Radiowave Propagation: Physics and Applications

10

CHARACTERISTICS OF THE IONOSPHERE

10.1 INTRODUCTION¹

The region of the Earth's atmosphere from approximately 40 km altitude out to several Earth radii is known as the ionosphere. In this region, solar and cosmic radiation ionize neutral particles to produce regions of space containing large numbers of positively charged ions and free electrons, with peak electron densities of about N_e = 10⁶ particles/cm³ at an altitude of about 300 km. The presence of free electrons has a significant effect on wave propagation at HF and lower frequencies, allowing for propagation over long distances through ionospheric reflections, as we will discuss in Chapter 11.

The physics of the ionosphere, essentially involving the chemistry of the upper atmosphere and its interaction with solar radiation, has been studied extensively [1], but can be very complex. Factors that can influence the ionosphere include the Earth's magnetic field, the amount of ionizing radiation obtained from the Sun (which can increase dramatically in periods of high “solar activity”), and atmospheric chemistry and mixing. In this chapter, we will study one of the basic theories that describes the electron/ion production and recombination rates in an ionospheric “layer” [2,3]. This theory, developed by Chapman, will turn out to be applicable mainly to the so-called E and F₁ regions of the ionosphere, but provides useful insight into the behavior of other regions as well. We will also consider the basic structure of the ionosphere ...