These satellites orbited the earth over the poles and took measurements in the auroral zone at heights of 1700 km and 1000 km respectively. This is the upper ionosphere where a large proportion of the atoms are ionized, creating a plasma. The study is restricted to the subauroral region. This area is expected to be less dynamic than the area in which aurora is usually visible. Nevertheless exciting phenomena occur there as well.
Data shows localized thinning in plasma density, known as cavities.
There are many physical processes that can lower the local density of plasma and thus create cavities, for example wave-particle interaction, electric fields and plasma transport. The thesis deals with several different types of cavities and localized higher density ion clouds, and gives possible explanations as to why these occur. Both Freja and Astrid-2 were relatively cheap satellites. They have supplied a wealth of very interesting data and contributed to an increased understanding of our universe and some of the processes which affect its environment. We put forth that both types of fine structure may be explained by considering wave excitation in the presence of density structures that impose an eigenmode structure on the waves.