1.2 The importance of kinetic physics
Kinetic processes prevail in the solar corona and solar wind. Since the plasma is tenuous,
multi-component, non-uniform, and mostly not at LTE (Local Thermodynamic Equilibrium) or collisional
equilibrium conditions, multi-fluid theories or kinetic physics are required for an adequate description of
many coronal and solar wind phenomena. The coronal plasma is stratified and turbulent, and strongly
driven by the underlying photospheric magnetoconvection, which is continuously pushing around the
magnetic field lines reaching out into the corona. Thus the field contains ample free energy for
driving plasma macro- and micro-instabilities. Consequently, magnetohydrodynamic as well as
kinetic plasma waves and associated wave-particle interactions are expected to play a major
role.
Certainly, Coulomb collisions also matter, which are kinetically described by the Fokker–Planck operator
(see, e.g., Montgomery and Tidman, 1964
). However, excitation, scattering and absorption of waves, either
of fluid or kinetic type, will dominate over collision effects. The consequences for the velocity distribution
function (VDFs) are often described by a quasilinear diffusion operator involving the wave spectra. The key
problem then is to understand the transport properties of the weakly collisional corona (and solar wind),
which requires consideration of multiple scales, spatial non-uniformity and most likely also temporal
variability.
The solar wind consists of electrons, protons, alpha particles and heavy ions. Kinetic plasma physics
deals with their collective behaviour as a statistical ensemble. Space-borne particle spectrometers enable us
to measure the composition and three-dimensional velocity distribution functions (VDFs) of the particles.
The Vlasov/Boltzmann kinetic plasma theory provides the adequate means for their theoretical description.
Key issues of kinetic physics are to address the coronal origin and acceleration of the wind and the spatial
and temporal evolution of the particles’ VDFs. They are shaped through the forces of the Sun’s
gravitational field, the average-macroscale and fluctuating-mesoscale electric and magnetic fields of
interplanetary space, and through multiple microscale kinetic processes like binary Coulomb collisions and
collective wave-particle interactions. Although, coronal expansion is irreversible, the solar wind microstate
carries distinct information about the coronal plasma state in the source region, and thus in situ
measurements allow for inferences and provide a kind of remote-sensing diagnosis of the coronal
plasma.
