Chapman's (1957) conductive model of the solar corona is characterized by atemperature varying as r**(-2/7) with heliocentric distance r. The densitydistribution in this non-isothermal hydrostatic model has a minimum value at123 RS, and increases with r above that altitude. It is shown that thishydrostatic model becomes convectively unstable above r = 35 RS, where thetemperature lapse rate becomes superadiabatic. Beyond this radial distance heatconduction fails to be efficient enough to keep the temperature gradientsmaller than the adiabatic lapse rate. We report the results obtained byLemaire (1968) who showed that an additional mechanism is then required totransport the energy flux away from the Sun into interplanetary space. Hepointed out that this additional mechanism is advection: i.e. the stationaryhydrodynamic expansion of the corona. In other words the corona is unable tostay in hydrostatic equilibrium. The hydrodynamic solar wind expansion is thusa physical consequence of the too steep (superadiabatic) temperature gradientbeyond the peak of coronal temperature that can be determined from white lightbrightness distributions observed during solar eclipses. The thermodynamicargument for the existence of a continuous solar wind expansion which ispresented here, complements Parker's classical argument based on boundaryconditions imposed to the solutions of the hydrodynamic equations for thecoronal expansion: i.e. the inability of the mechanical forces to hold thecorona in hydrostatic equilibrium. The thermodynamic argument presented here isbased on the energy transport equation. It relies on the temperaturedistribution which becomes super-adiabatic above a certain altitude in theinner corona.
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