Alfvén waves in the solar wind

Identification of Alfvén waves in the solar wind by means of spacecraft measurements was first achieved in late 1960s. Subsequent study of these waves has found that Alfvén waves in interplanetary space is a very complicated topic. Two and half decades after the first discovery there are still many issues unresolved. The only agreement reached by scientists is that the predominant component of the Alfvén waves observed in the solar wind is of solar origin. What is the generation mechanism and how these waves evolve in interplanetary space are still unclear. But on the other hand there exists another family of Alfvén waves which are not of solar origin. The best example is those waves generated in the exosphere of a comet. For these waves the physics is much better understood. In the present paper we discuss the two families of Alfvén waves separately.