied by the fields and plasma instabilities,
turbulence can play a role in the generation of such suprathermal
tails. Solar wind observations are used as boundary
conditions to determine the VDF in the other regions of the heliosphere.
Consequences of suprathermal particles are also illustrated
for other space plasmas like the plasmasphere and the polar wind of
the Earth and other planets.