  Chapman's (1957) conductive model of the solar corona is characterized by a
temperature varying as r**(-2/7) with heliocentric distance r. The density
distribution in this non-isothermal hydrostatic model has a minimum value at
123 RS, and increases with r above that altitude. It is shown that this
hydrostatic model becomes convectively unstable above r = 35 RS, where the
temperature lapse rate becomes superadiabatic. Beyond this radial distance heat
conduction fails to be efficient enough to keep the temperature gradient
smaller than the adiabatic lapse rate. We report the results obtained by
Lemaire (1968) who showed that an additional mechanism is then required to
transport the energy flux away from the Sun into interplanetary space. He
pointed out that this additional mechanism is advection: i.e. the stationary
hydrodynamic expansion of the corona. In other words the corona is unable to
stay in hydrostatic equilibrium. The hydrodynamic solar wind expansion is thus
a physical consequence of the too steep (superadiabatic) temperature gradient
beyond the peak of coronal temperature that can be determined from white light
brightness distributions observed during solar eclipses. The thermodynamic
argument for the existence of a continuous solar wind expansion which is
presented here, complements Parker's classical argument based on boundary
conditions imposed to the solutions of the hydrodynamic equations for the
coronal expansion: i.e. the inability of the mechanical forces to hold the
corona in hydrostatic equilibrium. The thermodynamic argument presented here is
based on the energy transport equation. It relies on the temperature
distribution which becomes super-adiabatic above a certain altitude in the
inner corona.
