Why does the thermometer show a low temperature in the thermosphere?

The thermometer shows a low temperature in the thermosphere because temperature is a measure of the kinetic energy of particles, and despite the thermosphere having very high energy particles, they are sparse and spread out. This means there are not enough particles in this layer to transfer heat effectively to a thermometer. As a result, while individual particles can be very hot, the overall temperature reading remains low due to the low density of matter.