Have you ever tried to cook rice pudding?
All the energy that escapes from the
Sun is produced in the core region by the
process of nuclear fusion.
This energy is slowly carried towards the solar
surface where 99.9% of it is radiated away.
This provides the vital sunlight that we need for life
on Earth.
Here we shall only talk about the way that energy is transported in the outer 25% of the Sun. We start in a shell of about 200,000 km deep just below the surface (photosphere). The energy is moved towards the photosphere by convection ( the green region in the top right corner of the diagram). Convection is a strange phenomena, but you must already know what it is!
Have you ever tried to cook rice pudding?
A rice pudding provides us with a very simple way to show the effect of convection. As it is heated from below the milk and rice start to rise up in the middle of the saucepan. At the top surface of the milk, the vertical rise is changed into a horizontal motion away from the center towards the side of the saucepan. During the horizontal motion energy escapes into the air above it. The milk and rice cools down and eventually sinks down again.
Why does the milk (and the gas in the Sun) behave in this way? This is very complicated, but the basic idea behind it is simple. When the saucepan is heated from below the milk close to the bottom becomes hotter than the milk nearer the top. When the milk becomes hotter it expands and therefore becomes lighter than the colder milk above it. It thus starts to rise towards the surface. At the top surface it is cooled by the cold air above it. As it cools it contracts, gets heavier than the hot milk and sinks down again - the same principle is used to get hot air balloons airborne, the hot air in the balloon is lighter than the surrounding cold air and it can therefore lift the balloon of the ground!
As the rice pudding continues cooking, the rice slowly absorbs the milk and the convective motion is slowed down. Eventually the pudding gets so thick that the convection stops. At this stage the large scale convective motion is no longer able to transport the heat from the cooker to the surface quickly enough and the milk starts to evaporate close to the bottom of the saucepan. This forms hot air bubbles that rise through the rice pudding. They burst when the layer of pudding above them can no longer withstand the pressure from below - just as when you blow air into water!
The solar convection zone moves energy in exactly the same way and the large scale motions at the solar surface contain a lot of energy that may be used indirectly to heat the corona.
Here is a movie that shows the convective motions at the photosphere.
White is hot
and dark is cold photospheric plasma.
The average diameter of the white cells
is 700 km and their life time
about 5 minutes.
The movie those represents a several minutes observation.
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