The influence of plasma density irregularities on whistler-mode wave propagation

Whistler mode (W-mode) waves are profoundly affected by Field-Aligned Density Irregularities (FAT) present in the magnetosphere. These irregularities, present in all parts of the magnetosphere, occur at scale lengths ranging from a few meters to several hundred kilometers and larger. Given the spatial sizes of FAT and typical wavelength of W-mode waves found in the magnetosphere, it is convenient to classify FAT into three broad categories: large scale FAT, large scale FAT of duct-type, and small scale FAT. We discuss experimental results and their interpretations which provide physical insight into the effects of FAT on whistler (W) mode wave propagation. It appears that FAT, large or small scale, influence the propagation of every kind of W-mode waves originating on the ground or in space. There are two ways FAT can influence W-mode propagation. First, they provide W-mode waves accessibility to regions otherwise not reachable. This has made it possible for W-mode waves to probe remote regions of the magnetosphere, rendering them as a powerful remote sensing tool. Second, they modify the wave structure which may have important consequences for radiation belt dynamics via wave-particle interactions. We conclude with a discussion of outstanding questions that must be answered in order to determine the importance of FAT in the propagation of W-mode waves and on the overall dynamics of wave-particle interactions in the magnetosphere.