Which layer is not part of the five principal layers of the atmosphere arranged from lowest to highest?

The correct answer is the ionosphere because it is not considered one of the five principal layers of the atmosphere when they are arranged from lowest to highest. The five main layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

The troposphere is the lowest layer where weather occurs and where we live. Above that, the stratosphere contains the ozone layer, which protects us from harmful UV radiation. The mesosphere is located above the stratosphere and is where meteors burn up upon entering the Earth's atmosphere. The thermosphere is the layer that contains a small proportion of the atmosphere but is very hot due to the absorption of high-energy radiation. Finally, the exosphere is the outermost layer where the atmosphere thins out and merges with outer space.

The ionosphere, while an important region of variations in the Earth's atmosphere, particularly for radio wave propagation, is not categorized among the primary atmospheric layers. It actually exists within the thermosphere and extends into parts of the mesosphere and exosphere, rather than being a separate principal layer.