Introduction

Although the earth's magnetic field has a very important influence on the propagation of radio waves in the ionospheric plasma, it is nevertheless of interest to study propagation when its effect is neglected. This was done in the early days of research in the probing of the ionosphere by radio waves, and it led to an understanding of some of the underlying physical principles; see, for example, Appleton (1928, 1930). In this chapter, therefore, the earth's magnetic field is ignored. For most of the chapter the effect of electron collisions is also ignored, but they are discussed in §§ 12.3, 12.11. The effect both of collisions and of the earth's magnetic field is small at sufficiently high frequencies, so that some of the results are then useful, for example with frequencies of order 40 MHz or more, as used in radio astronomy. For long distance radio communication the frequencies used are often comparable with the maximum usable frequency, §§ 12.8–12.10, which may be three to six times the penetration frequency of the F-layer. They are therefore in the range 10 to 40 MHz. This is large compared with the electron gyro-frequency which is of order 1 MHz, so that here again results for an isotropic ionosphere are useful, although effects of the earth's magnetic field have to be considered for some purposes.

This chapter is largely concerned with the use of pulses of radio waves and the propagation of wave packets, and uses results from ch. 10, especially §§ 10.2–10.6.