Alfvén waves in the Earth's magnetosphere

The environment surrounding the Earth that is dominated by the terrestrial magnetic field is called the magnetosphere. It comprises a highly structured magnetic field and plasma distribution. Transient events in the magnetosphere can arise from interaction with the solar wind or internal reconfigurations via reconnection, which are a natural source of magnetohydrodynamic (MHD) waves. We shall focus on the global scale waves that have wavelengths similar to the size of the magnetosphere. The different wave modes naturally couple to each other due to the structured nature of the magnetosphere. In particular coupling of the fast magnetoacoustic mode and the Alfven mode are common throughout the magnetosphere and are used in magneto-seismology studies. Alfven waves can also be excited when unstable particle distributions arise. Whatever the source of the Alfven waves, they are an agent for carrying field-aligned currents which can become particularly intense near the Earth. Consideration of these strong currents requires a two-fluid or kinetic treatment which indicate the need for a region (at altitudes of 1-2Re) where electrons are accelerated to keV energies and can produce optical auroral emissions. Alfven waves can also modify the ionosphere and induce ground currents, both of which have important implications for Space Weather. We review recent advances including the theory of Alfven wave excitation in a 3D equilibrium and particle acceleration.