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.
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