Conclusions and perspectives

8.2 Conclusions and perspectives

We have discussed the kinetic physics of the solar corona and solar wind. Modern in situ solar wind plasma measurements and coronal plasma observations, remotely through imaging and spectroscopy, indicate significant deviations of the plasma from thermal equilibrium. The existing ample evidence asks for a substantial revision of the conventional transport paradigm. We conclude that understanding the thermodynamics (heating) of the solar corona and the dynamics (acceleration) of the solar wind will ultimately require to consider kinetic issues and to go beyond a fluid description. In the following we define promising central areas for future research:

New multi-fluid or kinetic models involving plasma particles and waves need to be developed, allowing us to understand better the thermal kinetics and plasma dynamics of the Sun’s corona and the solar wind.
Novel kinetic concepts, capable of describing the self-consistent evolution of wave spectra and particle VDFs and their spatial and spectral transfer and coupling, are required to obtain a deeper physical insight into the coronal heating mechanism.
Small-scale dissipation processes of waves and fluctuations should be studied in depth, because turbulent wave energy transport, as well as cascading and dissipation in the kinetic domain, are expected to play a key role in the thermodynamics of tenuous space plasmas.

In the case of the solar corona and wind we are dealing with a complex kinetic system, which to understand requires a multiple-scale analysis and multiple-species description (electrons, protons, and heavy ions). They all have to be considered, since each species has a unique and prominent role to play, like electrons in heat conduction, protons and alpha particles in mass and momentum transport, and heavy ions in radiative cooling and wave-induced preferential heating. This heat is then gradually shared with protons and electrons by weak but unavoidable Coulomb collisions.

Therefore, it is time to develop a novel wave-based plasma transport scheme, which will enable us to achieve a new type of fluid closure, or to obtain the appropriate transport coefficients. They will complement or replace the ones presently available and in use for Coulomb-collision mediated transport.

Finally, it is worth stressing that theoretical progress will require adequate observations and new data. Up to now, the solar wind was only observed in situ at heliocentric distances larger than $0.3 AU$ (Helios perihelion). This is far away from the inner corona where the solar wind originates. Furthermore, even the present-day remote-sensing observations (such as made by SOHO and TRACE) of the solar corona do not provide the needed comprehensive diagnostics of the coronal plasma state. We can still not fully probe those regions where the corona is heated and the wind becomes supersonic. Therefore, novel observational strategies, including in situ measurements to be made at the closest possible (Solar Probe) distances from the solar surface, are required to make progress in the empirical phenomenology and physical understanding of coronal heating and solar wind acceleration.