7.1 INTRODUCTION

The Earth's upper atmosphere includes both neutral and ionized gases. The ionized gas is called a plasma, and the partially ionized region of the atmosphere is called the ionosphere. The ionospheric plasma environment is quite unusual; it is relatively cold, and consists of both molecular (e.g., and NO ⁺) and atomic ions (e.g., O ⁺, H ⁺, and He ⁺) [1]. When the ionospheric plasma is at rest, radio waves with frequencies above 30 MHz will almost always pass through the ionosphere unimpeded. However when the plasma is disturbed, plasma density irregularities can degrade the ionospheric radio channel at frequencies up to 2 GHz.

Plasma density irregularities frequently occur in the ionosphere at altitudes above 90 km. These field-aligned irregularities (FAI) are ion sound waves (near 100 km) or low-density cavities or bubbles (at higher altitudes) that are created by a variety of plasma instabilities [2, 3]. Scientists study these irregularities for a variety of reasons, ranging from pure scientific inquiry to practical applications.

Scientific investigation of plasma turbulence in the lower ionosphere relies heavily upon VHF and UHF radar remote sensing. The neutral atmosphere is too thin to loft balloons above 40 km, and too thick for durable satellites below about 200 km. The entire in situ sounding rocket data set for the lower ionosphere probably does not exceed 2-h duration.

A variety of radio methods are used to study the ionosphere, including techniques for measuring D-region...