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, 1964Jump To The Next Citation Point). 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.

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