Key concepts

• satellite radio communication and navigation

• ionosphere

• aurora

• potoionization

Introduction

Earth's upper atmosphere plays an important role in ground-based and satellite radio communication and navigation, and its density determines the lifetime of low-Earth orbiting (LEO) satellites. The upper atmosphere is composed of primarily neutral atoms and molecules in a region called the thermosphere. Within the thermosphere the amount of ionized gas becomes appreciable and forms a region called the ionosphere. The thermosphere and ionosphere overlap in altitude, but because they describe two different particle populations (neutral and ionized), they are often “divided” since what influences the structure and motion of one usually does not necessarily directly drive the other. However, the two populations are coupled through particle collisions (neutral-ion interactions), which means that you usually cannot neglect one or the other. Since the thermosphere-ionosphere system is so important to radio wave propagation and LEO satellite lifetimes, it is one of the crucial areas of study for space weather.