Why is the air density low in the thermosphere?

The air density in the thermosphere is low due to its high temperature and the significant distance from the Earth's surface. In this layer, solar radiation causes gas molecules to absorb energy and become highly energized, leading to their expansion and increased kinetic energy. As a result, the molecules are spread further apart, reducing the overall density of the air. Additionally, the thermosphere contains very few particles compared to lower atmospheric layers, contributing to its low density.