Multispacecraft Observation of Unidirectional and Bidirectional Alfvén Waves within Large-scale Magnetic Clouds

Recent years have seen growing evidence of the existence of Alfvén waves within interplanetary magnetic flux ropes, which are believed to be an important aspect of dynamics connecting the Sun and the heliosphere. Previous studies, due to localized observation by single spacecraft, focused on sunward or antisunward Alfvén waves propagating along with magnetic field lines. In this Letter, for the first time, we use multispacecraft observations to verify and analyze two large-scale magnetic clouds (MCs), when the spacecraft had quite different spatial separations. What surprises us is that not only unidirectional but bidirectional Alfvén waves exist in the large-scale MC, which is rooted to the Sun. We speculate that unidirectional Alfvén waves within an MC are generated by distortions produced within a preexisting flux rope, and bidirectional Alfvén waves are emitted from the center of reconnection and then travel outward along with two loop legs of an MC.