Excitation of an Alfvén wave in the magnetosphere's tail with the help of a proton accelerator.

The excitation of Alfven waves at large geocentric distances by proton beams with an energy of about 50 keV is calculated. The calculations are based on the assumptions that the magnetotail is a cylinder with a homogeneous magnetic field and plasma concentration, that the plasma is an ideally conducting incompressible fluid, and that the beam moves less rapidly than the velocity of the Alfven wave dispersion. Reasons for the amplitude of the Alfven waves being proportional to the pressure gradient in the magnetotail proton beam moving towards earth are discussed.