ABSTRACT
Magnetohydrodynamic waves in solar–terrestrial plasmas have very long wavelengths. This means that the study of propagation in a uniform medium is seldom applicable to real problems, except in a qualitative way. The dimensions of the system are often of the order of the wavelength, so that the nature of the boundary conditions is important. In addition, spatial gradients in unbounded plasmas introduce additional complexities. The unshaded region is the region of closed field lines in the magnetosphere. Sunspots and coronal loops are both complicated three-dimensional structures, consisting of regions of increased magnetic field and plasma density, elongated along the average direction of the magnetic field and of limited size perpendicular to this direction. Box models of the magnetosphere are very useful but it is often necessary to be more realistic. The inner magnetosphere has a magnetic field that is approximately dipolar. In general, its modes of oscillation are complicated and require numerical computation.
