Introduction

Early investigation of the terrestrial ionosphere through its effect on radio waves resulted in description by means of layers, principally the D, E, and F layers, the latter subdivided into F1 and F2 (see Vol. I, Fig. 12.1). This terminology continues to influence our current concept of the nature of energy deposition in atmospheres, although the misleading term “layer” has given way to “region”. The term “layer” arose from the observation of systematic variation in the height at which the critical frequency of reflection occurs in ionospheric radio sounding; this method cannot detect ionization above the peak of a region, which explains the appearance of layers. Radar and spacecraft measurements now give a more complete picture of peaks and valleys and reveal the complex morphology of the ionosphere. Chamberlain and Hunten (1987) provide a referenced discussion of the historical literature.

An overview of the altitude dependence and variability of Earth's ionosphere is given in Fig. 13.1, showing the diurnal and solar-cycle changes and the locations of the named regions. Space and planetary exploration has also found that the Earth's ionosphere is unique, just as its atmosphere is unique, and for some of the same reasons as we shall touch upon in this chapter.

An additional historical artifact in terminology is the word ionosphere itself. Because the atmospheric ionization was discovered before the neutral thermo sphere in which it is contained, anything above the stratosphere is often referred to as the ionosphere, resulting in a common misconception that this region of the atmosphere is mostly ionized.

Introduction

The tenuous, partially ionized plasma in planetary upper atmospheres is vulnerable to explosive and dynamic events from both the Sun and the lower atmosphere. The power of the Sun is continuously bombarding the atmospheres of planets with photons, energetic particles, and plasma. Some of the most dramatic solar events are the sudden release of electromagnetic energy during solar flares, and plasma from interplanetary coronal mass ejections (ICME). The intense solar radiation from a flare is the first to impact a planetary system, shortly followed by the arrival of relativistic energetic particles. Some time later, hours to days depending on the planet's distance from the Sun, the bulk of the plasma arrives to interact with, in some cases, the planetary magnetosphere; energy is then channeled into the upper atmospheres and ionospheres. The upper atmospheres are subjected to dramatic changes in external forcing by these types of events, by as much as a factor of two in total energy deposited, by an order of magnitude for individual processes, and by several orders of magnitude in some wavelength bands.

The upper atmospheres of planets are also being pushed and jostled by energy and momentum propagating upward from the dynamic chaotic lower atmospheres. The total solar irradiance driving the lower atmospheres is invariant except for the fraction of one percent changes observed over a solar cycle. Estimates have been made of the impact of longer-term changes in solar radiative output on Earth's climate, an area that is explored further in Vol. III.