Why is the thermosphere so hot?

Question

Why is the thermosphere so hot?

Answer 1

Temperatures climb sharply in the lower thermosphere (below 200 to 300 km altitude), then level off and hold fairly steady with increasing altitude above that height. Solar activity strongly influences temperature in the thermosphere. The thermosphere is typically about 200° C (360° F) hotter in the daytime than at night, and roughly 500° C (900° F) hotter when the Sun is very active than at other times. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher.

Answer 2

The thermosphere has a high temperature, because the smalll amount of oxygen present is heated up rapidly by solar radiation, causing the molecules to move about rapidly (temperature is the measure of the average kinetic energy of the molecules).  The tempersture is very high (up to 2000 deg F), but the amount of heat energy would not be great, because of the thinness of the atmosphere.  Lower down there are more molecules, so on average they are not excited as much, and the temperature (aveage kinetic energy) is lower; but the lower atmosphere contains a lot of heat - enough to power the world's climate.