Why are we interested in how hot the corona is? There two very important reasons!

• All energy from the sun is produced in the solar core - far below the solar atmosphere.
  We would naturally expect the temperature to decrease as the distance from the center of the Sun increases.

• The laws of physics suggest that energy from a gas of a given temperature cannot normally be used to heat another gas to a higher temperature.

This may be difficult to understand, so take the example where you have a saucepan containing hot water at 60 degrees centigrade. If you have a good thermometer you could measure the temperature of the water and the air above it. You would then find the air just above the water at the most would reach the same temperature as the water - it will never be hotter - and that the temperature would decreases with increasing height above the hot water.

The situation in the solar atmosphere should be very similar. However, the temperature does not continue to decrease with height! Somehow the corona is heated substantially and the natural question is therefore: