Observing the Corona Online University shirt sun shows. That the coronary solar corona outside the sky is millions of kelvin hotter than the panoramic image shown below. This is in stark contrast to the interior of the sun where the temperature (predicted by the models) is a reduced function of the radial distance from the core to the photosphere. What causes this is an open question in Astrophysics and a field of active research.
The Corona Online University shirt mechanism responsible for coronary heating cannot be heated (radiant heat, convection or conduction) because it will require the flow of heat from the colder zone to the warmer zone in violation of the second law of thermodynamics. That is, some non-thermal mechanisms are needed to bring the energy from inside the cooler sun to the hotter corona.
But the difficulty lies in explaining how they can deliver the right amount of energy to the right place so that the observed rim temperature records can be drawn in theory.
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