Guided propagation of Alfvén waves in the magnetosphere

The guided propagation of Alfvén waves in a cold plasma is discussed from several points of view. First the field of a line source is calculated at infinity. This calculation leads to a divergence in the radiated power in two directions forming very small angles with the magnetic field.

The divergence is removed if the line source is replaced by a strip source of finite width. It is shown that propagation from a strip source only a few kilo-metres wide is strongly guided by the magnetic field, with almost no spreading, along a field line extending up to, say, 4 earth radii for wave periods of about 100 sec.